The Bristol-Plainville-Southington area straddles the boundary between the New England Upland and the Connecticut Valley Lowland sections of the New England physiographic province. The western parts of Bristol are Southington lie in the New England Upland section, an area of rugged topography underlain by metamorphic rocks of Palezoic age. The eastern part of the area, to the east of a prominent scarp marking the limit of the metamorphic rocks, is in the Connecticut Valley Lowland and is underlain by sedimentary rocks and interbedded basaltic lava flows of Triassic age. The lowland is characterized for the most part by broad valleys and low intervening linear hills, but in the eastern parts of Plainville and Southington, basaltic rocks form a rugged highland. The bedrock is largely mantled by glacial deposits of Wisconsin age. On hills the glacial deposits are mainly ground moraine, and in valleys mainly stratified. The metamorphic rocks comprise the Hartland Formation, Bristol Granite Gneiss of Gregory (1906), and Prospect Gneiss. These formations contain water in fractures, principally joints occurring in regular sets. The rocks generally yield supplies of 5 to 15 gpm (gallons per minute) to drilled wells averaging about 140 feet in depth. The rocks of Triassic age in the area are the New Haven Arkose, Talcott Basalt, Shuttle Meadow Formation, Holyoke Basalt, and East Berlin Formation. The formations contain water principally in joints and other fractures and, to a lesser extent, in bedding-plane openings and pore spaces. Drilled wells penetrating these rocks generally range from 100 to 200 feet in depth and yield an average of nearly 20 gpm. The maximum yield obtained from a well in these rocks is 180 gpm. The ground moraine of Pleistocene age is composed principally of till. The deposit averages about 24 feet in thickness, and wells penetrating it average about 16 feet in depth. The ground moraine yields small supplier of water suitable for household use when tapped by shallow large-diameter wells. The stratified glacial deposits, which are as much as 300 feet thick, comprise ice-contact and proglacial deposits and deposits of generally obscure origin termed 'undifferentiated stratified deposits.' The ice-contact and undifferentiated stratified deposits, some of which underlie proglacial deposits, are coarse grained and contain gravel beds from which supplies of as much as 1,400 gpm can be obtained. The proglacial deposits are, on the whole, finer grained than the other stratified deposits, but in places they allow development of wells producing as much as 500 gpm. However, the stratified glacial deposits throughout much of the Bristol-Plainville-Southington area are fine grained and provide only small supplies.