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 originally was made influence its ability to hold 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, allows very little water to flow through. The carbonate aquifer, which is composed of layers of limestone and dolomite, is the principal source of ground water in west central Ohio, including Union County.
Limestone consists of fossilized sea shells, shell fragments and consolidated limy mud.
Its main mineral is calcium carbonate, CaCO 3 . Both limestone and dolomite are commonly referred to as limestone or carbonate rocks. The limestone and dolomite formations, which underlie most of the west central portion of Ohio, were deposited between about 400 and 500 million years ago. In most areas of this region, these formations are covered by a layer of glacial till, which is an unsorted mixture of clay, sand, gravel and boulders deposited by a glacier. Limestone formations are good sources of ground water because of their naturally formed solution channels, joints and fractures.