On the way to Missoula we passed through a daylight road cut on Interstate 90 near the hamlet of Alberton approximately 16 miles north of Missoula. I'd passed through this road cut before, but this time I was ready and pulled out my camera for a couple of pictures as we passed through the exposed silts on either side of the road. Roadside geology on the fly.
Layered silts and clays at the Nine Mile Section on Interstate 90 northwest of Missoula, Montana
These distinctly layered silts and clays represent alternating paler river silt deposits and the darker lake deposits. Chambers and Alt report 36 distinct layers. The lake sediments were laid down on the bottom of Lake Missoula each time the lake filled with water when the Clark Fork River was dammed by glacial ice between 18,000 and 15,000 years ago. As can be seen in the distinct silt layers, the river was dammed and then drained on numerous occasions - something like 40 times.
Looking at the slopes of the mountainsides east of Missoula on either side of Hellsgate where the Clark Fork River flows out into the broad valley where Missoula is located horizontal lines can be seen on the slopes. These lines mark the former shoreline of glacial Lake Missoula. They are wave cut terraces marking the various lake levels as the lake filled with water when the Clark Fork was dammed. The entire Clark Fork valley filled with water along with tributary river valleys such as the Bitterroot Valley south of Missoula. In all the lake covered as much as 2,900 square miles.
Faint horizontal lake shore terraces on the slopes above Missoula. Montana
The lake filled until it floated the ice dam blocking the river and then the entire lake drained rapidly across eastern Washington, down the Columbia Gorge through present day Portland shaping vast tracts of the landscape of Washington and Oregon. Water backing up the Yakima, Walla Walla and Willamette valleys silts that created some of the best farmland in the world.
The rich farmland of the Willamette Valley was the impetus of the early settlement of the Pacific Northwest by Americans. This in turn led to the bustling City of Portland. And from Portland two bands traveled up the former flood route to the former lake bed in Missoula last week. We very much enjoyed the Decemberists and Typhoon during our stay in Missoula. Typhoon was the opening band. I loved the audacity of a 12 or was it 13 member band.