Lyon County is in southwestern Minnesota, about 150 miles southwest of Minneapolis and St. Paul The basement rocks in the area consist of granite and quartzite of Precambrian age. These materials are in turn overlain by shale and sandstone of Cretaceous age, glacial drift of Pleistocene age, and alluvium of Recent age. Ground water is available primarily from aquifers in Pleistocene and Cretaceous strata from depths ranging from 15 to 500 feet below the land surface.
The county is divided into areas of ground-water availability based on the quality and quantity of ground water available from the different geologic units. The glacial drift which covers all of the area, yields very hard water from sands and gravels occurring in melt-water channel deposits or as small, isolated melt-water bodies. Wells in the drift commonly yield from 2 to 30 gpm (gallons per minute), but sustained yields of as much as 500 gpm are obtained in areas where thick melt-water channel deposits occur. Cretaceous strata underlie about two-thirds of the county and yield water from poorly consolidated sandstone, The water ranges in hardness from soft to very hard and is sometimes high in chloride content. Wells in Cretaceous strata commonly yield from 2 to 7 gpm; however, in areas where the sandstone is in contact with the underlying weathered granite, sustained yields of as much as 75 gpm are obtained. The geographic and stratigraphic distribution of the geologic units suggests that additional water supplies may be available from Pleistocene and Cretaceous strata in areas not yet fully explored.
