Montana Earth Science Picture of the Week

This photo, taken in Glacier Park, shows scratches on bedrock that were made as a valley (a.k.a. alpine) glacier moved across here. Large rock fragments stuck to the bottom of the ice caused the gouges, which are called “glacial striations.”

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How to “track” a glacier . . .
Striations help determine which direction the ice was flowing as it moved across an area. In Glacier Park where valley glaciers existed in stream drainages it’s obvious that the ice flowed downhill. However, with the much larger continental glacier that grew southward from Canada during the last ice age, striations 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.

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. 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 was 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.

Below: The glacier that formed the Polson moraine did not cause the polished gouge in this outcrop several miles south of Polson. It was caused by a large rock stuck to the underside of glacier that flowed across this area during an earlier ice age about 70, 000 years ago. The yellow arrow marks the striation and shows flow direction.

Photo by Bruce Baty

Term: moraine