Quaternary Research Center

Mt. St. Helens and Mima Mounds Tour

Mt. St. Helens May 19, 1980 with eruption column.

Ronald S. Sletten (Department of Civil Engineering) and Lawrence C. Bliss (Botany Department) University of Washington, Seattle, WA

Ken Schlichte (Resource Planning and Asset Management Division) and Pat Pringle (Division of Geology) Department of Natural Resources, Olympia, WA

Peter Frenzen, Mount St. Helens National Volcano Monument, Amboy, WA

David J. Marrett, Dept. Crop and Soil Sciences, Oregon State University, Corvallis, OR

Chien-Lu Ping, Palmer Research Center, University of Alaska-Fairbanks, Palmer AK

Darcy Mitchem, Mount St. Helens National Volcano Monument, Toutle, WA

Prepared for the: American Society of Agronomy, Crop Science Society of America, Soil Science Society of 86th America Annual Meeting, Seattle, 1994, Sunday, November 13, 1994. Sponsored by Division S-5, Carolyn Olson, chair

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Acknowledgments

We wish to acknowledge the assistance of a number of individuals for providing information background information for this tour. All our requests for information were quickly attended to by representatives of the Soil Conservation Service, including: Terry Aho and Karl Hippel, Spokane, WA; David Guenther and Darin Houpt, Kelso, WA; Mike Blakeley, White Salmon, WA; and Kirsten Stuart and other personnel at the SCS Lincoln, NB. We are thank Steve Lowell, Wash. State Dept. of Transportation, for providing a copy of his field guide of the travel log for Spirit Lake Memorial Highway produced for the Highway Geology Symposium Field Trip, August 18, 1994. We are grateful for support received from the personnel of the Mt. St. Helens National Volcanic Monument.

We are also especially grateful to A. Link Washburn for visiting the Mima Mounds site and sharing his insights, and to F. C. Ugolini for his inspiration and for first introducing several of the authors of this guide to the Mima Prairie and Mt. St. Helens.

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Overview

We will visit two unique sites in western Washington: Mt. St. Helens and the Mima Prairie Mounds. Our first stop will be at the Mima Prairie Mounds, located in a rare area of native prairie in the southern Puget Sound basin. The Mima Prairie and Mounds formed in outwash deposits of the latest continental glaciation. Here at the original type location, the origin of the Mima Mounds, and of other Mima-like mounds elsewhere, has been a mystery since their discovery in 1840. We will visit an exposed section revealing several large mounds in profile; theories of genesis will be presented. Our next stops will be on Mt. St. Helens, which erupted catastrophically on May 18, 1980. This natural disaster disturbed a large area and provided a unique opportunity for ecological, geological, and pedological research. During the bus ride and on the mountain, scientists involved with research on Mt. St. Helens will discuss the geology, primary succession, early pedogenesis, and the Andisol order. We will visit a recently opened visitor's center on the west side of Mt. St. Helens, which on clear days provides great views of the mountain and of the mudflows down the Toutle river drainage. A local resident present during the eruption will provide a personal perspective of the event and its consequences for the nearby communities.

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Itinerary and map of travel route

7:00 Leave Seattle Convention Center

8:30 Arrive at the Mima Prairie (Littlerock)

10:00 Depart Mima Prairie

11:15 Arrive at Mt. St. Helen's National Volcano Monument (Silver Lake)

12:15 Depart MSHVC

12:30 Arrive to mudflow deposits on South Fork of Toutle (lunch)

1:30 Depart mudflow

2:30 Arrive to Coldwater Ridge Visitor's Center

4:30 Depart CWRVC

6:30 Return to Seattle Convention Center

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Introduction and Overview

Seattle and its greater metropolitan area

The following information on history and development is excerpted from: Paul (1987).

Seattle is the largest metropolis in the northwestern United States and is the seat of King County, Wash. Seattle is located on a narrow, hilly isthmus between Puget Sound on the west and Lake Washington on the east, and has a population of 516,259 (1990 census) in the area of the city proper and 1,972,961 in the greater metropolitan area. The city is the focal point of a highly urbanized zone that fronts the eastern shore of Puget Sound and incorporates several smaller nearby towns and cities.

The central area of Seattle is crowded onto the constricted isthmus, but its suburbs sprawl to the north, south, and east of Lake Washington, which is spanned by two bridges. Spectacular mountains dominate the horizon: the Cascades to the east (with Mount Rainier on the southeast) and the Olympics to the west. Although the city receives an average of only 890 mm (35 in) of precipitation annually, mostly in winter, there is measurable rainfall on an average of 150 days a year. Temperatures are normally mild throughout the year.

Seattle's deep and well-protected Elliott Bay harbor requires no dredging, and it has long been a major Asia-oriented port as well as the principal gateway to Alaska. Its containerized shipping facilities are also among the largest in the world. Since the 1920s, however, the Boeing aircraft company has been the mainstay of the city's economy. Shipbuilding and the making of wood products are other major industries. Seattle is the home of the University of Washington (1861) and Seattle University (1891). The city has a symphony orchestra, professional theater companies, an opera company, and numerous museums, including the Seattle Art Museum, which in 1991 moved into a new building. Professional football, baseball, and basketball teams are also in Seattle.

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History

The earliest known northwesterners have inhabited western Washington since the end of the Pleistocene glaciation, about 10,000 years ago. The native inhabitants established many villages and subsisted primarily by fishing (Schwantes 1991).

Seattle was first visited by Europeans in 1792 led by Vancouver and settled by Europeans in 1851. Seattle was laid out 2 years later as the seat of King County and named for a friendly Indian chief, Seattle (also called Sealth or See-yat). The city's economy was driven by the lumber industry until the 1880s, when the arrival of railroads stimulated economic expansion. Trade with Asia began in the 1890s, and the Yukon gold rush made Seattle an important commercial center. Further stimulus came with the opening of the Panama Canal in 1914. A major shipbuilding center during both world wars, Seattle also experienced a boom from aircraft manufacturing during World War II.

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Geological history of northwest

Much of the following discussion is paraphrased and extracted from Galster and Laprade (1991).

The landscape of the Puget Lowland is the product of a long history of mountain-building and subsidence, glaciation and volcanism, erosion and deposition. Agriculture is facilitated by the abundance of water and rich valley soils, while upland soils have provided abundant supplies of timber. The most fundamental physiographic divisions are the Puget Lowland and the Cascade Range. The terrain of the Puget Lowland is made up of a series of rolling plateaus cut by steep-sided valleys. The drift plains slope gently west and northwest from the Cascade Range foothills (approx. 800 ft. elevation) to bluffs overlooking Puget Sound; they are built of unconsolidated sediment deposited during glacial and nonglacial periods in the past 2 million years. The fill ranges from a thin veneer to a depth of 3,600 feet in the deepest basin. The surface features of the drift plains are mostly inherited from the ice sheet that last flowed over them about 13-16 thousand years ago (Ka): elongate hills (drumlins) are arranged in the direction of ice flow, and marshes and lakes have formed in closed depressions between them and in late-glacial outwash channels.

