#24 - How Glaciers Helped Form Flathead Lake

The Rocky Mountain Trench . . .
The Mission Valley lies at the southernmost end of an impressive feature called "the Rocky Mountain Trench." The trench, which formed with the Rocky Mountains roughly 60 million years ago, extends all the way into the southern Yukon as a straight, steep valley. During the last ice age an enormous glacier filled the trench. As the glacier pushed southward into the Flathead area, the Mission Range caused it to split (red arrows), diverting one branch into the Swan Valley. Glaciers end when they reach a point where the climate is warm enough to melt the front of the ice back at the same rate it is moving forward. Since the narrower Swan Valley is more shaded, its branch flowed 50 miles farther south to the Clearwater Junction area.

Formation of the moraine dam . . .
Where a glacier ends, the ice melts, dropping rock material that was stuck to its bottom and sides. If the climate is stable for a time (months, years, decades) the rock material will build up forming a ridge at the end of a glacier, called the end moraine. Moraines help geologists determine how far the ice advanced and provide other insights. One such moraine can be found at Polson (yellow line on above image). If you're driving north on U.S. Highway 93, you first see Flathead Lake when you reach the top of this huge moraine. So, Polson marks the southernmost extent of the glacier during the last ice age (15,000 years ago).

As luck would have it . . .
The large size of the Polson Moraine suggests that the ice front must have remained in that position for quite a few years. Once the climate began to get warmer the ice probably melted within a couple thousand years. Geologists believe that the part of the glacier in the Mission Valley survived a few centuries longer than other parts of the glacier. Had the ice not persisted there, the present lake basin would have filled with outwash sediments. Eventually the stagnant mass of ice also melted, forming a lake behind the moraine. As soon as the level of water reached the top of this "moraine dam", it poured across and began to cut a channel through it. Moraine-dammed lakes can be relatively short-lived because water fills up behind the moraine and starts to flow over the top. This running water typically cuts through the moraine, and the lake waters drain away. However, this was not the fate of the Polson Moraine. By chance there happened to be a bedrock hill buried beneath the moraine where the water starting cutting down through the moraine. As a result the meltwaters did not drain completely, allowing Flathead Lake to exist today in the valley once occupied by a glacier.

Above: Diagram from Roadside Geology of Montana, Hyndman and Thomas, 2020 - Click on it to enlarge.

Below: This is Polson (on the southern end of Flathead Lake) with the Mission Mountains on the left. The Polson Moraine is the ridge behind the town, covered with grasses and trees. Click on photo to enlarge.

Source: Alt, David. "The Making of Flathead Lake" Profiles of Montana Geology; published by the Montana Bureau of Mines and Geology in cooperation with the Montana Magazine 1984

Terms: outwash plain, kettle lake