The north-central Connecticut of this paper is an area of about 640 square miles extending from the Connecticut-Massachusetts State line south to Middletown. It is underlain by unconsolidated deposits of Pleistocene and Recent age, which mantle an erosional surface formed on consolidated rocks of pre-Triassic and Triassic age.The unconsolidated deposits include ground moraine and glaciofluvial, glaciolacustrine, and eolian deposits. The ground moraine (till) occurs throughout most of the area but yields only small quantities of water. The glaciofluvial deposits comprise sand, silt, and some gravel of ice-contact and outwash-plain origin and range in thickness from less than a foot to more than 150 feet. These deposits store large amounts of water and yield 250 to 600 gpm (gallons per minute) to industrial and irrigation wells.The glaciolacustrine deposits consist of varved clay and silt with a capping of sand of fluvial origin. The clay and silt are essentially impermeable. They confine water in underlying outwash deposits, and also hold up perched water in overlying fluvial sand. The sand yields as much as 100 gpm to a properly constructed well.The consolidated rocks are sandstone and shale, interbedded layers of basalt, and crystalline rocks. On the average, these rocks yield 10 to 15 gpm to wells, but the sandstone and shale yield larger quantities, as much as 550 gpm, to a few wells. The ground water in north-central Connecticut is of the calcium bicarbonate type, has a low mineral content, and is suitable for most purposes. The sandstone and shale generally yield moderately hard to very hard water.