The Cascade Mountains rise in the east to 3,000 to 9,000 feet and are composed of a variety of bedrock types. In general, the South Fork Snoqualamie River divides the high, rugged North Cascades, made of mostly older metamorphic and intrusive rocks, from the gentler southern Cascades, which are dominantly Tertiary volcanic and sedimentary rocks. Although recent uplift of the Cascades occurred along a north-south axis, a significant secondary topographic grain was formed by older folders and faults that trend northwest-southwest. Continental ice sheets formed in the mountains of British Columbia and flowed into the Puget Lowland several times in the last 2 Ma. Little evidence remains of any of these except the most recent (Fraser) glaciation, which eroded or buried the deposits of previous events. The ice sheet reached its southern limit south of Olympia before 14 Ka and the glacier blocked drainage to the north; this led to the primary drainage being toward southwest in the Chehalis River. The Puget lobe yielded large amounts of sediment as it advanced, as did rivers from local Cascade and Olympic mountain glaciers.

Extent of the Cordilleran ice sheet, Glacial Lake Missoula, and cataclysmic flooding during the late Pleistocene (approximately 15,000 years ago)

During the brief maximum glaciation of the Puget Lobe, around 14-15 Ka, the ice sheet stood about 3,000 feet thick over Seattle. Flowing ice molded the till into elongate drumlins, aligned north-south in the west and northwest-southeast toward the eastern edge of the lowlands, reflecting the varying ice-flow conditions.

Stratigraphic sequence for the Seattle Area. Time-stratigraphic units denoted by asterisk have not been identified in metropolitan area. Reproduced with permission from Galster and Laprade (1991).

South of the glacial limit there are extensive outwash and alluvial deposits overlying oceanic basaltic and other volcanic bedrock. Along the I-5 corridor, we will primarily be on old outwash deposits and more recent alluvium. We will be driving over a series of terraces, formed in early to middle Pleistocene, as we descend to the Cowlitz River; these include the Logan Hills outwash formation (>1 Ma), Wingate outwash (500 Ka), Hayden Creek outwash (140 Ka), and Evans Creek outwash, and finally the modern flood plain of the Cowlitz River. The soils of the surrounding low lying hills (Willapa hills) consists of Ultic Palexeralfs, Palehumults, Ultic Haploxeralfs, Argixerolls (base rich due to basalt and tephra). The soils formed in the valley alluvium, outwash, and possibly mudflows are dominated by Fluvaquents and Ultic Haploxerolls.

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Northwest climate

Precipitation comes primarily as rain, with 75% occurring from the beginning of October to the end of March. Snowfall accounts for less than 5% of the precipitation in the lowlands, but increases rapidly with increasing elevation. This leads to an environment with mild wet winters and cool dry summers. Seattle, although infamous for being wet receives only about 34 inches of precipitation annually. The perception of wet weather is due to the high percentage of overcast days, the highest in the continental states.

Mean temperature and precipitation for Seattle/Olympia area
	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Avg
Mean T °F	41	44	46	52	57	61	66	65	61	54	47	44	53
Mean Prec. In.	5	4	3	2	2	1	1	1	2	3	5	5	34
Mean temperature and precipitation for Wind River
	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Avg
Mean T °F	32	36	39	45	53	59	65	64	58	49	39	35	48
Mean Prec. In.	19	13	12	6	3	2	1	2	3	8	15	18	102

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References

Galstar, R.W. and W.T. LaPrade 1991. Geology of Seattle, Washington, United States of America. Bulletin of the Association of Engineering Geologists 3:235-302.

Jackson, P.L. and Kimerling, A.J. (eds.) 1993. Atlas of the Pacific Northwest (8th edition). Oregon State University Press, Corvallis, OR, 152 p.

Paul, Charlotte, 1987. Seattle Guidebook, 6th rev. ed., Satterfield, Archie.

Schwantes, C.A. 1989. The Pacific Northwest. University of Nebraska Press.

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Mima Prairie and Mounds (Stop 1)

Geology of the Prairies

The last glacial advance into the Puget Sound Lowland, the Vashon Stade of the Fraser Glaciation, reached its maximum extent approximately 14,000 years ago. Deglaciation appears to have involved a succession of meltwater-channel and ice-margin-lake stages which probably lasted less than 1000 years. Glacial outwash was deposited by meltwater from the retreating ice mass in the meltwater-channels. The prairies of the Puget Sound Lowland are located on these glacial outwash deposits.

The high flow of the meltwater is reflected in the coarse texture of these glacial outwash deposits. The glacial outwash deposits of the Mima Prairie area consist primarily of well-sorted sands and gravels. The majority of the smaller silt- and clay-size particles remained in suspension in the meltwater and were deposited elsewhere. The gravelly Spanaway soils are found on the upper mounded terrace of Mima Prairie, and the gravel-free Nisqually soils are found on the lower, unmounded terrace surfaces.

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Origin of the Prairies

Conifer forests are thought to have dominated the area of the present prairies soon after the recession of the ice and the deposition of glacial outwash (Tsukada et al. 1981). This forested period was terminated on portions of the Puget Sound area by the development of drier and/or warmer climatic conditions. The climatic conditions may have started to become drier or warmer as early as 10,000 years ago.

During the drier and/or warmer conditions, the forest ecosystem was replaced by prairie ecosystems on portions of the glacial outwash terrain in the Puget Sound Lowland. The forest ecosystems were replaced by prairie ecosystems during this period of drier and/or warmer climatic conditions because of the doughtiness of these coarse-textured, somewhat excessively drained outwash deposits.

The climate appears to have remained drier and/or warmer until about 5,000-7,000 years ago when it began to moderate toward present conditions. The increased moisture and/or cooler temperatures associated with present climatic conditions have not resulted in a complete replacement of the prairie ecosystem for two major reasons: 1. Undisturbed prairie vegetation tends to completely cover the soil surface, minimizing suitable seedbed and limiting the establishment and survival of seedlings from adjacent forests; 2. Aboriginal burning minimized the encroachment of forest vegetation by destroying forest seedlings at the forest-prairie interface.

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Soils of the Prairie

The Spanaway Soil Series includes the soils formed under native prairie vegetation on somewhat excessively drained, gravelly outwash terraces (Soil Survey of Thurston County, Washington, 1991). The horizons that make up the profile of the unmounded Spanaway soil display a great deal of contrast in both color and texture. The A horizon at the surface of the Spanaway soil is a black gravelly sandy loam. The B horizon from 15 to 20 inches in depth is dark yellowish brown very gravelly sandy loam. The C horizon from 20 to 60 inches in depth is a dark yellowish brown extremely gravelly sand. The horizonation of the mounded Spanaway soil is dramatically different and will be discussed below.

The Spanaway A horizon's black color is primarily a result of the high charcoal content. The high charcoal content in the Spanaway A horizon is the result of centuries of aboriginal burning of the prairies. The charcoal contributes to the dark color of the Spanaway A horizon, but it is relatively inert and adds little to the nutrient status of this soil. Only small amounts of charcoal have penetrated to the lighter colored B and C horizons of the Spanaway soil.

Textural contrasts between the Spanaway soil horizons appear to have been influenced by differences in the rate of movement of the glacial meltwater that deposited them. The Spanaway C horizon formed in extremely gravelly glacial outwash material which was deposited from rapidly moving glacial meltwater. The upper soil horizons formed in finer textured outwash material (less gravel and more silts and clays) deposited later by slower moving meltwater.

