Parking is located just inside the Whittleton Campground for Henson Cave Arch. There is no access fee and there is a small parking area just past the entry gatehouse on your right. From the parking lot, you will head towards the bathrooms in the circle drive and will notice a bridge that is at the beginning of the trailhead. Waypoints are provided for both the parking and the trailhead. It is about .4 miles each way.
The Red River Gorge Geological Area is famous for all the sandstone arches and formations. But today you will learn about a less common bedrock arch. This arch is actually made of limestone, rather than the expected sandstone.
So why is an arch located here and why is it so different?
The bedrock that you see at this location is an upper Mississippian limestone in the Slade formation. It was previously called Newman Limestone. Mississippian Newman Limestone of southeastern KY is divided into informal lower and upper lithologic members. Lower member is composed principally of limestone with lesser dolomite and shale, and minor amounts of siltstone and sandstone. Upper member consists of shale with varied amounts of interbedded limestone, dolomite, siltstone, and sandstone. Most of the arches that you see in the RRG area are made through the weathering process of the more resistant sandstone. However at this location, slightly acidic water have created the arch, pit, and nearby cave.
So is this really an arch? When you descend the stairs and look at the formation, it does look like an arch. From the top, the trail actually walks over the top of the arch. However, this formation was likely formed by a collapsed ceiling to the cave or karst feature. As the limestone eroded from below, the weight of the ceiling bedrock caused it rock to collapse to the bottom of this pit. You can still see the loose rock at the bottom.
Water continues to erode this area. From the top, you can see the water fall over the edge of the bedrock and fall to the bottom. However, the water isn't pooling at the bottom. The water is actually leaving the pit through a hidden underground passage. The passage must be substantial to drain the pit instead of allowing the water to collect.
Limestone can be dolomitic or calcitic. Calcitic is a calcium carbonate mineral whereas dolomitic is made of calcium magnesium carbonate. Dolomitic will not typically have fossils since the process of forming the rock tends to eliminate the fossil material.
Logging tasks:
1. Do you think this is actually an arch? Explain why or why not.
2. Explain in your own words how this geologic feature formed.
3. Do you think the limestone is dolomitic or calcitic? Explain why.
4. Post a picture of yourself (face not required) or a personal item at the site.
Sources: personal knowledge, KY Geological Survey, and USGS.