Paper No. 5
Presentation Time: 8:00 AM-5:00 PM
RECORDS OF MULTIPLE PINEDALE GLACIER RETREAT/READVANCE PULSES, LEMHI MOUNTAINS, EAST-CENTRAL IDAHO
The Lemhi Mountains comprise a narrow, linear, NNW-trending faultblock range with peaks rising above 11,000 feet. Flank incision by deep canyons provided catchment areas and shadowed environments that permitted establishment of extensive Pleistocene snowfields and development of cirques that nourished valley glaciers flowing down to or near the mountain front, especially on the cooler northeasterly-facing slopes. Although there is areally limited evidence of Bull Lake and older glaciations, it is the depositional record of the episodic retreatal phases of Pinedale ice that presents the greatest detail. Numerous lateral and end moraines were formed, large portions of which have been preserved because sparse postglacial runoff accomplished little erosion on the valley floors. These records have been mapped in detail for eight canyons that lie along a 17-mile segment of the range. Extracted from an abundance of moraines on the basis of indisputable evidence of over-riding or cross-cutting relationships, a minimum number of retreat/readvance episodes was determined for each canyon. Listed from south to north with author, thesis date, and number of Pinedale episodes, the mapped canyons are:(1) Bell Mountain Canyon (Knoll, 1972) 6, (2) Steep Canyon (Knoll, 1972) 8, (3) Mammoth Canyon ( Knoll, unpublished) 7, (4) Long Canyon (Knoll, 1972) 7, (5) Spring Mountain Canyon ( O'Connell 2000) 13, (6) Meadow Lake Canyon (Knoll, 1972) 4, (7) Mountain Boy Canyon (Butler, 1982) 5, and (8) Hilltop Canyon ( Butler, 1982) 1. General absence of datable material precluded establishment of absolute chronologies and firm intercanyon correlations.