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Looking southeast at Behind The Rocks, a series of parallel sandstone walls called 'fins'.
The fins exist today because a certain sequence of events occurred in the geologic past. Behind the Rocks lies on the edge of a salt anticline - a result of underground salt flowage. Over millions of years, salt deposits were covered by thousands of feet of 'sedimentary' rock. The entire Colorado Plateau began to uplift about 10 million years ago. Because of the uplift, the rocks were subject to massive erosion. As erosion occurred, it uncovered the old salt anticlines. These salt deposits were very easily eroded and carried away, creating the fissures you see between the fins. The fins themselves are composed of harder sedimentary rock, and remain standing today.
Whenever salt anticlines occur in the Colorado Plateau, similar processes have created fins. Fins can also be seen in the Arches National Monument, and the Needles section of Canyonlands National Park. Because the fins are usually very narrow, they are often the targets of internal erosion; this creates the arches we enjoy today.