Kassia Groszewski, December 2011
Faculty Advisor: Dr. Erika R. Elswick
The drainage divide of the Tobacco Root Mts. bounds the upper watershed and is underlain by granite and granite-gneiss. Tertiary age deposits composed of weathered sediments fill in the lower watershed and create a dual aquifer system in the valley. The region has a long history of mining – gold, copper, iron, and zinc associated with the Tobacco Root Batholith. Most mines in the watershed are now abandoned. The ore minerals are the result of magma and hydrothermal intrusion into the gneiss – water in the fractures became supersaturated and then precipitated the ore minerals.
Glacial activity in the upper reaches of the watershed and freeze–thaw weathering produced silica–rich sediments from the igneous and metamorphic rock. The Tertiary fill in the lower reaches is composed of the weathered sedimentary materials carried down from the upper watershed and deposited by braided stream systems NE of the Elk Creek fault in the basin. Tertiary fill gravel, sand, silt and clay beds are well to poorly–sorted and variable in thickness.