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report by Horn (1983) contains ground -water pumpage data that are very useful in understanding and interpreting water levels and water -level changes. Delin and Woodward (1982) constructed regional water -level maps for both aquifers in southeastern Minnesota, which includes the 'Fain Cities Metropolitan Area. The author is grateful to all the well owners who allowed measurements of water -levels in their wells, Special. thanks are given to those municipal: ties, industries, and others who turned off their pumps the night prior to a water - level measurement in their well in order- to allow adequate time for water - levels to recover from pumping. The ground -water system underlying the 'Twin Cities contains 14 geologic units that Guswa and others (1982; table 1) combined into 9 hydroyeologic units-5 aquifers and 4 confining units (fig. 2). The five uppermost hydro - geologic units are not continuous in the Twin Cities area because of erosion of bedrock units or nondeposition of drift or both Bedrock valleys deeply incise the six uppermost hydrogeologic units. 'These valleys either contain drift or form present-day stream valleys and significantly affect flow in the Twin Cities ground -water system. The Prairie du Chien -Jordan aquifer comprises two geologic units with different lithologies and correspondingly different hydraulic properties. The Prairie du Chien Group of Ordovician age is predominantly dolomite; it contains fractures, joints, and solution cavities that control the flow of water through it. The Jordan Sandstone of Cambrian age is a fairly uniform, highly permeable sandstone; flow through it is primarily intergranular. The two units act as a single aquifer because no extensive confining unit separates them. Drift -filled bedrock valleys, present-day stream valleys, or both, inter- rupt. the continuity of the Prairie du Chien -Jordan aquifer in many parts of the study area. In the major stream valleys, where the overlying confining units have been removed by erosion (fig. 2), a good hydraulic connection exists between the aquifer and the major streams. The degree of hydraulic connection between the Prairie du Chien -Jordan and these st t: eams depends on the hydraulic conductivity of the valley -fill deposits. The Mount Simon -Hinckley aquifer comprises two sandstone formations, the Mount Simon Sandstone of Cambrian age and th. Hinckley Sandstone of Precambrian age. 7bey are cont'nuous throughout the study area and have similar hydraulic characteristics. A thick confining unit, the Eau Claire Sandstone, which overlies the Mount Simon -Hinckley aquifer (fig. 2), retards 'low between the Mount Simon -Hinckley and the overlying aquifers and the surface -water system. Norvitch and others (1973) present a detailed discussion of the geologic and hydrologist properties of the Prairie du Chien -Jordan and Mount Simon - Hinckley aquifers. 4