I was just in Des Moines a couple weeks ago. What your finding is referred as glacieal alluvium, which are rocks of many kinds transported far from the north by the glaciers of the ice age. When I driving back to Colorado I was spotting some of the much larger boulders in the fields along I80. Those they call glacieral eractics. It amazes me what all happened in the past across the state of Iowa and how the glaciers moved giant boulder like those hundreds of miles from their source.