VALLEY GEOLOGY
The jagged rocks of these cliffs show the scars of the largest flood on geologic record, the Ice Age flood of Glacial Lake Missoula about 15,000 years ago. Then most of western Montana was covered by glaciers. A glacier blocked the Clark Fork River at its entrance to the Pend d'Oreille Lake in Idaho. The water backed up, creating Glacial Lake Missoula, which covered most of the Clark Fork Valley, including the current site of Missoula. When the glacier gave way, the water rushed down the valley at 45 miles an hour, with a volume about 500 times the outflow of the Missouri River. Glacial Lake Missoula filled 41 times in about a thousand years. Walls of water 2,000 feet high carved the channeled scablands of eastern Washington.
The rushing water scrubbed the soils from the narrow necks of the valley, exposing the bedrock of Belt mudstone, which covers much of western Montana and parts of Idaho and British Columbia. The mudstone was formed in Precambrian times about a billion years ago. Then, blue-green algae were the only life on earth. The weathered rock outcrops are stained red by iron oxide present in the rock.
From the marker