Legacy of Ice and Rock

Legacy of Ice and Rock. . , The late-Wisconsin Glacier was no ordinary sheet of ice! The glacier was one mile thick and it stretched from eastern Ohio to the Missouri River. It caused North America's most recent Ice Age, about 14,000 years ago. , As the glacier pushed south, it ran into highland plateau called the Coteau des Prairies. The ice couldn't overrun the Coteau and was forced to split in two, with some ice going east and some going west. The eastern flow is called the Des Moines Lobe. The western ice flow, which followed the James Lowland, is called the James Lobe. , The James Lobe covered most of eastern South Dakota, stopping its westward push at the Missouri River. As the glacier moved southward around the Coteau, it covered the area that's now Sioux Falls. This mountain of ice blocked the southerly flow of the ancestral Big Sioux River and meltwater from the glacier. , As the glacier melted, its meltwater joined with the Big Sioux River. The water found a trough, or low area in the surface of the bed rock and created its new course, forming the giant S-curve that now flows through Sioux Falls. The meltwater also eroded sediment covering the bedrock. Where there were high ridges or outcroppings of bedrock, in places like Falls Park, meltwater dramatically exposed the Sioux Quartzite. , A sheet of ice changed the course of the Big Sioux River and created the area now known as Falls Park. , 1. Before the latest glacier, the Big Sioux River ran straight south. A thick layer of sediment left by earlier glaciers covered the rolling upper surface of the quartzite bedrock. , 2. During the late-Wisconsin glacier, the James Lobe wrapped around the south side of Sioux Falls, blocking meltwater coming down the river. The water found a new course along the glacier's edge. , 3. Meltwater swept sediment away, eroding down to bedrock in places like Falls Park. The river and meltwater found a trough in the bedrock, creating a new river course.