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Characteristics of the Mima Mounds

The Mima Mounds of the Puget Sound Lowland are reported to vary from 8 to 70 feet in diameter and from 1 to 7 feet in height. The height of all the mounds within a specific area tends to be uniform. The mounds occur at densities as great as 8-10 per acre.

Viewed from above or in air photos, the mounds are generally circular to elliptical in outline; and certain groups of mounds show elongate and curving trends, apparently reflecting drainage patterns.

Cross-sections of the mounds and the adjacent intermound areas commonly reveal a biconvex outline of the black Spanaway A horizon. The A horizon commonly extends deeper into the extremely gravelly C horizon near the center of the mound, reaching thicknesses of as much as 7 feet. In contrast, the A horizon in the intermound area is commonly less than 6 inches thick.

The A horizon soil within the mound contains some small rounded pebbles, but is well mixed and show no stratification in contrast to the bedded, rounded outwash gravels in the C horizon below. Mound roots are extensions of the black A horizon soil into the bedded outwash gravel and have been observed and described under several mounds.

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Origin of the Mima Mounds

Many hypotheses have been proposed over the years to explain the origin of the Mima Mounds. Hypotheses on the origin of the Mima Mounds range in character from whimsical theories (i.e. "buffalo wallows" or "sucker nests") to credible theories with significant scientific support. Washburn (1988) prepared a critical review of the most promising hypotheses regarding the origin of the Mima Mounds and the reader who desires additional information on these hypotheses is encouraged to review this publication.

Washburn indicates that for Mima-like Mounds (that is, mounds apparently similar to the Mima Mounds of the Puget Sound Lowland, but occurring elsewhere) there are two hypotheses that are potentially widely acceptable, but remain to be proved. One of these hypotheses explains the Mima and Mima-like Mounds as the work of fossorial (burrowing) rodents, such as pocket gophers. The other hypothesis explains these mounds as the result of runoff erosion combined with vegetation anchoring.

Washburn feels that runoff erosion combined with vegetation anchoring may best explain the Mima Mounds. He suggests that a former forest existed and could have served as anchoring vegetation, retaining soil particles in place against the forces of erosion and creating the mounds, although other types of anchoring are not excluded.

The fossorial rodent hypothesis was primarily developed by Dalquest and Scheffer (1942). They cite the following evidence:

The Mima Mounds are constructed by entirely by soil materials small enough to be moved by gophers.

Materials too large to be moved by gophers appear beneath the mound or in the intermound region.

Mound roots extending into the gravel bed correspond to the size and shape of tunnels and nest excavations occupied by living gophers.

Mima Mounds are found only on prairies where gophers now live or quite certainly once lived, but are absent from prairies which, though geologically similar, yield no traces of gophers.

The characterization features of the mounds (namely, aerial distribution, size, and shape) are in conformity with the habits of pocket gophers.

The thickness of the black A horizon within the mounds (as much as 7 feet) is much greater than that on unmounded prairies (15 inches) or in the intermound areas (less than 6 inches). The greater thickness of the A horizon and the high degree of soil mixing within the mounds indicate that a major redistribution of A horizon soil material has occurred since the soil parent material was deposited by flowing water. These soil characteristics strongly support the fossorial rodent hypothesis for mound formation.

While the gopher theory is appealing, there remain several reasons to question whether it is correct, including:

No modern analogs of large mounds are known to have been formed by gophers;

The presence of gophers does not prove conclusively that the soil was mounded by gophers, the mounds may have been occupied after they were formed;

Many Spanaway soils occur in an unmounded phase and if the gopher population was as prolific as would not required, there is no known basis given for why gophers preferentially occupy only some areas.

Perhaps the most recent mound hypothesis is that of Berg (1990), who observed that small-scale Mima Mounds can be produced experimentally by subjecting a plywood board covered with a thin veneer of loess to impacts that produce vibrations in the board. Based on these observations, Berg suggests that Mima Mounds may have formed as a result of seismic activity in conjunction with unconsolidated fine sediments on a relatively rigid planar substratum.

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Vegetation of the Mima Mounds

Prairies on glacial outwash gravel occupy large areas of west-central Washington. These prairies, including the Mima Mound prairie became established about 9,000 yr. BP, prior to the entrance of Douglas fir into this area. Summer drought, initial low soil fertility and coarse-textured substrate, along with frequent fires enabled these prairies to exist. With fire suppression and reduced grazing, trees began to invade about 1850. With increase agriculture and disturbance, weedy species have invaded including: Scot's broom (Cytisus scoparius), St. John's wort (Hypericum perforatum), and Cat's ear Hypochaeris radicata).

The common native species include grasses Festuca idahoensis, Poa pratensis, and Agrostis diegoensis. Common forbs are Cammassis quamash, Dedecatheon hendersonii, and Viola adunca in the spring. Other common forbs include Achillea millefolium, Rumex acetosella, and the seedy species Hypericum perforatum and Hyopchaeris radiata. The mosses Rhacomitriuum canescens and Polythichem juniperinum and the lichen Cladonia mitis occur throughout.

In addition to natural fires, the Indians burned these prairies to aid the growth and abundance of the camass bulbs that provided an important source of starch. As you walk over the mounds, note some species are more abundant in the troughs between mounds and other species are more common in the slopes and tops of these mounds.

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Timing of the Formation of the Mima Mounds

This section follows Hallet and Sletten (1994).

Mima Prairie of western Washington is the type locality for conspicuous mounds composed of A horizon soil material that dot prairies in this area. These mounds range in height up to 7 ft and are spaced typically 30 ft apart. They are restricted to recessional outwash deposits dating to about 14 Ka following the maximum extent of the Vashon Stade of the Fraser glaciation. Assuming that soil formation was spatially uniform and that the A-horizon material was subsequently heaped into mounds, an estimated period of several thousand years must have elapsed between deglaciation and the onset of mound formation for a ~0.5 m thick A horizon to develop over the featureless gravelly substrate. This A-horizon thickness is similar to that found in unmounded prairies in the area, and it is sufficient to form mounds of realistic dimensions.

The conspicuous micro-relief characteristic of the mounds suggests that insufficient time has elapsed since their formation for significant degradation to occur due, for example, to soil creep, or that they are sustained in spite of erosive wear. A simple diffusion model of the mound relief suggests that, in the absence of constructional processes or of continued preferential erosion of low areas, the mounds "diffuse out" quickly, attaining heights only a tenth of their original heights within a few millennia.

This work favors formative mechanisms that either operated much more recently than the Vashon deglaciation or are continuing to operate. These include convergence due to fossorial rodent activity, and seismically induced soil mounding.

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Reference List for Mima Mounds

Berg, A.W. 1990. Formation of the Mima Mounds: a seismic hypothesis. Geology 18:281-284

Dalquest, W.W. and V.B. Scheffer 1942. The origin of the Mima Mounds of western Washington. Journal of Geology 50: 68-84

Hallet, B.H. and R.S. Sletten 1994. Mima mounds: constraints on the timing of formation. The Geological Society of America 1994 Annual Meeting, Seattle, WA October 24-27, 1994.

Tsukada, M., S. Sugita, and D.M. Hibbert 1981. Paleoecology in the Pacific Northwest. I. Late Quaternary vegetation and climate. Internationale Vereinigung fur Theoretische und Angewandte Limologie, Verhaudlunger 21: 730-737.

United States Dept. of Agriculture 1990. Soil Survey of Thurston County, Washington. 283 p.

