GEOLOGY OF BAD MEDICINE LAKE

Bad Medicine Lake is on the southwestern flank of a glacial highland called the Itasca moraine. The moraine is composed of glacial sediment (clay, silt, sand, gravel, and boulders) deposited by ice that advanced from the northeast about 25,000 years ago. The Itasca moraine is the highest land formation in northwestern Minnesota and stands 350 meters higher than the Red River valley, which is 100 kilometers to the west. More recent glacial advances from the northwest, north, and northeast eroded the Itasca moraine and deposited more glacial sediment, which created the very rough knob-and-kettle highland topography we see today.

Following a more recent glacial advance, meltwater was discharged at high pressure from beneath the glacial ice, and with great force it cut the picturesque narrow valley that contains Bad Medicine Lake. The lake is con-fined to a six-kilometer segment of the steep-walled valley and has a small drainage basin with no inlet or outlet stream. Fed by groundwater springs, Bad Medicine Lake is noted for its exceptional water quality and clarity. Groundwater also flows out of the lake through the surrounding sand and gravel.

A continental drainage divide called the Laurentian Divide is about six kilometers west of here. Locally, it is oriented northeast to southwest. Surface water west of the divide drains into the Red River of the North and, eventually, into Hudson Bay. Surface water on this side of the divide drains southeastward into the Mississippi River and on to the Gulf of Mexico. Surface water divides are different from groundwater drainage divides, which are formed by underground features. But in this case, the groundwater from Bad Medicine Lake also flows to the south and east.

Erected by the Geological Society of Minnesota in partnership with the Minnesota Department of Transportation and the Minnesota Geological Survey. 2003