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The geology of the watershed includes consolidated bedrock formations overlain by unconsolidated <br />glacial sediments (also known as quaternary deposits). Unconsolidated glacial sediments are from glacial <br />deposits left from the quaternary geologic period and modified by post -glacial erosion and soil formation <br />processes. Most of the quaternary deposits in the watershed were deposited approximately 12,000 to <br />20,000 years ago by the Des Moines lobe (Grantsburg sublobe) of the Wisconsin Glaciation (the most <br />recent local glacial episode) (Meyer and Swanson, 1992). The Grantsburg Sublobe deposited a silty till that <br />was reworked by glacial meltwater over much of the watershed into sand plain, sandy lacustrine and <br />valley train deposits. These glacial deposits, along with older, buried glacial deposits, range from 100 to <br />250 feet in thickness within the watershed. The watershed is almost entirely located within the Anoka sand <br />plain, a flat, sandy lake plain and terraces along the Mississippi River. This is a highly permeable sand layer <br />with generally high water table. <br />More information about the surficial geology of the LRRWMO is available from the Anoka County <br />Geologic Atlas at: / : - . / <br />Consolidated bedrock formations (bedrock deposits) are much older and lie below the glacial deposits. <br />They include overlapping sequence of sandstones, limestones, dolostones, and shales. Most bedrock units <br />in the watershed were deposited during the Paleozoic era marine environments about 450 to 530 million <br />years ago. The uppermost layer of bedrock varies with location but is generally belongs to the St. <br />Lawrence formation or Tunnel City group (formerly Franconia formation). Buried bedrock valleys that cut <br />down to the Wonewoc sandstone and Eau Claire formation occur in the north and southeast portions of <br />the watershed (reference geologic atlas). The Eau Claire Formation acts as a confining unit between the <br />overlying Wonewoc sandstones and the underlying Mt. Simon Sandstone -Hinckley Sandstone aquifer. <br />2-13 <br />