In general, if a formation is capable of yielding enough water to support a well or spring, it is called an aquifer. The material from which the formation originally was made influences its ability to store and transmit water. For example, sand and gravel 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 regional carbonate aquifer, which is composed of layers of limestone and dolomite, is the principal source of ground water in west-central Ohio, including Darke County.
Limestone is consolidated limy mud or calcareous sand, with fossil shells and shell fragments.
The main mineral in limestone is calcium carbonate, CaCO 3 . Limestone and dolomite are technically carbonate rocks, though both are commonly referred to as limestone. The limestone and dolomite formations, which underlie most of the western portion of Ohio, were deposited about 400 to 450 million years ago. These formations are covered by a layer of glacial till, which is an unsorted mixture of clay, silt, sand, gravel and boulders, deposited by glacial processes that occurred approximately 10,000 to 20,000 years ago. Limestone formations usually are adequate sources of ground water because of their naturally formed solution channels, joints and fractures, which provide water storage capacity and pathways for water movement.