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Iowa Geologic Map

Geologic Maps of the 50 United States


Iowa's gentle landscape and deep soils hide almost all of its bedrock, but drillholes and excavations will reveal rocks like these. (more below)
Iowa's rocks

Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman (fair use policy)

Click the map for a larger version
Only in Iowa's far northeast, in the "Paleozoic Plateau" along the Mississippi River, 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.

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 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 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 about Iowa Geology

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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