Bedrock Geologic Map of Iowa
Image courtesy Iowa Geological Survey Bureau (fair use policy)
Iowa's gentle landscape and deep soils hide almost all of its bedrock. Only in the far northeast, in the "Paleozoic Plateau," do you find bedrock and fossils and the other delights of the eastern and western states. There's also a tiny bit of ancient Precambrian quartzite in the extreme northwest. For the rest of the state, this map has been constructed from outcrops along riverbanks and many boreholes. The topographic shading reflects the contours of the bedrock, not the surface.
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 peaceful history occurred about 74 million years ago when a large comet or asteroid struck, leaving behind a 35-kilometer feature called the Manson Impact Structure. It's invisible at the surface—only 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 map of surface deposits in Iowa shows a far different region.
See a somewhat larger version with the original image.
More Iowa resources on About.com:
Iowa Geography & Maps
Iowa State Symbols & Facts
Iowa Campgrounds
Iowa National Parks
Iowa State Parks
Iowa Scenic Roads
Iowa Family Destinations
Iowa Skiing
Iowa Archaeology
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