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. Delaware County is underlain with four distinct aquifer systems. Buried valley aquifers, in southern Delaware County, consist of thin lenses of sand and gravel interbedded with thick layers of clay. The carbonate aquifer, which is composed of layers of limestone and dolomite, is the principal source of ground water in the western half of the county.
Limestone consists of fossilized sea shells, shell fragments and consolidated limy mud.
Its main mineral is calcium carbonate, CaCO 3 . Sandstone aquifers are found in the eastern third of the county. The poorest aquifer that may be encountered in the county is shale, which can be found in the central third of the county. As mentioned earlier, the movement of water through layers of shale is minimal. The exact yield will depend on the properties of the geologic formation at the specific location of the well.