Sandstone arches are found in abundance in the Red River Gorge.
Primarily a sandstone formation, the Red River Gorge is in the eastern part of Kentucky and part of the Daniel Boone National Forest Reserve. The gorge contains about 150 natural sandstone arches, the largest number east of the Mississippi river, sculpted over a million years by wind and water. The natural rock caves and shelters were in use by the paleoindians as early as 13,000 years ago when the continental ice sheets still dominated the northern half of the continent.
Geology of Red River Gorge
The initial layers of rock were laid down as part of an inland sea and then a huge delta, as the Appalachian mountains were worn down by time. This initial sedimentary layer had several components, but the crucial one for the Red River Gorge area was the very hard layer of conglomerate sandstone. The formation resisted erosion and channeled the streams coming from the Appalachians and from the several continental ice sheets that built and then retreated.
Conglomerate Sandstones
The sandstones are composed of a variety of sizes of pebbles, but almost all of them are silicate rocks, basically quartzite and quartz-based. Once fused into a rock, the sedimentary layers were subsequently uplifted by the intrusion of a major magma flow, which underlies much of Kentucky. The uplift set the stage for the primarily water erosion of the sandstones into the numerous bridges and arches found today.
Limestone and Shale
The most important secondary rock laid down in layers with the sandstone is limestone. These layers are generally beneath the conglomerates and since limestone is both worn by erosion more easily and subject to chemical erosion by carbonic acid it eroded much more quickly. In many places this left either rock shelters or cave systems. As is common in limestone there also are deposits of chert. Several locations exist where layers of shale are exposed, deposited when the area was part of the inland sea and again eroding more quickly than the overlying sandstone.
Erosion and the Natural Bridge
The formation of arches, and the Natural Bridge in particular, depends on the initial fracturing of the formation during uplift and then weathering and erosion over a long time period. Parallel fractures in the rock are eroded by the combination of freeze-thaw fracturing and water erosion. With a creek on either side of the formation eventually two rock shelters are worn into the underlying strata from each side and they finally join leaving the arch above.
Tags: River Gorge, arches found, continental sheets, laid down, more quickly, Natural Bridge
