A Peak Through The Rocks

The Geography of Lost Lake, MT

If you are looking for something unusual, the geological marvel of Lost Lake in Chouteau County is precisely that. Scattered over the plain at the head of the lake lie rock formations taller than a man. The rocks become more clustered at the head of the lake until one is finally standing on rock cliffs that form a horseshoe and drop hundreds of feet to the lake below. Exact measurements of the width and depth of the headwall have never been made.

Bureau of Land Management geologist Jim Mitchell says that the horseshoe drop was once a waterfall. Only one other stranded waterfall is known in this area, Dry Falls in Washington, which was left when the ancient Columbia River returned to its original course.

The creation of Lost Lake dates back into antiquity. Geologists estimate that 50,000 to 70,000 years ago, during one of the advance glacial advances, a large lake was created in the Great Falls area. Alternate melting and freezing caused vast quantities of water to flow from the lake. This overflow is said to have formed the Shonkin Sag where Lost Lake is located. The sag is nearly a mile wide and over 500 feet deep in places, too sizeable to be formed by a river.

Mitchell says Lost Lake is part of the pre-glacial Missouri River Channel which was altered by earth and stone left by the glacier’s advance. Since river channels melt rapidly, considerable erosion in the rocks at Lost Lake occurred. The rock formations which appear to be randomly located across the plain at the head of the lake are actually the tip of a volcanic dike or arm which has been uncovered by water erosion. This volcanic or igneous rock is part of an igneous intrusive body associated with the Highwood Mountain complex. The volcanic arm can best be seen from the air, but because of the buildup of soil it is impossible to tell the extent of the underlying volcanic formations, Mitchell says.

The rock formations are vastly different at the headwall and lower end of the lake, Mitchell says. The lower end is sandstone and shale composite, while the headwall formations are potassium rich salt volcanics which are found only on the continental margins.

Fish cannot survive in the lake because there is no freshwater inflow and the salts are highly concentrated. The average depth of this relatively shallow lake is 25 feet according to landowner Bill Harrer.

Lost Lake Ranch Headquarters

Other wildlife is present, however. Ducks and birds, deer, snakes, rabbits and other small animals are commonly seen. Plant life thrives in the moist pockets in the rocks. What appears at first as barren is very much alive.

From this upstream boulder-strewn terrain, which appears similar to the shadow created by wind erosion in th Dakotas, the land slopes downward. Following the road, you veer away from the lake only to have it eventually reappear as you approach Harrer’s Lost Lake Ranch. If you’re in Fort Benton, Loma, Carter, or Highwood, MT, we hope you contact us soon.

Here at the downstream or mouth end, the lake is less mysterious, but no less beautiful. In the distance stands the tall columnar jointed horseshoe encircling the upper end of the blue lake, which nestles in its earthen hollow.

Lost Lake lies on private land and permission to view the area should be obtained from landowners Bill Harrer and Carl Grossman.