#51 - Sediment from Glacial Lake Great Falls

The Big Chill . . .
Over the past several hundred thousand years there have been glaciations (commonly referred to as ice ages) about once every 100,000 years. During the peaks of those ice ages (called glacial maximums) glacial lakes (see map below) formed in parts of Montana as the continental glacier grew far enough south to block the flow of rivers. In the case of Lake Great Falls, the glacier grew far enough south to reach the Highwood Mountains east of the present-day city of Great Falls. There the glacier and the mountains teamed up to block the flow of the ancient Missouri River, causing the Glacial Lake Great Falls to form. It is difficult to know exactly when the lakes were present, how often they formed, and how often outburst floods occurred - it's complicated! Estimates for the presence of the lakes generally fall within the range of 11,500 to 20,000 years ago.

Lake Bed Sediments in the Helena Valley . . .
Evidence of shorelines along hillsides around the city of Great Falls indicates that the lake reached a maximum depth of 600 feet there (3,900 feet above sea level); high enough to flood the lower part of the Helena Valley 80 miles to the south of Great Falls. The photo at the top of this page was taken at Lower Holter Lake (part of the Missouri River) between Helena and Great Falls. The cut bank shown in the photo is made of sediment that was deposited at the bottom of Lake Great Falls some time during the last ice age.

Sources: 1. Alt, David and Donald W. Hyndman, (1986) Roadside Geology of Montana, Mountain Press Publishing, Missoula, Montana, 268-270.

2. Stickney, M. C. (1987) Quaternary Geology and Faulting in the Helena Valley: Road Log No. 3, Montana Bureau of Mines and Geology Special Publication 95: Compiled by Richard B. Berg and Ray H. Breuninger.

Term: glacial outburst flood

Map courtesy of Roadside Geology of Montana - Alt and Thomas