Near Greenough in Missoula County, Montana — The American West (Mountains)
Souvenirs of the Ice Age
Glaciers formed in this area multiple times over about 300,000 years during two ice ages: the Bull Lake and Pinedale. During the Bull Lake Ice Age, about 140,000 years ago, the glaciers flowed farther south in the valleys south of Ovando and Helmville. Between 60,000 and 15,000 years ago the ice cap formed again. Southward spreading glaciers carried gravel and sand into the Ovando area, about one to two miles south of Highway 200. Melting glaciers left behind mounds of rock and soil debris, called moraines. The low areas between the moraines mark places where the glacial till wasn't deposited, leaving hollows that filled with water. Many of the ancient hollows can still be seen today as wetlands, shallow ponds and small lakes. Large depressions north of
The smooth broad plains to the north and east of here are glacial outwash plains. They were once the location of raging streams that carried meltwater loaded with mud, sand, gravel from the front of melting glaciers. These valleys are now the agricultural land along the Clearwater and Blackfoot rivers.
(sidebar at center)
• Although scientists don't know everything about ice ages, cyclic movements in the earth's orbit and spin probably cause glacial-to non glacial cycles.
• North America may have experienced as many as twenty ice ages, but scientists only know a great deal about four of them.
• The Cordilleran Ice Sheet covered northwestern Montana, the Idaho panhandle, all of British Columbia, much of Alberta, southern Alaska and parts of the Yukon.
• Glaciers carry rocks in from areas that can be faraway from Montana. The rocks form deposits of everything from rounded gravel to huge boulders. Look for big boulders that look out of place next to the highway. They will have a variety of different colors, banding, and textures indicating different types of bedrock.
Erected by Montana Department of Transportation.
Location. 47° 0.144′ N, 113° 22.134′ W. Marker is near Greenough, Montana, in Missoula County. Marker is at the intersection of State Highway 200 at milepost 31 and State Highway 83, on the right when traveling east on State Highway 200. Touch for map. Marker is at or near this postal address: 44601 Montana Highway 200, Bonner MT 59823, United States of America.
Other nearby markers. At least 3 other markers are within 12 miles of this marker, measured as the crow flies. Big Blackfoot Milling Company (within shouting distance of this marker); A Vast Network of Indigenous Trails. (within shouting distance of this marker); Sand Park Cemetery (approx. 11.7 miles away).
More about this marker. This marker is located at the Clearwater Rest Area.
Categories. • Natural Features •
Credits. This page was last revised on September 9, 2018. This page originally submitted on September 9, 2018, by Barry Swackhamer of San Jose, California. This page has been viewed 59 times since then. Photos: 1, 2. submitted on September 9, 2018, by Barry Swackhamer of San Jose, California.