United States Dept. of Agriculture 1990. Soil Survey of Thurston County, Washington. 283 p.

Washburn, A.L. 1988. Mima Mounds: An evaluation of proposed origins with special reference to the Puget Lowland. Washington Division of Geology and Earth Resources Report of Investigations 29, Washington State Department of Natural Resources.

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Mt. St. Helens

For the second topic of the field trip we will have several planned stops. Some of the sites are designed to provide background information and scenic views of the mountain and mudflow.

As it turns out, we have had the earliest snowfall in the Cascades in about 25 years and this resulted in last-minute modifications to the planned tour. As these last-minute additions to this guide are being made, it is noted that the weather forecast is for more snow on the weekend of this tour. Therefore we will have to be flexible in our planning and we may have to make some last minute modifications. Please try to return to the bus at the time requested, since there are a number of very interesting sites that we plan to visit if we stay on schedule.

The sites that we will visit are described sequentially below. In addition, there is a road log of the Mt. St. Helens memorial highway following the descriptions of each stop.

On the next page is a satellite image of Mt. St. Helens[1]. For your reference, north faces the binding of this guide. The blast was directed toward the northwest and much of the gray area to the north is due to tephra deposits. Mudflows can be seen on the North and South Fork of the Toutle River on the west side of the mountain. Other mudflows occurred on the southeast side of the mountain. Coldwater Lake is located to the northwest and Castle Lake is a smaller lake. Both of these lakes were created by the dam of debris avalanche. The level of Spirit Lake (north of crater) was raised significantly and a tunnel was bored in to the lake bottom to lower water level and reduce the potential of an even more massive mudflow.

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(Stop 2) Mt. St. Helens National Volcanic Monument Visitors Center, Silver Lake

The main visitor's center for the Monument is operated by the United States Forest Service and provides an excellent opportunity for orientation to the Mount St. Helens National Volcanic Monument. The Visitors Center has a number of very interesting exhibits, displays, and educational materials that will describe the events that led to the May 18th eruption of Mount St. Helens and the subsequent impacts. In addition, slide and movie presentations are shown in the theater, generally on the half hour. We will stop here for one hour and we will not present any discussion here; you are encouraged to visit the numerous displays.

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The eruption of Mt. St. Helens

On the morning of May 18, 1980, Mount St. Helens erupted in the most violent volcanic event yet to have occurred within the continental United States. A catastrophic earthquake-induced landslide/debris avalanche and the associated volcanic eruption of Mount St. Helens destroyed approximately 220 square miles of timberland, and resulted in the deaths of 57 people. The major portion of the debris avalanche was deflected to the west by an east-west trending ridge located immediately north of the mountain, and traveled down the North Fork of the Toutle River valley approximately 14 miles. The debris avalanche buried the existing Toutle River valley to depths varying from 60 to 600 feet. Subsequent dewatering of the massive debris avalanche resulted in the formation of lahars (volcanic mudflows) at the distal end of the debris avalanche. These lahars flowed a substantial distance down-stream, eventually entering the Columbia River, 60 miles to the southwest.

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Formation of the National Volcano Monument

Since the eruption, Mount St. Helens has received not only national but international attention and recognition. In October of 1981, the Mount St. Helens Land Management Plan included the designation of 84,700 acres of land as an "Interpretive Area" to protect the distinctive features and processes for public education, recreation, and research. In August of 1982, the United States Congress passed an Act (Public Law 97-243) establishing the 110,000 acre Mount St. Helens National Volcanic Monument.

The primary access into the Mount St. Helens National Volcanic Monument was intended to be provided by State Route 504. This highway route was considered to be the best way to meet the objectives of the Congressional Act, which are the public understanding, use, and enjoyment of the area. State Route 504 has been designated a Scenic and Recreational Highway, as provided by Washington State Law (RCW 47.39.020), and provides the most direct route for tourists and recreational traffic to access the Mount St. Helen's National Volcanic Monument.

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(Stop 3) 1980 and Pine Creek Mudflows on S. Fork of Toutle River

This is the first stop where we will see the mudflow from the 1980 eruption up close. There are 3 sites that we will visit here: (1) cross-section of Pine Creek age mudflow deposits, ca 2500 years old; (2) soil developed on Pine Creek mudflow (Greenwater soil series); and (3) 1980 mudflow overlain by tailwater deposits (Carrolls series).

At this stop we will split our group into 3 smaller groups since the area around the sites that we will visit is limited, and we will rotate each group so that everyone will have an opportunity to visit all three sites.

The Pine Creek age mudflow represents an event many times greater than the 1980 event as will be obvious when you see both of the mudflows together. It was the recognitionn of large mudflows like this one that prompted the steps that were taken to lower the water level of Spirit Lake. Above the Pine Creek mudflow are the Greenwater soil series (description in appendix). These soils show OABC horizon and are classified as Entisols due to their coarse texture.

At an elevation below the Pine Creek mudflow is a section revealing the 1980 mudflow. This section is particularly interesting since it occurs on top a pre-1980 asphalt road. The section reveals deposits from mudflows that came down both the South and North Fork of the Toutle River. We are currently on the South Fork; on about noon on the day of the eruption a mudflow came down this drainage. About 3:00 on the same day a mudflow came down the North Fork of the Toutle River; the slow flowing material created a dam across where the South and North Forks meet. The backwater from this blockage flowed up the South Fork led to the deposition of finer grained deposits on top of the South Fork mudflow. The upper layer reveals some stratification, typical of fluvial deposition, and there are fragments of pumice rock that floated on the water and was deposited here.

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(Brief stop 4) Hoffstadt Bluffs Viewpoint

For a description of this site, see the entry in the highway travel log below and Pat Pringle's guide.

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(Brief stop 5) Elk Rock Viewpoint

Again, see the highway travel log below and Pat Pringle's guide for information on this stop.

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(Stop 6) Coldwater Ridge Visitor's Center

We will spend several hours here and several brief discussions will be given, either at the site or in the visitor's center, depending on the weather conditions. We must maintain flexibility here since weather conditions at this time of the year can be quite comfortable or they can be quite stormy.

Due to the early snow at the Coldwater Ridge Visitor's Center this year, we had to make some last-moment modifications in site selection. We had planned an optional 2 mile walk through the debris avalanche on a new trail being developed by the Monument personnel, but the trail is now snow-covered. Instead of this, we will make several presentations inside the visitors center (by the large windows overlooking the mountain) and visit several areas in the vicinity of the visitor's center.

We will examine two soils in different road cuts, both of which are overlain by recent blast deposits. One section is near the visitor's center and we will walk to this site after our discussions inside. The other section is located in the Coldwater Lake parking lot and we will ride the buses to this site. Also at the Coldwater Lake soil section, we will have a close view of the debris avalanche.

A few general comments on the status of "soil surveys" for the counties surrounding Mt. St. Helens: Skamania and Cowlitz county were remapped shortly after the eruption of Mt. St. Helens. The Skamania County Soil Survey was published in 1989, while the Cowlitz County Soil Survey is currently in press and expected to be completed in 1996 (Terry Aho, personnel communication). After the 1980 eruption of Mt. St. Helens, the soil were given a new soils series name if they contained more than 0.5 m of new blast or debris flow material, while soils with less than 0.5 m of new material retained their original soil series nomenclature (Terry Aho, personnel communication). Since the remapping, there has been significant erosion and changes to the landscape in some cases. For soils with little new material overlaying the old soils, often the original vegetation survived and emerged through the new deposits.

