Bedrock Geology of Iowa

The bedrock geologic map portrays the current interpretation of the distribution of various bedrock stratigraphic units present at the bedrock surface. The bedrock surface is buried by unconsolidated surficial sediments (mostly Quaternary) over most of its extent, but this surface coincides with the modern land surface in areas of bedrock exposure. The map is consistent with all available data including drill records and well samples, as well as surface bedrock exposures (both natural and man-made) and shallow-to-bedrock soils units (NRCS county soils maps). Mapped stratigraphic intervals are portrayed primarily at the group level (i.e., a grouping of bedrock formations), each characterized by distinctive lithologies (rock types) summarized in the map key and associated metadata. The distribution of bedrock units was mapped to conform to the current map of bedrock topography (elevation of the bedrock surface). The structural configurations of relevant stratigraphic datums were intercepted with the bedrock topographic surface to produce the map contacts. The line style shown on the bedrock geologic map qualitatively reflects both data density and degree of certainty of individual stratigraphic contacts. Detailed line work is possible in areas of modern bedrock exposure, but more generalized line work (smooth and more sweeping forms) is portrayed in areas of sparser data control. The new bedrock map is, in part, a revised and updated compilation of seven multi-county bedrock maps prepared between 1998 and 2004 as part of Iowa's STATEMAP program (funded through U.S. Geological Survey). These maps were further supplemented with other STATEMAP bedrock compilations for portions of northeast and eastern Iowa, although much of the bedrock geology shown for northeast Iowa represents new and previously unpublished information. Bedrock faults are displayed in the map as sharp linear features offsetting mapped stratigraphic units.