FORMATION AND CLASSIFICATION OF NATURAL ARCHES IN THE RED RIVER GORGE GEOLOGICAL AREA, EAST-CENTRAL KENTUCKY - PRELIMINARY INVESTIGATIONS

There are well over 400 documented natural arch locations in the Red River Gorge Geological Area in east-central Kentucky, and it is considered to have the most abundant cluster of natural arches east of the Mississippi River. Natural arches form by a combination of physical and chemical weathering on a rock mass to selectively remove rock. Erosive agents and processes combine the action of water, gravity, temperature change, and tectonic pressures on a rock. Wind is not a primary agent in the formation of natural arches, but serves as a modifying factor. An important factor in the formation of natural rock openings is the presence of pathways for water movement. Pore openings in a rock can increase the rock’s porosity, or water may also move along joints or other discontinuities in a rock mass. Rock types, composition, and mechanical properties also play a role in the formation of natural arches.

The Natural Arch and Bridge Society has created a classification scheme based on observable attributes such as context (surroundings in which the arch occurs), morphology (shape of various parts of the arch), metrics (size of various parts of an arch), geology (type of rock or formation in which the arch occurs), and anthropomorphic factors (actual or perceived relationships between arches and humans). The most common types of natural rock openings in the Red River Gorge are (alphabetically) alcove arches, buttress arches, cave arches, pillar arches, shelter arches, and waterfall bridges.

Natural arches in the Red River Gorge form on top of and at the base of narrow, joint-bounded, sandstone-capped ridges in the cliff-forming Pennsylvanian Corbin Sandstone. Natural arches also occur in the Mississippian Newman Limestone, which forms discontinuous cliffs along the flanks of ridges and also underlies the valleys. Certain types of natural arches also occur along different parts of the landform and the landscape. Generally, shelter arches form on ridgetops, alcove and buttress arches form along the sides of cliffs, pillar arches form along the base of cliffs, whereas bridges span drainages and cave arches usually form in limestone.

Future work will entail refining the classification of ridgetop arches and collecting samples for OSL dating to determine possible age for arch formation and landscape evolution.