How were arches formed? Over time, parallel fractures in surface rock eroded to form "fins." As the process of erosion continued, arches were created where the fins were worn completely through.

Arch Formation

The red rocks of Arches National Park were deposited in layers over millions of years. Later, erosion carved them into the features seen today. Water, ice, and very hot and cold temperatures cause erosion.

Seas covered this region 300 million years ago. When they dried up they left behind thousands of feet of salt. Later, layers of sand and other sediments covered the salt. When buried, the sediments were compressed into sedimentary rock. The buried salt moved under the pressure of the overlying sediment like toothpaste in a tube. As the salt moved, it ran into underground faults or barriers that caused the salt to go upward. Near the earth’s surface, the salt formed what looked like long giant bubbles in the upper layers.

The overlying rocks did not bend as easily as the salt did, and so they cracked in parallel fractures. Over time, water ran into these cracks and dissolved much of the salt below. The rock on top soon had nothing to hold it up. It collapsed on itself, similar to a badly-baked loaf of bread.

Water carried away sand grains one by one and widened the cracks into narrow canyons. Today, the thin rock walls between these canyons are called fi ns, because they look something like a fi sh’s fi ns. In cooler months, water in the form of rain and snow enters tiny cracks in the fins, freezes and expands, and causes little pieces of sandstone to flake off. After a long time, the tiny cracks break all the way through the fin and an arch is formed. Arches are still forming today, and many ages and sizes of arches are found throughout the park.

In 1973 it was decided that in order for a hole in the rock to be considered an arch it had to have an opening of at least three feet. Even if the opening is one inch high, as long as it is three feet long, it counts as an arch. While you are visiting the park, keep your eyes open because if you find a new arch, you get to name it. Many arches have been named for their shapes such as Pork Chop arch and Piano Leg arch. There are over 2000 documented arches in the park and probably some more to be found!

(Information courtesy of the National Park Service)