The Eastern Arm of Glacial Lake Missoula
On September 14th, a beautiful fall day, the little Montana Natural History Center bus set off with a small group to explore nearby Glacial Lake Missoula sites in the rarely visited eastern arm of the lake and go as far as the eastern ancient shoreline near Drummond, Montana. This was always relatively shallow water, only about 300 feet deep, so what we saw was primarily sediments and erratics/dropstones. The Clark Fork River backed up behind the narrow Hellgate Canyon and left deep deposits of the fine grained lake sediments in what is now East Missoula. This material was used in the 1800s to make bricks for the streets and buildings of the growing town and also by the railroads to build depots and other structures along the route, as it was actually cheaper to use than wood. Further upstream in the same general area we examined the jumble of various kinds of rocks strewn widely in the Canyon River development – 200 or so have been recorded. These are close to the confluence with the Blackfoot River, and some may have been carried along from that drainage. The eastern most varved lake deposits dated at 20,000 years ago were examined even further upstream.
Along the way we learned about signs of volcanic activity in this area, placer gold deposits, rocks indicative of diamonds, ancient fault lines causing current slumping of hillsides, and other geology of the area. The future location of a high water stone marker and signage along the interstate at a currently closed rest stop was also inspected.