Discussion at Coldwater Ridge Visitors Center

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Early stages of soils formation in new blast materials

Elemental composition and mineralogy of the blast and tephra deposits

The blast deposits from Mt. St. Helens contained both newly formed and old rock. The tephra composition changed from early in the eruption being dominated by lithic fragments of the old cone to a dacitic composition later during the eruption; the latter reflecting formation of fresh magna. The tephra was mostly glassy with some plagioclase, and lesser amounts of iron oxide and hornblende (Hooper et al. 1980). The old cone material contained hydrothermally-formed clay minerals including smectite and trioctohedral vermiculite (LaManna and Ugolini 1987).

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Weathering of the tephritic deposits

Initially the tephra was leached of high concentrations of Si, Cl, Ca, and SO₄, on the order of 100's of mg/L for the latter two ions. After the initially rapid leaching, weathering proceeded at relatively high rates as measured by analysis of soil solutions collected by tension lysimeters (Nuhn 1987). Within a few years, extractable Fe, Al, and Si were significantly greater in depressional areas and formation of soil crusts was observed here. Some evidence of alteration from inherited smectite to poorly formed kaolinite was noted after 5 years (Nuhn 1987).

Buffering in these soils is poor until sufficient time has elapsed for leaching of soluble ion and formation of oxides and amorphous aluminosilicates. In the vicinity of the crater, acid rain deposition with pH values as low as 3.6 has been measured. While the soils have low buffer capacity, they will weather easily and the pH is neutralized quickly in the soil.

In one study of soils sampled in 1980, 1981, 1982, and 1992, Aho (1994) found that the only clay minerals present were smectite that was inherited with the initial blast deposits.

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Vegetation establishment, Mt. St. Helens

The May 18, 1980 eruption resulted in the destruction of vegetation over a 150-square mile area. Large areas had all vegetation destroyed with pumice and tephra laid down on the north side of the crater. Most of these areas still have limited vegetation, with a species of lupine predominating in some areas. As the lupines die, they are often replaced by grasses, and several species of forbs. Where springs occur there is much more vegetation including willow shrubs. Fireweed (Epilobium angustifolium) and pearly everlasting (Anaphalis margaritacea) are slowing invading these nutrient poor systems.

On mudflows on the southeast side of the volcano, the surfaces remained quite barren for the first 5 years, except where plants of Lupine lepidus. L. latifolius, Penstomen cardwellii, Arctostaphylos uva-ursi, and Spraguea umbellatum. Pinus contorta, Pseudotsuga menziessii, and Tsuga heterphylla along with Alun sinuata are now establishing in large numbers.

The debris flow down the Toutle Valley has diverse habitats now occupied by a variety of species including red alder, willow, and fireweed, with cattails in wet sites.

The large areas of forest blowdown were slow to show initial recovery. Fireweed and pearly everlasting established within 5 years, often from nearby clear-cuts where these species already occurred. Many of the herbaceous species were killed either by heat or by burial but many blueberry shrubs (Vaccinium membranaceum and V. ovalifolium) survived. These species are now very evident along with fireweed in many areas. Survival of species as well as early invaders were very similar in the narrow bands of standing dead trees at the margins of the devastated areas. Where soils were left intact, except with the addition of new tephra, plant establishment is much more rapid than nearer the volcano where nearly everything was destroyed. In those areas there is still little plant recovery 15 growing seasons later.

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Classification of Volcanic Soils - Development of the Andisol Order

The Andisol order was developed from the reclassification of Andepts, a suborder of Inceptisols, and some soils in the Andic subgroups. The reclassification was first proposed by Dr. Guy D. Smith in April, 1978. An International Committee on the Classification of Andisols (ICOMAND) was formed in 1979 and chaired by Dr. M. L. Leamy, New Zealand Soil Bureau, to consider and test the proposal. Since then there have been 10 circular letters issued by ICOMAND as the results of a series of field testing and workshops sponsored by USDA - Soil Conservation Service and the Soil Management Support Service (SMSS) of the U.S. Agency for International Development (AID). The last circular letter (#10) was issued in February 1988, and proposed to the USDA Soil Conservation Service to incorporate the Andisol order in the 1989 edition of the Keys to Soil Taxonomy. USDA-SCS adopted the proposal in October 1989 and distributed the Andisol amendments to National Soil Taxonomy Handbook 430-VI, issued November 13. The recommendation of ICOMAND was incorporated in the 4th edition of the Keys to Soil Taxonomy in 1990.

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Andisol: Distribution and Concept

Andisols cover more than 124 million hectares or approximately 0.8 percent of the earth's surface. By far the most striking pattern in the distribution of Andisols follows the circum-Pacific Ring of Fire, that concentration of active tectonic zones and volcanoes along the western coast of the North and South American continents, across the Aleutian Islands, down the Kamchatka Peninsula of Russia, through Japan, the Philippine Islands, and Indonesia, across Papua New Guinea, the Solomon Islands, Vanuatu and other Pacific Islands to New Zealand. Other distinctive patterns are associated with the Rift Valley of Africa, the west coast of Italy, in the Hawaiian Islands, the West Indies, Iceland, the Canary Islands, and other island locations. In the U.S., the Andisols occur in southern Alaska, especially the Cook Inlet, Alaska Peninsula, Aleutian Islands and limited areas in the Wrangell Mountains and southeast Alaska.

The central concept of an Andisol is that of a soil developing in volcanic ejecta. The dominant process in most Andisols is one of weathering and mineral transformation. However, accumulation of humus complexed with aluminum and iron is characteristic of Andisols in some regimes. Andisols occupy a central position in the range of weathering of primary volcanic material from fresh tephra to clay-dominated soils. In the surface horizon, humus accumulates and forms complexes with Al and Fe from the weathering of tephra. In the subsurface horizons the weathering of primary aluminosilicates has proceeded only to the point of formation of short-range-order minerals such as allophane, imogolite, and ferrihydrite. Thus the exchange complexes of Andisols are dominated by Al, Fe-humus complexes and short-range-order minerals.

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Andisol-Spodosol Transition

Generally, Spodosols formed in tephra share the common properties of Andisols: exchange complexes dominated by Al, Fe-humus complexes and short-range-order minerals, low bulk density, high variable charge, high water retention and thixotropic (smeary). The difference is that in Andisols, the Al, Fe-humus complexes are formed in situ (A horizon), whereas such complexes in Spodosols are translocated from an eluvial horizon (E) into the underlying illuvial horizon (Bhs, Bs). Thus the separation of Andisols from Spodosols depends on the presence of an E horizon. However, such separation is not clear in southeast Alaska because of the transition problems of the two orders. The E horizons often are interrupted or destroyed by bioturbation or masked by the oversupply of humus in the hemlock forest. In addition, there are soils formed in igneous glacial till under highly leached environment in southeast Alaska; weathering of primary aluminosilicates in the parent material also leads to the formation of short-range-order minerals. These soils show transitional properties between Andisols and Spodosols.

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Classification

The classification of Andisols is based on Andic soil properties throughout a thickness of 35 cm within the upper 60 cm of the soil. Andic soil properties result mainly from the presence of significant amounts of allophane, imogolite, ferrihydrite or Al-humus complexes.

