| Geology of the County | ||
|
In Marshall County, water is obtained from consolidated sedimentary rocks of Mississippian age, and from unconsolidated sediments of Cretaceous, Tertiary, and Quaternary ages. Geologists call the oldest rocks found at the surface in Marshall County the Fort Payne Formation. These Mississippian rocks were deposited 350 million years ago in the bottom of a warm, shallow sea. 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 of the Jackson Purchase and some of the Mississippian Plateaus with sands, clays, and gravels. The Tertiary Period began 70 million years ago, with deposits of marine and fresh- to brackish-water sediments. 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, Quaternary sediments have been deposited along the larger streams and rivers. Geologic Formations in the County Limestones Sandstones For more information, see the definitions of geologic terms and rock descriptions, a geologic map of Marshall 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"
|
||