Geology of the County

In Estill County, water is obtained from consolidated sedimentary rocks ranging in age from Ordovician to Pennsylvanian, and from unconsolidated sediments of Quaternary age. The oldest rocks found on the surface in Estill County were deposited in shallow seas 490 million years ago during the Ordovician Period. Above the Ordovician rocks, the Devonian New Albany Shale, 400 million years old, was formed 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. The Mississippian sandstones and siltstones are the result of a great influx of mud, silts, and sands brought in by rivers and streams from uplands many miles to the northeast and deposited as a great delta. The Mississippian limestone found in Estill County was deposited 350 million years ago in the bottom of a warm, shallow sea. At the end of the Mississippian Period, 320 million years ago, the seas receded and sediments of the Pennsylvanian Period were deposited. The warm climate of the Pennsylvanian allowed extensive forests and great coastal swamps to form at the edges of water bodies. Marine waters advanced and receded many times, which produced many layers of sandstone, shale, and coal. Vegetation of all sorts fell into the water and was buried under blankets of sediments, which over long geologic time were compressed into coal. The nonvegetative sediments such as sand, clay, and silt were compressed into sandstone and shale. 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)

Slade Formation (in southeastern corner) (Mpn)

Grundy Formation (contains Lee-type sandstone of the former Lee Formation) (Plc)

Interbedded clay shales, siltstones, and sandstones
Borden Formation (MDbb)

Coals, sandstones, and shales
Breathitt Group (Pikeville Formation) (Pbl)

Fractured shales
Boyle Dolomite (MDnb)
New Albany Shale (MDnb)

Clay shales
Crab Orchard Formation and Brassfield Dolomite (Scb)

Interbedded limestones and shales
Drakes Formation (Od)

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

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