Geology of the County

In Letcher County, water is obtained from consolidated sedimentary rocks ranging in age from Devonian and Mississippian to Pennsylvanian, and from unconsolidated sediments of Quaternary age. The oldest rocks in Letcher County, the Devonian black shales, are found along Pine Mountain. The black 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) which 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 was deposited 350 million years ago in the bottom of a warm, shallow sea. At the end of the Mississippian, 320 million years ago, the seas receded and sediments of the Pennsylvanian were deposited. The warm climate of the Pennsylvanian grew extensive forests and great coastal swamps 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 non-vegetative sediments such as sand, clay and silt were compressed into sandstone and shale. Over the last one million years unconsolidated Quaternary sediments have been deposited along the larger streams and rivers.

Geologic Formations in the County
Unconsolidated deposits

GRUNDY, ALVY CREEK FORMATIONS(contains Lee type quartzose sandstones of the
former Lee Formation) (PMl)

Coals, sandstones, and shales
BREATHITT GROUP (Pbu, Pbm, Pbl) (Princess Formation, Four Corners Formation,
Hyden Formation, Pikeville Formation)

Fractured shales

Interbedded limestones, sandstones, 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.

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