| Geology of the County | ||
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Water in Allen County is obtained from consolidated sedimentary rocks of Ordovician, Silurian, Devonian, and Mississippian age, and from unconsolidated sediments of Quaternary age. Geologists call the oldest rocks found at the surface in Allen County the Cumberland Formation. This formation was deposited in warm seas during the Ordovician Period, 450 million years ago. Lying on top of the Ordovician rocks are Silurian rocks. Above them is the Devonian Chattanooga Shale, which is 400 million years old, and 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 away and deposited as a great delta. The Mississippian limestone found in Allen County was deposited 350 million years ago in the bottom of a warm, shallow sea. Over the last million years, the unconsolidated Quaternary sediments have been deposited along the larger streams and rivers. Geologic Formations in the County Limestones Fractured shales 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. Previous--Next--Back to "Groundwater Resources in Kentucky"
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