Geology of the County

In Montgomery 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 Montgomery County were deposited in shallow seas 490 million years ago during the Ordovician Period. 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 limestones found in Montgomery County were 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 Period were deposited. The warm climate of the Pennsylvanian allowed extensive forests to grow and great coastal swamps to develop 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 (Mn)
Lexington Limestone (Ol)

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

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

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

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

Clay shales
Crab Orchard Formation and Brassfield Dolomite (Scb)

Interbedded limestones and shales
Drakes Formation (Od, Odb)
Grant Lake Limestone/Fairview Formation/Calloway Creek Limestone (Oaf)
Clays Ferry Formation (Okc)

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

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