Thus a soil formed in fresh tephra cannot be classified as Andisol until the tephra weathers and produces weathering products to meet the minimum expression required for classification. Such minimum expression is based on the presence of short-range-order minerals as Al and Fe extracted by acid-oxalate solution (Al + 1/2 Fe > 2%), low bulk density (0.9 g/cm³), high phosphate retention (>85%) or if the oxalate- extractable Al + 1/2 Fe is less than 2%, and P-retention is less than 85%, then it requires a proportionate amount of volcanic glass present in the silt and sand fractions of the soil.

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Soil profile at Coldwater Ridge Visitor's Center

Near the Coldwater Ridge Visitor's Center we will walk about 5 minutes to see a road-cut revealing a buried soil. This soils is classified as the Elkprairie series (ashy over medial Vitrandic Cryorthents, see appendix for full description). There is approximately 25 in (60 cm) of blast deposit material overlaying this soil; none of the original vegetation survived. The tephra deposited here displays gradation typical of Stokes Law settling in fluids, the coarser fragments and blast deposits overlay the remnants of an organic mat. The texture becomes progressively finer toward the upper soil surface (see appendix description). The O horizon in these soils is very thin; the official soil series description notes that the tephra is directly on top of an A horizon. The initial blast may have stripped or burned the O material, and possibly some of the A horizon.

In the vicinity, in similar parent material but with less tephra, we may find the Vanson soil series (Appendix 2). The Vanson meets the criteria for Spodosols (ashy over medial-skeletal Andic Haplocryod).

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Soil profile and debris avalanche in Coldwater Lake parking lot

In the Coldwater Lake parking lot we will examine a section revealing a soil that has been mapped as the Swift series. This soil occurs in an area on a steep slope and the amount of blast material here (about 3-4 in) is significantly less than at the section on Coldwater Ridge. The blast deposits are dark and probably represent more cone material rather than newly formed tephra. There was quite some variability in the distribution of blast material over short distances; generally steep slopes and slopes facing the blast source received or retained significantly less material.

A description of this soil is given in Appendix 2. This soil is classified as "ashy over loamy-skeletal, mixed frigid Typic Udivitrand". The soil profile may appear somewhat different from the description in the appendix since there is significant movement of colluvium. Since the blast deposits were thin here, some of the original vegetation survived, unlike the soil profile we visited on Coldwater Ridge.

We will also see debris avalanche material at this site and large hummocks that are composed of cone material. The debris avalanche material is mapped as Delemeter series (Appendix 2). The debris avalanche produced a surface with hummocks often composed on old cone rock, including hydrothermally altered minerals.

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Perspective of the eruption from a local resident

Darcy Mitchem is a local resident of the Toutle area and work for the MSHNVC as an interpretive guide. She will discuss "what is was like" to live near Mt. St. Helens afte the 1980 eruption.

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Reference List for Mt. St. Helens

Aho, T. 1994 Mt. St. Helens 1980 Tephra, Pedogenesis in the first decade. Agronomy Abstracts, Seattle 1994.

Burk, R.L., K.R. Moser, D. McCreath, N.L. Norrish, and R.L. Plum 1989. Engineering Geology of a Portion of the Spirit Lake Memorial Highway, in Engineering Geology in Washington State, Washington Division of Geology and Earth Resources Bulletin 78, Olympia, Washington.

Foxworthy, B.L., and M. Hill 1982. Volcanic Eruption of 1980 at Mount St. Helens- The First 100 Days, U.S. Geological Survey Professional Paper 1249.

Hooper, P.R., I.W. Herrick, E.R. Laskowski, and C.R. Knowles 1980. Composition of the Mt. St. Helens ashfall in the Moscow-Pullman Area on 18 May 1980).

LaManna, J.M. and F.C. Ugolini 1987. Trioctohedral vermiculite in a 1980 pyroclastic flow, Mt. St. Helens, Washington. Soil Science 143:162-167.

Lipman, P.W. and D.R. Mullineaux, editors. 1981. The 1980 Eruption of Mount St. Helens, U.S. Geological Survey Professional Paper 1250.

Nuhn, William W. 1980. Soil genesis on the 1980 pyroclastic flows of Mount Saint Helens. Thesis (M.S.)--University of Washington.

Pringle, Patrick T. 1993. Roadside Geology of Mount St. Helens National Volcanic Monument and Vicinity. Washington Department of Natural Resources, Division of Geology and Earth Resources, Information Circular 88, 120 p.

Ugolini, F.C., R. Dahlgren, J. LaManna, W. Nuhn, J. Zachara 1991. Mineralogy and weathering processes in recent and Holocene tephra deposits of the Pacific Northwest, USA. Geoderma 51: 277-299.

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Appendix 1. Mt. St. Helens Memorial Highway travel log

The travel log given below notes sites of particular interest along the Mt. St. Helen's memorial highway. You will find a more detailed description and very nice diagrams in Pat Pringle'g guide that was passed out along with this guide.

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Exit from Interstate 5 to Mt. St. Helens Memorial Highway

We will turn off of Interstate 5 at Castle Rock (Exit 49) and proceed up State Route (SR) 504, which provides a direct western approach to the Mount St. Helens National Volcanic Monument. Because of the enormity of the eruption of Mount St. Helens, our field trip will only provide a glimpse of the events that occurred in the Toutle River valley and surrounding environment. As a result of the catastrophic events of May 18th, more than 30 miles of State Route 504 was destroyed.

Between the Mount St. Helens National Volcanic Monument Visitors Center, and the Corps of Engineers Sediment Retention Structure, State Route 504 essentially traverses the valley bottom of the Toutle River.

Just east of the visitors center at MP 8.1 the highway passes Silver Lake, which was formed by several enormous lahars approximately 2500 years ago. These lahars are thought to have been generated by a catastrophic drainage of a lake (perhaps Spirit Lake) or lakes, located to the east of Silver Lake. The peak discharge of the largest of these catastrophic lake outbursts has been estimated at 9 million cubic feet per second, or a flood stage discharge equivalent to that of the Amazon River.

At MP 11 SR-504 crosses the Toutle River at Coal Banks. The original bridge at this location was destroyed on May 18, 1980 when the log-choked lahar floated the superstructure of the bridge off of its foundation supports. The bridge was replaced the following year by a longer and higher bridge so that any future lahars would pass, harmlessly, beneath the new bridge.

Between MP 16.5 and MP 16.9 the highway follows the north bank of the North Fork of the Toutle River. The alder trees that are present on the river banks became established after the May 18, 1980 eruption of Mount St. Helens and occupy the top of the 1980 lahar.

Between MP 19.3 and MP 20.1 is an area known as Maple Flats. This area was inundated by the 1980 lahar. On the north bank of the river near MP 19.8 deposits of ancient lahars can be seen in the exposed cut bank. The roadway in this vicinity is located on a meander bend on the south side of the river and was severely damaged by the erosive force of the 1980 lahar. Attempts to rebuild this section of the roadway were hampered by not being able to maintain rip rap protection of the embankment through this area. This problem was eventually solved by encapsulating the rip rap in WW II submarine nets obtained from the U.S. Navy.

