EPISODIC PINEDALE GLACIER RETREAT, SPRING MOUNTAIN CANYON, LEMHI RANGE, EAST-CENTRAL IDAHO

On the eastern flank of the Lemhi Range, Spring Mountain Canyon has an especially detailed record of retreat of its Pinedale glacier. The half-mile-wide canyon floor has an over-all gradient of about 450 feet per mile from the base of the cirque headwall at 9400 feet to the canyon-mouth alluvial fan apex at 7400 feet. Several steps, both bedrock and moraine, punctuate the longitudinal profile. Narrow cirques of the main canyon and its tributary, Horseshoe Gulch, cut deeply into the sheltering main divide that rises above l0,400 feet.

Late Pleistocene and Holocene aridity permitted remarkable preservation of even minor moraines, both straight segments and loops; deglacial meltwater was scarce. Within the lower half of the main canyon 60 mappable moraine segments were delineated, 43 in the upper half, but only l4 in steeper Horseshoe Gulch. Most of the deposits lie on the northerly side of the valley where melting was concentrated, the southerly side being generally in shadow. Based on conservative interpretation of cross-cutting and over-riding relationships of moraine segments, 6 readvances are identified in the lower canyon and at least 7 in the upper canyon. Numerous stillstands are also indicated.