Geologic Tour of Mantle Rock Nature Preserve

Introduction

Mantle Rock Nature Preserve is located off Ky. 133 in Livingston County, Kentucky. The property, owned by The Nature Conservancy since 1988, is a preserve to protect the area’s rare and fragile sandstone glade. The main geologic attraction is Mantle Rock natural arch, which, at a length of 165 feet, has the longest span of any arch in Kentucky. The arch is located on the 2.8-mile Mantle Rock Trail, which includes part of the  Trail of Tears . Other geologic features along the trail include soils above bedrock, erosional features such as honeycomb and differential weathering of the sandstone, depositional features such as crossbedding, and structural features such as joints that are related to faulting in the area. 

Bedrock Geology

A geologic map shows the bedrock geology of the area. Different colors represent different rock units mapped at the surface (Amos, 1966; Martin, 2002). On the geologic map for Mantle Rock Nature Preserve, the red-dashed line represents the Trail of Tears and the blue-dashed line represents Mantle Rock Trail. Mantle Rock is an alcove type of natural arch; it formed in the Pounds Sandstone Member of the Caseyville Formation through weathering and erosion. The sandstone is of Early Pennsylvanian age and preserved in a downfaulted block of the Earth’s crust. Faulting and related fracturing have influenced the local geology and the formation of the arch.

Geologic Setting

Mantle Rock is formed in the Early Pennsylvanian (320 million years old) Pounds Sandstone Member. The Pounds is composed of sandstone and shale deposited as sand and mud by ancient rivers. Crossbedding in the sandstone indicates the river flowed to the south and southwest.

Faulting in the area has occurred at different times throughout Earth history, resulting in offset rock layers. Faulting during the Middle Permian (270 – 280 million years ago) was associated with the movement of fluorine-rich hydrothermal fluids. Those fluids deposited minerals, including fluorspar (also called fluorite), along faults and fractures throughout the area. In fact, this part of Kentucky is referred to as the Western Kentucky Fluorspar District and is known worldwide for its beautiful fluorite crystals. Fluorite and fluorspar are the same mineral, but the mining industry uses the word ‘fluorspar’.

The area has been exposed to surface erosion and weathering since the Permian Period. During the Pleistocene Epoch (2.6 million years ago to 11,700 years ago), areas north of the Ohio River in Illinois were covered by glaciers several times. The Ohio River was originally formed from the meltwaters of one of these glaciers. Windblown silt, called loess, from glaciers in Illinois and regions farther north was deposited in upland areas in Kentucky along the Ohio River Valley. The loess is approximately 35 feet thick in these upland areas and over 10 feet thick in places along the Trail of Tears portion of the Mantle Rock trail system.

Fractures

There are two types of fractures that separate Earth’s crust: joints and faults. Joints are fracture planes with no movement on either side of the fracture. Faults are fracture planes that move (up, down, or sideways) on either side of the fracture. The most noticeable fractures along Mantle Rock Trail are joints.

On this geologic map, faults are shown as black lines, and they commonly separate map units. Joints are shown by a straight red line indicating the direction of the joint. The red “square” in the middle of the line is the map symbol for a vertical joint, and the symbol with a “half square” indicates the direction the joint is dipping. The yellow-highlighted, gray-dashed line represents the boundary of Mantle Rock Nature Preserve.

Faults occur in the Mantle Rock area but are not well exposed at the surface. Mantle Rock lies within a downthrown fault block called a graben. The southeast bounding fault, which trends approximately 40° northeast–southwest, of this large-scale graben is less than half a mile to the southeast of Mantle Rock. The stresses responsible for the faulting of the rocks in this area are also responsible for the formation of the joint sets. 

Soils and Sediment

Most of the bedrock in the park and surrounding area is covered by soil, except along cliff faces. Soils consist of weathered bedrock grains, organic material, and secondary minerals formed through weathering and soil formation processes. Soils are defined in different ways. Soil types are commonly based on color, grain size, and soil processes (which are partly a function of climate).

Sandy soils are common where vegetation and climate break down sandstone into a layer of loose sand. Sandy soils tend to drain rapidly, so they are relatively dryer than other soils in the area are, and commonly support different types of plants than other soils. In the preserve, glades (open areas in the forest) with sandy soils occur on bluffs, which support drought-tolerant (xeric) plants such as the prickly pear cactus. In some areas, a layer of windblown silt from nearby loess deposits covers deeper sandy soils with a thin layer of silty soil, creating different conditions that support different plant cover.

Soils can be mapped along with other surface sediments based on the relative transport of material on a slope. Soil developed in flat, upland areas that hasn’t been transported is called residuum or residual soil (indicated on a geologic map with the symbol Qr). Soil and individual rocks along hillslopes that have been transported downslope by gravity and water are called colluvium (Qc). And the rocks in a stream channel and the soil along the banks of the stream are called alluvium (Qal).

Trail Guide

We selected several points of geologic interest along Mantle Rock Trail. The trail is 2.8 miles long and is rated as moderate in difficulty. The trailhead from the parking lot leads to the original Trail of Tears. Mantle Rock Trail breaks from the Trail of Tears, then makes a loop around the ridge and reconnects with the Trail of Tears along McGilligan Creek. Cell-phone service in the area is limited, so refer to this information before your trip. Please follow the posted visitation rules: Stay on the trails, no camping, and please dispose of your trash responsibly. Enjoy your hike!

We hope you enjoyed this geologic tour of Mantle Rock Nature Preserve. If you want to learn more about the geology of the area, please visit the  KGS website  and the  KGS geologic map service . 

Acknowledgements

This geologic tour was created in cooperation with Riverlands Alliance, a nonprofit agency that promotes and protects public lands in western Tennessee and western Kentucky. This project would not have been possible without the assistance of Daniel Kahl (associate director, Community and Economic Development Initiative of Kentucky), William (Drew) Andrews (head of the KGS Geologic Mapping Section), and Doug Curl (head of the KGS Geologic Information Management Section). A special thank you goes to Bill Patrick for his photograph of Mantle Rock.

References Cited

Amos, D.H., 1966, Geologic map of the Golconda quadrangle, Kentucky-Illinois, and part of the Brownfield quadrangle, Kentucky: U.S. Geological Survey Geologic Quadrangle Map GQ-546, scale 1:24,000.

Kind, T.C., and Shelby, L., 1985, Mantle Rock and alcove arch, Livingston County, Kentucky: Transactions of the Kentucky Academy of Science, 46, p. 22-27.

Martin, S.L., 2002, Spatial database of the Golconda quadrangle, Kentucky-Illinois, and part of the Brownfield quadrangle, Kentucky: Kentucky Geological Survey, ser. 12, Digitally Vectorized Geologic Quadrangle Data DVGQ-546. Adapted from Amos, D.H., 1966, Geologic map of the Golconda quadrangle, Kentucky-Illinois, and part of the Brownfield quadrangle, Kentucky: U.S. Geological Survey Geologic Quadrangle Map GQ-546, scale 1:24,000.