Red River Gorge Geology Field Trip Guide (RRGGFTG)

Stop 8: Sky Bridge: A ridge top arch of the Lee Formation

Geological Synopsis: Sky Bridge is a ridge top arch of the Corbin Sandstone Member of the Pennsylvanian Lee Formation. It is a great place to see cross-bedding, joint control of arch orientation, liesegang iron staining, honeycomb weathering, and ant lions. It is easily accessible and not far from the parking lot.
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Quick Contents (all on this one page)

Introduction - Overview of Sky Bridge geology
Geologic Facts - Ridge top arch, Lee Formation, Joints, Liesegang bands, Cross-Beds, Honey Comb Weathering, and Conglomerates
Location - Where is Sky Bridge
Maps - Topo, Geologic, and Location maps for Sky Bridge
Image Gallery - Photos of the various features at Sky Bridge
Geologic Summary - Landscape and Geologic History of Sky Bridge
References

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Introduction

Sky Bridge is a ridge top arch in the Red River Gorge Geological Area of the Daniel Boone National Forest in Eastern Kentucky. This area contains numerous ridge top arches and steep cliffs as can be seen from the top of Sky Bridge (fig 1).

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Geologic Facts

Sky Bridge is composed of Middle Pennsylvanian aged quartz arenite from the Corbin Sandstone Member of the Lee Formation (Rice, 1984; Dever and Barron 1986). The arch is bounded by joints that are visible from above and below the arch (fig 2,3). The sandstone is medium to coarse grained and weathers to a grayish orange. It contains numerous cross beds which dip to the south west (fig 4) and large liesegang bands (fig 5,6). There is also a conglomerate layer (fig 7) that contains liesegang bands and half inch sized quartz pebbles. The matrix is fine grained and dark gray to black in color. This layer is unconformable at its base and grades up into sandstone. Heavy weathering has led to the erosion of less resistant material and to the formation of honeycombed sandstone (fig 8) below the arch. There is also a rock shelter present on the trail (fig 9).

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Location

Sky Bridge is located in the Red River Gorge Geological Area in Wolfe County Kentucky. It is approximately 6.5 miles off Mountain Parkway. From Mountain Parkway take the S-15 exit and head west on S-15. After approximately 0.17 miles make a right and continue north on S-15 for 0.8 miles. Then make a right onto Sky Bridge Road and stay on this road until you reach the parking lot. Sky Bridge is approximately 400 feet from the end of the parking lot. The trail is about 1 mile long and goes over and below the arch. The coordinates for Sky Bridge are N37°49.130' (37.81883°) and W83°34.736' (-83.57893°).

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Maps


Topo Map	Geologic Map (soon)	Road Map

References
www.topozone.com
www.mapquest.com

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Image Gallery


Nearby landscape from top of Sky Bridge showing the steep sandstone cliffs of the Lee Formation.	Wall of cross beds below the arch that have a south west dip.	Joint on the top of Sky Bridge that forms the edge in the background and cuts through the arch in the foreground. Average strike is N77^oE.

View of Sky Bridge from below as well as the joint that forms the edge of the arch.	Liesegang bands on the top of Sky Bridge. The large patches of the bands are the remnants from erosion	Liesegang bands on a vertical cliff in the rock shelter.

Conglomeratic layer with small quartz pebbles and liesegang bands. The layer grades up into sandstone	Erosion of less resistant material has led to honeycombed weathering beneath the arch	Rock shelter with spectacular liesegang bands and a conglomeratic layer.

Images taken by Tim R. Nelson on 26 March 2004.

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Geological Summary

Deposition of the Corbin Sandstone Member of the Lee Formation occurred in the Middle Pennsylvanian. During this time, the Appalachian orogen was being uplifted leading to increased rates of erosion. To the north, erosion of Paleozoic sediments and metamorphic rocks produced large quantities of quartz sand and gravels that were transported southwest by rivers. These rivers were restricted to Eastern Kentucky due to the elevated Cincinnati Arch and Jessamine Dome in central Kentucky. The rivers deposited sediment in a delta plane, which extended across all of eastern Kentucky. As the volume of sediments increased, the streams became braided and spread out across the flood plains leading to the deposition of broad sheets of pebbly sand and gravels, which may be the source of the conglomerate layer at Sky Bridge (Rice and Weir, 1984).

Since compaction and lithification of the sediments, rivers have gouged deep valleys in the Pennsylvanian rocks. As further weathering occurred, less resistant layers underlying the Corbin Sandstone Member eroded leading to the formation of rock houses and shelters. As the underlying rocks eroded the ridge forming sandstone collapsed along joints. The erosion of Sky Bridge is controlled by near vertical joint sets that are visible form the base and top of the arch. As further erosion and mass wasting occurred the underlying material completely eroded from beneath the more resistant upper layers leading to the formation of the narrow sandstone arch, Sky Bridge (Dever and Barron, 1986). Further erosion will cause this arch to collapse in the future.

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References

Dever, G.R.J., and Barron, L.S., 1986, Red River Gorge Geological Area (Daniel Boone National Forest) and Natural Bridge State Park, east-central Kentucky: Geological Society of America Centennial Field Guide--Southeast Section, p. 43-46.

Rice, C.L, 1984, Sandstone Units of the Lee Formation and Related Strata in Eastern Kentucky: Geological Survey Professional Paper 1151-G, United States Government Printing Office: Washington, p53.

Red River Gorge Geology Field Trip Guide (URL: http://www.uky.edu/AS/Geology/webdogs/rrggftg )
This RRGGFTG contribution authored by: Tim R. Nelson and modified on: 12 April 2004
GLY 350 Historical Geology, Class Project Website Spring 2004
Department of Geological Sciences, University of Kentucky