Pacific Northwest Gardening: Eastern Washington Scabland/ Gods Hardscaping in NW, 1 by Soferdig

Soferdig wrote:
This is the remainder of glacial Lake Missoula. Flathead lake remains because of a shelf of Granite that held the water back after the final retreat of ice and the big "Ice Cube" remained as the valley was filled in around with glacial deposits. The ice cube that formed Flathead Lake held back the silt, rocks and erratics that formed the valley floor. When it finally melted the lake was shaped in the form of the terminal morraine and Granite dam at the south end of the lake.