Sandstone Arches

Sandstone arches form as natural bridges due to a variety of environmental factors. Many times they form where cliffs are subject to erosion from the sea or other bodies of water. This erosion gets certain materials out, but researchers have recently discovered that the shape of sandstone arches is actually inherent to the rock itself. A great place to see these arches is at Arches National Park.
For more information see: How Arches Form