"Over 40,000 atoms counted this year."
EXPOSURE DATING AND VALIDATION OF PERIGLACIAL WEATHERING LIMITS,
John O. Stone, Colin K. Ballantyne and L. Keith Fifield
Geology 26, 587-590 (1998)
Periglacial weathering limits on two mountains in northwest Scotland
separate zones of contrasting exposure history. Exposure dating of bedrock
below the weathering limits gives ages consistent with Late Devensian deglaciation,
but six out of seven samples from above the weathering limits give minimum
exposure ages older than Late Devensian ice expansion. These results suggest
that mountain summits stood as nunataks above the last ice sheet surface
and rule out formation of weathering limits by periglacial rock breakdown
since deglaciation, trimming of frost debris during an ice-sheet readvance,
or enhanced weathering prior to climatic warming during ice sheet downwastage.
The dating results do not preclude the possibility that weathering limits
mark a former englacial boundary between passive cold-based ice on mountain
summits and erosive, warm-based ice at lower elevations, though Al-26/Be-10
ratios for high-level bedrock surfaces provide no evidence of prolonged
static ice cover.