#136 - Snake Butte Boulder Train

Say cheese! . . .
This photo was taken along Highway #66 south of the Ft. Belknap Agency near mile marker 36. The cows are posing by pieces of Snake Butte that were carried here by the Laurentide Ice Sheet. That is Snake Butte in the background, ~8 miles away. The boulders are part of the "Snake Butte Boulder Train" - pieces of the butte that have been scattered in a 1-2 mile wide swath extending to the southeast of Snake Butte for about 50 miles. In places the pieces of Snake Butte are few and far between, but many of them are very big like the those shown in the photo. The best place to see rocks of the boulder train is on the west side of the highway between mile markers 35 and 36.

I'm melting! . . .
During the last two glaciations (commonly called ice ages), the Laurentide Ice Sheet grew southward into Montana but the Bears Paw Mountains turned it toward the southeast as it grew into this area. The pieces of igneous rock that the butte is made of were plucked away from the butte as the ice flowed over it. They were transported by the glacier(s) as it continued to flow - Then as the ice age(s) ended the glaciers melted, dropping the rocks where they sit today.

Right: The map is from the new edition of Roadside Geology of Montana, Hyndman and Thomas, 2020 - Great Resource! Click on the map to enlarge.

Standing out . . .
Rocks transported away from the place where they formed by glaciers are called "erratics". There are MANY glacial erratics along the Hi-Line and beyond that originated in northern Canada where the Laurentide Ice Sheet started growing from. However, Snake Butte is made of a unique kind of igneous rock called Shonkinite - so its erratics are very easy to recognize among the others scattered throughout the area.

Term: Laurentide Ice Sheet

Below: Here are some big pieces of Snake Butte. In some places they are few and far between - in other parts of the boulder train there are many in one place. One location where they are abundant is on the west side of Highway #66 near mile marker 35.