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continue to this day in the streams and river valleys of the area. Some of the streams have cut down <br />into the rock. In several places, the streams deposited sand, gravel, and clay into these alluvial <br />valleys and the stream has changed course, leaving behind a "buried bedrock valley" with little <br />surface topographic expression. <br />A generalized cross section from northwest to southeast through the Twin Cities mcfropolitan area is <br />shown on Figure 2. The crystalline Precambrian bedrock consists of very old rocks (greater than <br />about 600 million year old) that are composed mostly of granite-like rocks. The Mt. Simon and <br />Hinckley Formations (which typically are not differentiated from one another) are sandstones. <br />Overlying the Mt. Simon-Hinckley Formations is the F,au Claire Formation, which is a relatively <br />low-permeability shale. The [ronton and Galesville Sandstones overlie the Eau Claire Formation, <br />followed by [he Franconia Formation, which is a sandstone in some locations and a shale or <br />NORTHWEST <br />RIVER <br />~\~~ <br />~\ s _ <br />`~ <br />\\\ <br />~~ <br />., \ <br />0 Glacial duff (sand. gravel. clay) <br />U Platteville formation (limestone) <br />0 St. Peter Sandstone <br />Prairie du Chien Group Qimestone) <br />0 Jordan Sandstone <br />® St. Lawrence Formation (shale) <br />® Franconia Fm. (sandstone & limestone) <br />Ironton 8 Galesville Sandstones <br />Eau Claire Formation (shale) <br />~ Mt. Simon & Hinckley Fms. (sandstone) <br />Precambrian Crystalline Bedrock <br />1... <br />Is <br />Figure 2 Schematic Geologic Cross Section Through the Twin Cities Area (not to scale) <br />SOUTHEAST <br />ST CROIX RIVER <br />limestone in others. The St. Lawrence Formation (a sandy shale) overlies the Franconia Formation. <br />The Jordan Sandstone (which is typically about 90 feet thick) and the Prairie du Chien Group <br />P:AMpls\23 MNV71A2371105 Watee Supply Altemati ve SWdyVfSnulDellvereb les'VAltcrnelives _Rcpon Fnal.doc 6 <br />