Near the end of the last Ice Age, a lobe of the Cordilleran ice sheet over 40 miles wide and 3000 feet thick covered this area. It blocked the Clark Fork River, impounding over 500 cubic miles of water in a huge Glacial Lake Missoula covering much of western Montana.

When the lake reached about 2000 feet deep, the pressure of the water against the ice broke down the dam, releasing the lake water and icebergs from the broken dam in a huge Ice Age Flood that drained away in only a few days time.

This cycle repeated 40-100 times over a 5000 year period as the advancing ice lobe dammed the lake and subsequently collapsed again and again.