Red River Gorge Virtual Field Trip

Red River Gorge Stop #3

Rock Bridge

Waterfall Arches

Rock Bridge is a classic example of a waterfall arch. This is one of very few natural bridges in the world that actually has water flowing under it, and this stream is what created the arch. The process which produces waterfall arches is very interesting. All waterfalls forms when a resistant rock, like sandstone, prevents a stream from eroding down. As the stream slowly erodes the resistant rock, the waterfall recedes upstream. It takes a special circumstance for an arch to be produced by this process.

If there is a weakness in the resistant rock, like cross-bedding or joints, the water will start to erode the weaker area faster than the surrounding rock. Eventually it will cut through the sandstone and start rapidly eroding weaker units, like shale, below the resistant bed. Soon there is enough room for the water to begin to flow under the piece of sandstone, which is now an arch. With time, the water will widen the stream bed, and continue to move the waterfall upstream. Occasionally pieces of rock will fall from the bottom of the arch, limiting its lifespan. How long do you think these arches stand? Also, how long do you think it takes for them to form? When will this one fall?

Rock Bridge 2

Build a Waterfall Arch!!