Above: The Front edge of a glacier
The formation of most of Iowa's soils was influenced directly or indirectly by glacial events. Glaciers slowly plowed across Iowa several times thousands of years ago. As they advanced from the north they dragged frozen soils and rocks with them. Some of this mixture of sand, clay, gravel, and boulders was pressed into the ground by the immense weight of the glaciers. These deposits made directly by the glaciers are referred to as glacial till. When glaciers melt, vast amounts of glacial meltwater wash out even more unsorted deposits. Together, the till and deposits are called glacial drift. Glacial drift in varying depths blankets nearly all of Iowa's limestone bedrock. Northeast Iowa has virtually no drift but it is over 600 feet thick in west central Iowa. Most of northern Iowa's soils were formed from glacial till.
From Mark Muller's "The World Beneath Your Feet: A Closer Look at Soil and Roots," Funded by the Iowa Living Roadway Trust Fund