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Corps of Engineer's Sediment Retention Structure (SRS)

A major post-eruption problem within the North Fork of the Toutle River is the erosion and transport of very large quantities of sediment from the volcanic ash, pyroclastic flows, and debris avalanche deposits that are present in the upper reaches of the valley. To mitigate the effects of this serious problem, the Corps of Engineers designed the SRS to greatly reduce the ongoing sedimentation of the river systems downstream of this structure. The 184-foot-high SRS is essentially a large decanting basin. The North Fork of the Toutle River flows through a series of vertically stacked, large diameter pipes located on the north side of the SRS. As the sediment builds behind the SRS, the lower pipes will be closed systematically, and the river allowed to flow through the pipes located above; thus allowing sediment to settle in the SRS impoundment area. The capacity of the SRS is approximately 285 million cubic yards and has an expected life of approximately 45 years. On the north side of the SRS is a 4500 foot long 400 foot wide spillway. Once all the vertically stacked pipes in the SRS have been closed, the spillway will become the permanent river channel. The construction of the SRS began in the Fall of 1986, and was completed in 1989.

The Toutle River bridge, at MP 21.3, is an 1150 foot long multi-span bridge. The foundation support for the interior piers are driven H-Piles. The south abutment is a spread footing on bedrock, while the north abutment footing is a spread footing in the approach embankment.

The bridge also marks the western end of the SR-504 reconstruction project. From this point on the highway is on a totally new alignment. The former SR-504 highway alignment, to the east, was located in the valley bottom and was buried by the 1980 debris avalanche and lahars for a distance of approximately 20 miles.

Just downstream of the Toutle River Bridge is a fish collection facility operated by the State Department of Fish and Wildlife. Since the SRS contains no fish passage to allow migrating anadromous fish access to spawning beds in the upper portions of the river valley, this facility was constructed to capture the fish returning to spawn. Once the fish have been collected, they are trucked above the dam and released so they can reach their spawning streams in the upper portions of the valley.

For the next 4 miles, between MP 21.5 and MP 25.5, the highway alignment encountered thick deposits of residual soils consisting of wet silty clays and clayey silts. Construction of this portion of the alignment required substantial amount of sub-excavation of unsuitable material from below proposed embankments and wasting of large quantities of the cut excavations. All embankments constructed along this portion of the alignment were designed with a shot rock drainage blanket at their base. The upper portions of the embankments were constructed of granular borrow material obtained from a large materials source located in the river valley. To help offset the waste/borrow haul costs, a waste site was established adjacent to the borrow source so that trucks could haul with full loads in both directions.

East Pullen Creek Bridge is located at MP 24.6. The bridge is a 372-foot-long, 4-span concrete girder structure. The end abutments are supported on spread footing in the approach embankments, and the two westerly interior piers are supported on spread footings founded in bedrock. Because of softer ground conditions in the easterly interior pier location, steel H-piles were driven to bedrock to provide the foundation support at this pier. The short span concrete girders were incorporated into this bridge because of restrictions along the narrow logging roads that accessed this bridge site.

Between MP 25.1 and MP 25.3 a landslide occurred approximately one year after the embankment was constructed. This landslide initiated as a sliding block downslope of the embankment after a period of high intensity rainfall. Over a period of two months, the landslide progressed upslope and severely affected the embankment for approximately 800 feet. Subsurface investigation and monitoring indicated that the landslide was failing on the contact between the underlying bedrock and the overlying soils. The landslide correction included a 900-foot-long, 30- to 35-foot-deep shot rock shear key. The shear key was constructed in short, 100-foot, segments to maintain upslope stability. Once the construction of the shear key was completed an earth berm was constructed on top of the shear key to provide additional normal load to the shear key. Since the landslide has been corrected, no movement has been detected in the slope inclinometer instrumentation located downslope of the shear key/earth berm.

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Hoffstadt Bluffs Viewpoint

The highway alignment at this stop traverses the base of Hoffstadt Bluffs. Immediately west of the Hoffstadt Bluffs Bridge is a large Quaternary landslide, as indicated by the lack of colluvium at the base of the bluffs. It is believed that the landslide failed when the base level of the main valley was somewhat lower than it is at the present time and that the landslide is now buttressed by late Quaternary alluvium and lahar deposits.

The bluffs are prone to large rockfall and debris flow events. To mitigate these geologic hazards the alignment was shifted out from the base of the bluffs to allow for a catchment area. During construction of this portion of the alignment, in the spring of 1990, a large debris flow generated near the top of the bluffs occurred after an extended period of high intensity rainfall. The debris flow destroyed two large backhoes that were parked near the base of the bluffs.

The Hoffstadt Bluffs Bridge is a 440-foot-long, 2-span structure. The end abutments of the bridge are spread footings founded on bedrock, while the interior pier is set on 4 - 6 foot diameter shafts.

In the valley to the southeast, is an area of low hummocks. These low hummocks mark the distal end of the 1980 debris avalanches, approximately 14 miles downstream of Mount St. Helens. The debris avalanche deposit is a result of the massive earthquake-induced failure of the north side of Mount St. Helens. The landslide has been estimated to be 0.6 cubic miles in volume and is the largest historic landslide in the world. From this point in the valley, the debris avalanche deposit remobilized to form a massive lahar which continued downstream to the Columbia River, 60 miles to the southwest.

The remnant of the N-1 Structure can be seen downstream of the distal end of the debris avalanche. The N-1 Structure was an emergency sediment retention dam built by the Corps of Engineers shortly after the eruption of Mount St. Helens in an attempt to reduce the amount of sediment flowing downstream. The sediment retention structure filled very rapidly, and in an attempt to provide additional capacity nearly 12 million cubic yards of material was removed from behind the structure. On March 19, 1982 the N-1 Structure was overtopped and breached by a lahar, and the dam was abandoned.

Cow Creek Bridge is located at MP 28.6. The bridge is an 864 feet long, 5 span steel girder structure. The interior piers are supported on large diameter shafts founded in bedrock. The easterly abutment is supported on steel H-Piles founded on bedrock, while the westerly abutment is supported on a spread footing in the approach embankment.

The Hoffstadt Creek Bridge is the largest structure on the Spirit Lake Memorial Highway. The bridge's total length is 2340 feet, and is 370 feet above Hoffstadt Creek. The western end of the Hoffstadt Creek Bridge represents the western limit of the lateral blast resulting from the eruption of Mount St. Helens, approximately 15 miles from the mountain. In this area the standing timber was killed by the extreme heat. Just east of here all standing timber was blown down. Of the 150,000 acres of private, state and federal forest lands that were devastated, 68,000 acres belonged to the Weyerhaeuser Company. Weyerhaeuser began salvage logging of its tree farm three months after the eruption, and in the next two years over 850 million board feet of timber was recovered. Since then the company has hand planted over 18 million seedlings to replace the timber that was destroyed.

As we cross the Hoffstadt Creek Bridge, the distal end of the debris avalanche can be seen in the valley to the south.

On the east end of the Hoffstadt Creek Bridge you will notice an earth berm to the south of the highway. This berm was constructed to mitigate the effects of an Elk calving area located between the highway and the North Fork of the Toutle River. Each spring Elk congregate in this area to bear their young.

Between MP 31.3 and MP 32.2, the highway traverses a lateral moraine from the Hayden Creek Glaciation (140,000 B.P.). Major alpine glaciers occupied the North Fork of the Toutle River valley at least twice during the Pleistocene and deposited glacial material as far down-valley as Hoffstadt Bluffs.

