Even if you have not seen it, you have probably heard of Rainbow Bridge in Utah. This is as close as we got back in 2004 when we hiked the short trail in from Lake Powell. Judging the size is difficult, but if someone were standing on top, you would see little more than a tiny speck.
Big South Fork National River and Recreation Area, Tennessee
Take a moment to examine this second picture. Although this natural rock arch is also large, it can be tricky to pick out in the middle of a forest amidst all the leaves and tree trunks. But it is there, and it is big. Not as big as Rainbow Bridge in Utah, but still an impressive piece of rock.
The Twin Arches in north-central Tennessee are part of the Cumberland Plateau formation, made up of layers of sedimentary rock deposited at the bottom of a large inland sea that covered part of eastern North America a few hundred million years ago. These rock layers are mostly sandstone, relatively soft and easily eroded away.
In some places, the sandstone is covered by a layer of conglomerate, a sedimentary rock made up of a conglomeration of pebbles that first accumulated along the bottom of rivers and streams that developed when the inland sea dried up. The conglomerate is much more resistant to erosion than the underlying sandstone.
Thus the formation of large rock overhangs, abundant in the area, as well as quite a few arches. The conglomerate on top resists erosion, while the sandstone around it readily washes away. If the flowing water washes underneath the conglomerate, it can erode the underlying sandstone and create a natural rock arch as seen in the picture.
Not only is it difficult to see these arches in the middle of a forest, and difficult to take good photographs of them, their location also made for a humorous moment on our recent hike through the area.
At one point one of our group was walking near the arch pictured above and through the trees saw what appeared at first glance to be a large splash of blue paint on the side of the rock face. Connie was incredulous that someone would deface such a unique natural landmark by painting a large piece of rock blue. Maybe she was thinking about the Carolina Tar Heels and their off year in college basketball this past season. In any case, she immediately turned around and made her way back to the rest of us and insisted that we go see what she could barely believe had happened.
We followed her path, and for a brief moment saw the splash of blue she mentioned, but a few steps farther along the path noticed a large patch of blue sky visible through the arch. Her splash of blue paint was a beautiful blue patch of sky.