Geology of the County

In Oldham County, water is obtained from consolidated sedimentary rocks of Ordovician, Silurian, and Devonian ages, and from unconsolidated sediments of Quaternary age. The oldest rocks found on the surface in Oldham County, the Drakes Formation, were deposited in shallow seas 490 million years ago during the Ordovician Period. In the Late Ordovician, the seas became relatively shallow, as indicated by the amounts of mud (shale) in the sediments. When the waters were clear and warm, a profusion of animal life developed, particularly brachiopods and bryozoa. Lying on top of the Ordovician rocks are the Silurian rocks, which were also deposited in warm seas, 430 million years ago. In Kentucky, the Silurian seas were commonly warm and clear, although the presence of some shale beds suggests that muddy conditions prevailed at times. Locally, numerous corals and brachiopods can be found in the Silurian limestones and dolomites. The Devonian New Albany Shale lies above the Silurian rocks. This shale, also called the black shale, was formed when the deep sea floor became covered with an organic black muck 400 million years ago. The muck is now hard black shale (an oil shale) and is one of the most distinctive of all geologic formations in Kentucky. Over the last million years, unconsolidated Quaternary sediments have been deposited along the larger streams and rivers.

Geologic Formations in the County
Unconsolidated deposits
Alluvium (Qa) and glacial sediments (Qg)

Devonian limestones (Sellersburg Limestone, Jeffersonville Limestone) (Dsj)

Interbedded limestones and shales
Louisville Limestone (Slw)
Waldron Shale (Slw)
Laurel Dolomite (Slb)
Osgood Formation (Slb)
Brassfield Formation (Slb)
Drakes Formation (Saluda Dolomite, Bardstown, Rowland Members) (Od) and Bull Fork Formation (Ob)

For more information, see the definitions of geologic terms and rock descriptions, a geologic map of Oldham County, a summary of the geology of Kentucky, and a discussion of fossils and prehistoric life in Kentucky.

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