Montana Earth Science Picture of the Week

The “Big Chill” . . .
This photo, taken 10 miles northeast of Helena, shows layers of sediment that were deposited at the bottom of Glacial Lake Great Falls 15,000 years ago during the climax of the last ice age. During this time an ice sheet covered Canada, extending southward into Montana. Central Montana's higher elevation combined with the edge of the glacier to form a dam along the Missouri River. The result was the formation of Glacial Lake Great Falls, a prehistoric lake whose size would rival some of the modern Great Lakes. Shoreline evidence along hillsides around the city of Great Falls indicates that the lake reached a depth of 600 feet there.

Seasonal sorting . . .
At its maximum extent, the waters of Glacial Lake Great Falls flooded the Helena Valley, over 75 miles to the south of Great Falls. During the ice age springs and summers, particles of silt and clay were transported to the lake by water that drained into it. The larger particles settled to the bottom, while the smaller particles of silt and clay stayed suspended. Then, during the winters the lake surface froze over, causing the lake water to become calm so that even the finest particles of silts and clays could settle to the bottom. Year after year this seasonal sorting of sediments formed the layers shown in this photo.

NOTE: The water shown in the photo is a part of the Missouri River known as Hauser Lake. It is about 8 miles northwest of Canyon Ferry Dam (also part of the Missouri River).

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.

Below: Sediment from Glacial Lake Great Falls can be seen in the cut banks along Lower Holter Lake between Helena and Great Falls (part of the Missouri River).

Term: varve