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The geologic map, cross sections,and stratigraphic column on this plate depict the type, distribution,and structure of the bedrock units in Washington County that are either exposed at the land surface or lie directly beneath unconsolidated Quaternary glacial sediments of variable thickness. The map shows how the bedrock surface would appear if it was viewed from an aerial perspective and the overlying Quaternary sediments were stripped away. The bedrock units near the land surface in Washington County consist of sedimentary rocks of Paleozoic age that form distinguishable and mapable layers designated as formations. These units are commonly exposed along the Mississippi and St. Croix River bluffs, within rock quarries, and along roadcuts within the county. The accompanying bedrock geologic cross sections add the dimension of depth and illustrate the stratigraphic, structural, and topographic relationships of the bedrock units,as well as the variable thickness of the overlying Quaternary sediments. |