Geologic Formations

The Boone's Knob formation overlooking the Kentucky River.

Library of Congress

Sedimentary Rock in the Civil War

All rocks within Camp Nelson National Monument are limestone, which is a rock made up of the mineral calcium carbonate (CaCO₃) from the bodies of marine animals both microscopic and large. The calcium carbonate in limestone is a relatively soft mineral, and one that’s susceptible to dissolving when in contact with acids like rainwater. The dissolution of limestone overtime creates what’s called “karst topography” which includes rolling hills, sink holes, caves, and natural springs, all of which can be seen in the national monument today!

 

Illustration of karst topography formation process.

Trista L. Thornberry-Ehrlich, Colorado State University.

Carved By Water

These features form when an acid, such as rainwater, flows into the ground and through the natural cracks and crevices in the limestone. Over time, these cracks may grow into large holes or caverns, which if open toward the surface, may cause the soil above to fall in forming a sinkhole. When groundwater flowing along limestone encounters a hard barrier like clay, the water may exit the ground at the surface, forming a natural spring like the Officer’s Spring here at Camp Nelson National Monument.

It’s these limestone-centric features, formed from the skeletons of millions of years old marine organisms, that defined the topography deemed tactically advantageous for the creation of Civil War Camp Nelson.

 

A map of Camp Nelson National Monument, and a geologic map with the national monument’s boundary superimposed.

NPS, USGS & Kentucky Geological Survey

Limestone Layers of Camp Nelson

Inside the park there are four different depositional units representing four different marine environments that became lithified (hardened into rocks). From the bottom (oldest) to the topmost (youngest) rock unit, these environments depict a shift, from shallower to deeper marine environments in what’s called a marine transgression.

The lowest and oldest rock unit is named the Camp Nelson Limestone, after the very Civil War camp this national monument now preserves. The Camp Nelson Limestone is the oldest rock exposed anywhere in the state of Kentucky and represents a shallow marine environment full of trace fossils (fossils that show evidence of animal behavior like burrows and tracks/trails), and some body fossils of snails and nautiloids (shelled squids).

The Oregon Limestone, though it doesn’t have any fossils in Camp Nelson, contains evidence of the environment it used to be in the form of mud cracks and breccia (angular chunks of rock preserved inside of a larger rock). These features let geologists and paleontologists know that when the Oregon Limestone was deposited this area was a tidal flat, experiencing dryness at low tide, and flooding at high tide.

The Tyrone Limestone is the most diverse rock unit within Camp Nelson, and preserves a shallow marine environment full of corals, snails, nautiloids (shelled squids), crinoids (sea lilies), brachiopods (lamp shells), bivalves (oysters, muscles, clams, etc.), ostracods (two-shelled crustaceans), and bryozoans (moss animals). This rock unit’s fossil diversity is reminiscent of the biodiversity at today’s reefs, where animals may gather for safety and an abundance of food. Also in this rock unit are a few thin layers of bentonite clay, which is clay formed from volcanic ash. Not only did this clue geologists in to volcanic activity during the Late Ordovician, but it allowed them to get age dates for those ash layers, determining that these volcanic eruptions occurred around 435.35 million years ago!

The Lexington Limestone is the topmost and youngest rock unit in Camp Nelson National Monument. This unit is the most fossiliferous in that it has the most fossils per square inch, however it’s made up almost entirely of broken brachiopod shells. Limestones made up of mainly shells happen in shallow marine environments where waves tend to break up and collect shells forming a unique kind of limestone called a “coquina”.