Striations of Snake Butte - Ft. Belknap Reservation|
Ouch! That's going to leave a mark.
This photo, taken on Snake Butte in north-central Montana, shows scratches that were made as the continental glacier flowed across here during the last ice age. Large rock fragments stuck to the bottom of the ice caused the gouges, which are called "striations."
How to "track" a glacier . . .
Striations help determine which direction the ice was flowing as it moved across an area. Striations caused by the continental glacier that grew southward from Canada during the last ice age help geologists locate "centers" where the ice started to grow from before it merged to form the single ice sheet that covered Canada. In fact, striations found in various parts of Canada indicate that there were three places in northern Canada where snowfall accumulations contributed to the ice sheet that eventually reached Montana. Striations on Snake Butte indicate that the glacier flowed toward the southeast as it flowed over the butte – probably because the Bears Paw Mountains (several miles southwest of here) forced the ice in that direction.
How far south? . . .
The huge glacier left other clues that help geologists determine how far south it advanced. For example, large boulders of granite and gneiss brought from Canada by the ice can be found as far south as the Missouri River in central and eastern Montana. As the glacier flowed across Snake Butte, it scattered huge rocks from the butte in a line extending almost 50 miles to the southeast, confirming the flow direction indicated by the striations. In some places ridges of till, called "terminal moraines", mark the farthest advance of the ice. The town of Polson is built on one of these moraines. The Polson moraine formed as ice at the end of a glacier melted, dropping any rock material that it was transporting. Moraines in southern Illinois indicate that the continental glacier grew much farther south in the Midwest than it did in Montana.