EXPOSURE DATING AND VALIDATION OF PERIGLACIAL WEATHERING LIMITS, NORTHWEST SCOTLAND

Periglacial weathering limits on two mountains in northwest Scotland s eparate zones of contrasting exposure history. Exposure dating of bedr ock below the weathering limits gives ages consistent with late Devens ian deglaciation, but six out of seven samples from above the weatheri ng limits give minimum exposure ages older than late Devensian ice exp ansion. 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 f rost debris during an ice-sheet readvance, or enhanced weathering prio r 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, although Al-26/Be-10 ratios for high-level bedrock surfaces provide no evidence of prolonge d static ice cover.