Laserfiche WebLink
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 metropolitan 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 Eau Claire Formation, which is a relatively <br />Cow-permeability shale. The Ironton and Galesville Sandstones overlie the Eau Claire Formation, <br />followed by the Franconia Formation, which is a sandstone in some locations and a shale or <br />NORTHWEST <br />ELK RIVER <br />APOLIS <br />SOUTHEAST <br />ST CROIX RIVER <br />f ~~ <br />4~ <br />It <br />I <br />0 Glacial drift {sand. gravel. clay) <br />[] Platteville Formation {limestone) <br />~ St. Peter Sandstone <br />Prairie du Chien Group (limestone) <br />~ Jordan Sandstone ` a ,~ <br />®St. Lawrence Formation (shale) <br />~ Franconia Fm. (sandstone & limestone) <br />~ _"~ Ironton & Galesville Sandstones <br />Eau Claire Formation (shale) FAULTS <br />C~ Mt. Simon & Hinckley Fms. (sandstone) ~ I <br />Precambrian Crystalline Bedrock <br />Figure 2 Schematic Geologic Cross Section Through the Twin Cities Area (not to scale) <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:\Mpls\23 MN\71\2371105 Water Supply Alternative Study\FinalDeliverables\Alternatives_Report_final.doc 6 <br />