Montana Earth Science Picture of the Week

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. Glacial lakes (see map near bottom of this web page), such as Lake Missoula and Lake Great Falls, were not always present during these ice ages. The formation of glacial lakes in Montana happened when a two conditions were present. In the case of Lake Great Falls, the continental glacier had to grow far enough southward to reach the Highwood Mountains east of the present-day city of Great Falls. When this happened the ice and mountains combined to block the flow of the ancient Missouri River, causing the lake to form. A second requirement is that climate had to be warm enough for melting to occur. During the coldest centuries of the recent ice age it was too cold and dry for there to be any runoff so rivers would have dried up. However, during warmer centuries within the ice age, rivers flowed and glacial lakes would have formed. It is difficult to know exactly when the lakes were present (and how often they formed), but estimates 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, taken 10 miles northeast of Helena, shows layers of sediment that were deposited at the bottom of Lake Great Falls some time during the last ice age.

Seasonal Sorting
During melting seasons, 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, letting the water to become so calm 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. To see a spectacular example of this seasonal sorting, select the link titled VARVES, which can be found beneath the map at the bottom of this page.

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 also be seen in the cut banks along Lower Holter Lake between Helena and Great Falls (another stretch of the Missouri River).

Term: varve