Exposure dating and validation of periglacial weathering limits, NW Scotland

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, although Al-26/Be-10 ratios for high-level bedrock surfaces provide no evidence of prolonged static ice cover.