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This photo, taken on Snake Butte in north-central Montana, shows scratches that were made as the Laurentide Ice Sheet (continental glacier) flowed across here during the last ice age. Rocks that were stuck to the bottom of the ice caused the gouges - called "striations." Snake Butte is about 10 miles south of Harlem, MT on the Ft. Belknap Indian Reservation. The Little Rockies can be seen in the distance.
Striations help determine which direction the glacier flowed as it grew across the land, and also help locate "spreading centers" where the ice grew FROM, before merging to form the single continental glacier that covered Canada. Striations found in various parts of Canada reveal that there were three places in northern Canada where snowfall accumulations contributed to the ice sheet that eventually reached the USA (Of course there was no USA at that time!). These striations on Snake Butte prove the glacier flowed toward the southeast as it moved over the butte - probably because the Bears Paw Mountains (several miles southwest of here) forced the ice in that direction.
Below: The black dashed line indicates how far south the ice advanced into Montana when the ice age peaked about 18,000 years ago.
1. Erratics – Metamorphic rocks from the Hudson Bay area can be found on Snake Butte (and throughout northern Montana). These rocks, which were embedded in the ice, were dropped here when the ice melted.
2. The Snake Butte Boulder Train – Large pieces of Snake Butte (igneous rock) have been scattered in a straight line extending to the southeast of Snake Butte. As the glacier flowed across the butte, pieces of the butte were carried away, and eventually dropped as the ice melted. Although they are few and far between, the line extends for almost 50 miles. The direction of the boulder train matches the direction indicated by the striations.
For much more about Snake Butte, CLICK HERE (includes a photo tour).