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of higher sea level, calcium-rich mud and clay was deposited in the sea bottom. As the seas periodically fell, beach and near-shore marine environments encroached over the mud, depositing sand. Over the course of millions of years, the sands and mud formed into rock. The calcium-rich mud formed limestone, the mud formed shale, and the sand formed sandstone. The sea-level fluctuations (called "transgressions" and "regressions"), combined with the rock-forming processes, are responsible for the "layer cake" stratigraphy of bedrock deposits that are found in the Twin Cities area. In the millions of years that followed, "tectonic" processes acted on the region to cattle the rocks to be further buried. The Twin Cities structural basin formed as much deeper crystalline rocks that are part of one arm of the Lake Superior rift basin dropped down. ~ The rocks near what is now Minneapolis slowly became deeper than surrounding areas. A series of faults step the bedrock down to this slightly deeper depth. These faults are most prevalent in southeastern Washington County and southwestern Scott County. There is some evidence to suggest that the transgression-regression sequences continued for periods later than 450 million years ago, depositing sediment that became younger rock formations. However, continental glaciation that took place beginning about 2 million years ago and ending about 200,000 years ago, "planed off 'much of these younger rocks, leaving behind a hodge-podge of clayey silt (called "glacial till") and gravelly sand (deposited from the flowing water of the melting glaciers). Meltwaters from the glaciers, on occasion, ponded into massive lakes, such as Glacial Lake Agassiz in northwestern Minnesota-eastern North Dakota. A natural rock and ice dam at the southeast end of Glacial Lake Agassiz failed catastrophically, sending massive torrents of water down what is now the Minnesota River Valley. Other, less energetic glacial streams incised tributaries through bedrock, forming the Mississppi River. These unconsolidated glacial deposits have not had enough time to form into rock. Processes of erosion and deposition by flowing water About 700-million years ago, the beginnings of a new ocean began to form in what is now part of Lake Superior. This process, however, stopped prematurely, leaving behind a rift valley, similar to the rift valley in Ethiopia. 'The remnants of this "failed rift valley" is the Mid-Continent Gravity Anomaly then trends from Lakes Superior, southwest through the Twin Cities and into Missouri. This arm of the failed rift valley has been the location of periodic tectonic activity aver geologic time. P:\Mpls\23 MN\71\2371 l05 Water Supply Alternative Study\FinalDeliverables\Alternatives_Report_final.doc 5