The basic geologic framework underlying Cerro Gordo County consists of an igneous or metamorphic Precambrian basement complex overlain by, in ascending order, consolidated sedimen tary rocks of Precambrian, Cambrian, Ordovician, Devonian, Mississippian, and Cretaceous age, and unconsolidated sand, gravel, and clay of Quaternary age. Structurally the county is in the northern part of the Iowan Basin, and the sedimentary units of Cambrian through Devonian age dip southward at 9 to 14 feet per mile. Three major glacial advances, the Nebraskan, Kansan, and Wisconsin, covered all or part of the county during the Pleistocene Epoch and left behind glacial drift con sisting of 0 to 125 feet of till and related outwash deposits. De posits of Holocene age, aside from surficial soil, are thin and scattered.

Ground water occurs in the sedimentary units of Cambrian through Mississippian age and in the glacial drift and outwash deposits of Quaternary age. The chief aquifers, in ascending order, are the deep Cambrian sandstones, Jordan aquifer, St. Peter Sandstone, Devonian and Mississippian limestones and dolo mites, and the Pleistocene glacial drift and related interglacial deposits. Units between the St. Peter Sandstone and the Devonian limestones, the Platteville, Decorah, Galena, and Maquoketa Formations, are of low permeability and are consid ered to be an aquiclude in comparison to the overlying and under lying units.

The deep Cambrian sandstones will yield about 60 gpm (gal lons per minute) to an individual well; the Jordan aquifer about 1,200 gpm; the St. Peter Sandstone 30 to 200 gpm; the Devo nian limestones and dolomites 120 to 200 gpm; and the glacial outwash or shallow bedrock generally will yield 10 gpm or more. The Jordan aquifer supplies large quantities of water for mu nicipal and industrial use at Mason City. Pumping tests on wells tapping the Jordan in that area show that the aquifer has a transmissibility of about 35,000 gallons per day per foot and a storage coefficient of 2 x 10^-4 Pumping in the Mason City area from 1912 to 1969 has produced about 200 feet of draw down in the areas of maximum withdrawal.

Ground water in Cerro Gordo County is of suitable chemical quality for domestic, industrial, or municipal use. The total dis solved solids ranges from 152 mg/1 (milligrams per liter) in water from the shallow bedrock to 885 mg/1 in water from the deep Cambrian sandstones. Water from the Jordan, which is the most productive aquifer in the county, generally contains less than 500 mg/1 dissolved solids.

Ground water is the major source of water supplies through out Cerro Gordo County. The majority of wells in the county are used for domestic and stock supplies, but the largest with drawals of water are for municipal and industrial supplies. Municipal pumpage in the county in 1968 was about 1.5 billion gallons; industrial pumpage was about 1.8 billion gallons and most of the water was obtained from the Jordan aquifer. If future development of water from the Jordan aquifer is to be to the best economical advantage, careful consideration should be given to the location, capacity, and pumping schedules of pro posed wells and nearby existing wells so that local overdevelop ment or excessive interference do not occur.