Ancient Rock
Some of the oldest rocks known on Earth can be found in the river valley. The earth is 4.5 billion years old and rocks in this area date back over three billion years. In Lac qui Parle State Park and the Gneiss Outcrops Scientific and Natural Area near Granite Falls you can see outcrops of 3.6 billion year old rock called gneiss (pronounced “nice”). Gneiss is a banded metamorphic rock formed when granite and other rocks were subjected to intense heat and pressure deep beneath the earth's surface (photo).

Glaciers
Glacial materials cover most of this ancient bedrock. The landscape we see today is rooted in Minnesota’s glacial history. For tens of thousands of years, massive glaciers advanced and retreated across the landscape (map). Most of the basin is relatively flat because these glaciers deposited rock, sand, and gravel hundreds of feet thick. This material is called glacial drift.

A distinctive topographic feature in the basin is the Coteau des Prairies which rise about 2,000 feet in the western portion of the basin. Early French explorers named the Coteau des Prairies which means Highland of the Prairies. This highland was created when glaciers deposited additional material in this area over a base of quartzite rock, creating a plateau that rises above the adjacent lowland. Headwater streams for several of the Minnesota River’s major tributaries, such as Lac qui Parle, Yellow Medicine, Cottonwood and Redwood Rivers, arise in this highland (map). Streams flow from the top of the plateau dropping through steep gorges.