|Geology of the County
In Washington County, water is obtained from consolidated sedimentary rocks of Ordovician, Silurian, and Devonian age, and from unconsolidated sediments of Quaternary age. The oldest rocks found on the surface in Washington County, from the Lexington Limestone, 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 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 suggest that muddy conditions prevailed at times. Locally, numerous corals and brachiopods can be found in the Silurian limestones and dolomites. Above the Silurian rocks lies the New Albany Shale, also called the black shale. The New Albany Shale was formed 400 million years ago during the Devonian, when the deep sea floor became covered with an organic black muck. 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
Interbedded limestones and shales
For more information, see the definitions of geologic terms and rock descriptions, a geologic map of the county, a summary of the geology of Kentucky, and a discussion of fossils and prehistoric life in Kentucky.