In general, if a formation is capable of yielding enough water to support a well or spring, it is called an aquifer. The types of geologic material from which the formation is composed influence its ability to store and transmit water. For example, sands and gravels allow water to flow through easily. By comparison, shale, which originated from compacted layers of mud and clay, generally allows very little water to flow through it unless the shale is highly fractured. The carbonate aquifer, which is composed of layers of limestone and dolomite, is a principal source of ground water in west central Ohio, which includes Clark County.
Limestone consists of fossilized sea shells, shell fragments and consolidated limy mud.
Its main mineral is calcium carbonate, CaCO 3 . Limestone and dolomite are collectively referred to as carbonates. The limestone and dolomite formations, which underlie most of the western portion of Ohio, were deposited between about 400 to 500 million years ago. In most areas of this region, glacial till covers the bedrock. Till is an unsorted mixture of clay, sand, gravel and boulders deposited by glaciers.