Iowa's bedrock ranges in age from Cambrian (tan) in the northeast corner through Ordovician (peach), Silurian (lilac), Devonian (blue-gray), Mississippian (light blue) and Pennsylvanian (gray), a period of some 250 million years. Much younger rocks of Cretaceous age (green) date from the days when a wide seaway stretched from here into Colorado.
Iowa is solidly in the midst of the continental platform, where shallow seas and gentle floodplains usually lie, laying down limestone and shale. Today's conditions are definitely an exception, thanks to all the water drawn out of the sea to build the polar ice caps. But for many millions of years, Iowa looked much like Louisiana or Florida does today.
One notable interruption in that peaceable history occurred about 74 million years ago when a large comet or asteroid struck, leaving behind a 35-kilometer feature in Calhoun and Pocahontas counties called the Manson Impact Structure. It's invisible at the surfaceonly gravity surveys and subsurface drilling have confirmed its presence. For a while, the Manson impact was a candidate for the event that ended the Cretaceous Period, but now we believe that the Yucatan crater is the real culprit.
The wide green line marks the southern limit of continental glaciation during the late Pleistocene. The map of surface deposits in Iowa shows a far different picture of this state.
More Iowa resources on About.com:
Iowa Geography & Maps
Iowa State Symbols & Facts
Iowa National Parks
Iowa State Parks
Iowa Scenic Roads
Iowa Family Destinations