Geology of the County

In McCracken County, water is obtained from consolidated sedimentary rocks of Mississippian age and from unconsolidated sediments of Cretaceous, Tertiary, and Quaternary ages. The oldest geologic formation exposed on the surface in McCracken County is the Tuscaloosa Formation of Cretaceous age. During the latter part of the Cretaceous, 130 million years ago, the Gulf of Mexico inundated much of the southern United States and covered all the Jackson Purchase and some of the Mississippian Plateaus with sands, clays, and gravels. The Tertiary began 70 million years ago, and deposits consisted of marine and fresh- to brackish-water sediments. The distribution of deposits indicates that the area was near the northern limit of the Gulf Embayment (also called the Mississippi Embayment). Portions of the embayment must have been swampy, because thin beds of lignite (brown coal) and carbonaceous clays occur in the western half of the eight-county Jackson Purchase Region. These geological deposits are a marked contrast to the underlying older hard rocks, because most of the Cretaceous and younger sediments remain unconsolidated and soft. 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 and glacial outwash sediments (Qa)
Terrace gravel deposits and continental deposits (QTcl)
Tuscaloosa Formation (Kt)
Chert rubble
Limestone, chert, shale (Paleozoic bedrock)

Claiborne (Tjc) and Wilcox (Tw) Formations
Porters Creek Clay (Tp)
Clayton and McNairy Formations (TKcm)

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

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