4 Quaternary Geology, Geomorphology, Archeological Geology

Wednesday, 7 May 2003
3:30 PM - 5:05 PM

MULTIPLE PRE-BULL LAKE GLACIATIONS, EASTERN SIDE OF LEMHI RANGE, EAST-CENTRAL IDAHO

Paper number: 4-3
Program: Technical Sessions
Time: 3:50 PM – 4:05 PM

Authors

Abstract

Valley glaciers occupied canyons cut into the easterly flank of the NNW-trending Lemhi Range. Fresh end moraines show that Pinedale ice reached mountain-front canyon mouths at 7200-7400 feet in especially favorable situations, but terminated upvalley in most canyons. Subdued forms attributed to Bull lake glaciers lie close in front at a few locations; elsewhere Pinedale ice extended beyond Bull Lake limits.

Paralleling the Lemhi Range near Gilmore is a 2-mile-wide graben valley, beyond which rises the fault-block complex of Middle Ridge, on the highest summits of which are 6-foot quartzite erratics derived from Lemhi sources. These could not have been emplaced on present terrain; ice streams from the Lemhis would have been diverted along the low graben floor. Therefore, this glacier advance predated uplift of Middle Ridge. A few miles north, roadcuts at 6600 feet expose till that is plastered against the Middle Ridge fault, hence is post-faulting in age. Bulldozer excavations into a broad pediment(?) remnant north of Middle Ridge expose till, outwash, paleosols, and congeliturbates of unknown, but possibly intermediate age.