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In the valley of Cataract Creek in the Tobacco Root Mountains, Montana, two moraines were deposited by Pleistocene niche or slab glaciers that emanated from poorly developed cirques (proto-cirques) along the sides of the valley at elevations of less than 8000 feet (2438 m) rather than by valley glaciers that emanated from well-formed cirques at elevations of 9000 feet (2743 m). The niche or slab glaciers seem analogous to those occurring in presently glacierized proto-cirques in Spitsbergen, Norway, and Scotland. Proto-cirques were formed in hollows at elevations of less than 8000 feet (2438 m) in this valley because the hollows were hemmed in and orographically influenced by valley glaciers on the south and east and by the main divide of the mountains on the north and west. The two valley glaciers generated a locally cold microclimate in the hollows, and their nearness to the main divide resulted in the hollows’ being well-situated for the accumulation of snow drifting across the divide. The fortuitous position of these hollows during the late Pleistocene resulted in incipient glacier formation at elevations lower than would be expected.

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