Des Moines Lobe
The last glacier to enter Iowa advanced in a series of surges beginning 15,000 years ago and reached its southern limit, the site of modern-day Des Moines, 14,000 years ago. By 12,000 years ago, the slowly decaying ice sheet was gone, leaving behind a poorly drained landscape underlain by pebbly clay as well as sand and gravel from swift meltwater streams. Today, broadly curved bands of ridges and knobby hills (moraines) mark the positions of stationary ice fronts. Clear Lake and Storm Lake lie along the lobe’s eastern and western borders. Iowa’s “Great Lakes,” smaller prairie potholes or kettle lakes, and abundant wetlands dot the terrain of this freshly glaciated landscape.