As the Laurentide Ice Sheet receded further, lower elevation outlets were exposed and the lake level dropped significantly. The St. Louis River flowed across the now-exposed lakebed, etching wide and changeable meanders into the soft sediments. On the south side of the river are several steep, curving valleys that are former meanders. Slumping of the unstable clay walls further widened the main valley. The river's downcutting eventually passed through the Pleistocene lake sediments and into the Precambrian bedrock, exposing the rocky, gorge that characterizes the western half of the park.