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Elsewhere across the state, the bedrock surface is covered with younger glacial-age materials. As a result, much of our information about Iowa's bedrock geology comes from rock samples brought up to the land surface during the drilling of wells. Iowa’s bedrock geology map shows rocks from younger periods overlapping older rocks. Most of the rock units are dipping gently to the southwest, and this bedrock structure, coupled with a long history of surface erosion, contributes to the irregular bedrock surface crossing rock units of different ages. Two small, but noteworthy features interrupt this general bedrock pattern. The first is in the far northwest corner of Iowa, where an ancient ridge of silica-cemented sandstone pokes to the land surface. At 1.6 billion years of age (Precambrian), these scattered outcrops of hard, reddish Sioux Quartzite are the oldest bedrock exposed anywhere in Iowa. Elsewhere beneath the state, the Precambrian rocks are usually igneous and metamorphic types, and they lie deeply buried by the thick sedimentary strata. |
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