Beginning at MP 32.5, the highway provides an excellent overview of the distal portion of the debris avalanche deposit. Note the hummocky nature of the surface, and in some areas along the south flank of the valley the debris avalanche deposit has run up a short ways on the valley walls. The North Fork of the Toutle River flows on the south side of the valley, and has highly reworked the debris avalanche deposit.

The North Fork of the Toutle River valley between the N-1 structure and the western boundary of the National Volcanic Monument was purchased from the Weyerhaeuser corporation by the Rocky Mountain Elk Foundation. The Rocky Mountain Elk Foundation then turned the land over to the Washington State Department of Game to be administered as an elk refuge.

The majority of the soils encountered along the highway between the Hoffstadt Creek Bridge and MP 35 are glacial tills. Just upslope of this location the glacial tills disappear, and coarse grained colluvial deposits are prevalent. This provides an indication of the thickness of the glacial ice that once occupied the North Fork of the Toutle River during the Pleistocene. The downvalley extent of this alpine glaciation is thought to be in the vicinity of Hoffstadt Bluffs. To the west of Hoffstadt Bluffs, glacial deposits are not present along the highway alignment.

The Bear Creek Bridge is located at M.P. 35.7. The bridge is a 500-foot-long, 3-span steel girder bridge. The interior piers are spread footings founded on bedrock, while the abutments are spread footings founded in the approach embankments. The area upslope of the highway alignment was identified during the Phase II geotechnical investigation as having slope angles and snow accumulation areas that were conducive to the initiation of snow avalanche. Although pre-eruption aerial photographs did not reveal evidence of snow avalanche tracks, the deforestation of the area was judged to make the area more susceptible to these events.

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Elk Rock Viewpoint

This is the near the highest point on the highway (Elev. 3800).

In the river valley to the southeast and south is the hummocky surface of the debris avalanche. Constrictions in the valley at this point tended to "dam" the debris avalanche as it flowed downstream. This "damming" effect slowed the debris avalanche and material dropped out of the avalanche.

The effects of the lateral blast are quite noticeable at this location. Directly to the east, in the Elk Creek drainage the directional effects of the lateral blast are indicated by the imbricated pattern of the blown-down trees. This lateral directed blast from Mount St. Helens was a result of the almost instantaneous "unloading" of the mountain by the massive failure of the north slope, which released the hydrothermal and magmatic systems of the volcano. The debris-laden lateral blast traveled northward in an 180 degree arc reaching speeds of 650 miles per hour, and leveling everything in its path for a radius in excess of 12 miles.

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North Fork Viewpoint

This viewpoint provides an excellent overlook of the upper portions of the North Fork of the Toutle River Valley. On the morning of May 18, 1980, after several months of major volcanic activity, the world's largest historic landslide occurred on the north flank of Mount St. Helens. This retrogressive landslide failed at 8:32 AM after a magnitude 5.1 earthquake shook the mountain. The major portion of the resultant debris avalanche was deflected by the east-west trending Johnston Ridge (named for U.S.G.S. geologist David A. Johnston), located directly north of the volcano, and buried the upper valley to depths in excess of 600 feet. A portion of the main debris avalanche flowed into the tributary valleys of Coldwater and South Coldwater Creeks backfilling these valleys to depths in excess of 100 feet. Within South Coldwater Creek (directly north of Johnston ridge) the debris avalanche backflowed into the valley approximately two miles. A northward directed portion of the debris avalanche overtopped Johnston Ridge, and was deflected to the west by the north valley wall of south Coldwater Creek. This westward flowing debris avalanche was joined by the eastward flowing debris avalanche approximately midway within the South Coldwater Creek valley.

To the southeast is Castle Lake, a lake formed by the blockage of Castle Creek by a lateral levee of the debris avalanche. Within a year, Castle Lake filled to a depth of 105 feet behind this natural dam. As the water reached the top of this natural dam, a catastrophic breach of the dam was possible. To reduce the probability of this breach, the Corps of Engineers excavated an emergency spillway on the east end of the natural dam to control the level of the lake. Castle Lake has been at the center of a controversy that questions the stability of the natural dam during a major seismic event in conjunction with high groundwater and/or lake level conditions.

Just downslope of this viewpoint are the remains of the steel superstructure from the Coldwater Creek Bridge. The bridge superstructure was lifted off of its foundation by the force of the debris avalanche and deposited on a logging road.

At MP 42 is the Maratta Creek Bridge. The bridge is a 495-foot-long, 3-span plate girder structure supported by two interior piers on spread footings, and two end abutments founded on spread footings in the approach embankments. Near the completion of the construction of this bridge, both approach embankments began to settle. The settlement was accompanied by rotational movement of the footings to the extent that they were non-functional for the intended structure. As a result, the footings and approach embankments had to be dismantled and reconstructed. During the reconstruction of these approach embankments, a forensic geotechnical investigation was conducted by Golder and Associates to determine the cause of the failures. It was concluded through field observations and testing that the embankment settlement was because of variable and generally inadequate compaction (75 to 85 percent), that was not in compliance with project specifications.

At MP 43.4 is the U.S. Forest Services Coldwater Lake Visitors Center. This Visitors Center, which was opened to the public in 1993, focuses on the biologic recovery of the area. It is the intent of the U.S. Forest Service to allow the devastated area, within the boundaries of the monument, to recover naturally.

At MP 45.3 is the Outlet Channel Bridge. The bridge is a 492-foot-long, 3-span steel girder bridge. The interior piers are supported on spread footing founded on bedrock, while the abutment pier are supported on steel H-Piles driven to bedrock. The bridge crosses the Corp of Engineer's outlet channel to Coldwater Lake. This bedrock outlet channel was constructed shortly after the eruption to control the level of Coldwater Lake. The control of the lake level was necessitated by the concern for overall stability of the natural dam and the potential for a catastrophic breach of the loose debris avalanche deposit.

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Coldwater Ridge Visitors Center

At this point in the North Fork of the Toutle River valley the debris avalanche deposit is in excess of 1.5 mile in width, and has depths in excess of 200 feet. The 200-foot-deep Coldwater Lake was formed as a result of the damming effect of the debris avalanche deposit. Currently Coldwater Lake is approximately 3 miles long, and 1/2 mile wide. The debris avalanche backfilled into the Coldwater Creek Valley approximately 1 mile. A debris avalanche hummock can be seen extending above the lake surface to the north of the Coldwater Lake parking lot.

The Mount St. Helens seismic zone is an interpreted 60-mile long, nearly vertical, right-lateral strike-slip fault zone that trends to the north and is located in the Coldwater Lake area. The maximum recorded earthquake on this active fault zone was a magnitude 5.5 event which occurred on February 14, 1981 near Elk Lake, located to the north. Estimates of probable maximum magnitude along this seismic zone range between 6.2 and 6.8, with a near-source horizontal ground acceleration of 0.55 g's, and an estimated earthquake duration of 30 seconds.

The South Coldwater Creek Bridge, which is located just south of Coldwater Lake, was founded on very loose to loose, fully saturated debris avalanche deposits. The deposits were judged to be highly susceptible to seismically-induced liquefaction and dynamic settlement. To mitigate this deep foundation problem, blast densification was used successfully to "treat" the debris avalanche deposit to depths of 120 feet.

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