The Iowa Geological Surveyhas a lot of data available to download here.  PDF versions of many of their publications can be found in their List of Publications.

Their GIS data is minimally attributed for the most part, their public wells data set did have a more robust attribution scheme.  One cool feature I found in the wells data is that they provide a hyper-link for many features to an on-line site record.  Found both shapefiles and file geodatabases in the samples I downloaded.

More interestingly, I opened up their Bedrock Geological Map and noticed something odd to me.  See the bulls-eye just NW of center?  That, even to a non-geologist like me, does not look too geologic.

Turns out it is related to a meteorite impact known as the Manson Crater, once thought to be The Dinosaur Killer until it was determined to be too old.