Glacial Erratics

Nature Watch
August 20, 2008

By Susan Benson,
CNHM Director of Education

On a stroll through the backwoods of northwest Wisconsin, you may come across an unusual sight — a large rock, perhaps sitting all by itself in a meadow or on the forest floor. There may be no other rocks in sight. How did this one get here? You’re probably looking at a glacial erratic, a piece of rock that may have been transported great distances by the ice sheets that covered the region long ago. One long-distance champion is an erratic composed of solid native copper, probably originating from the Lake Superior region along the Upper Peninsula of Michigan, that was moved by glacial ice more than 600 miles to southern Illinois.

Glacial erratics can be any size from small pebbles to huge boulders. Most of them appear worn and rounded, sometimes including beveled or faceted surfaces. During the course of their journey, the rocks were jostled against other erratics or scraped against underlying bedrock, rounding off corners and planing smooth surfaces. Glacial transport also fractured some boulders, resulting in sharper edges.

Transportation by glacial ice produced other unique features, the most easily observed of which are glacial striations, series of parallel lines or grooves gouged across the rock face or inscribed on the underlying bedrock surface. These glacial furrows were produced when an erratic, frozen firmly in slowly moving ice, scraped against another erratic or against the bedrock surface over which the glacier was moving. These glacial striations can be used to identify the direction of ice movement.

The composition of glacial erratics can reveal their point of origin and give clues about the direction of ice movement. For example, a string of erratics of similar composition might be observed across a broad region. These are referred to as “boulder trains,” series of erratics that originated from the same source. Boulder trains appear as long lines or fans of erratics extending outward from their source in the direction of ice flow.

On your next hike, keep your eyes open for glacial erratics. Each one has a story to tell!

Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.