forming

   Arches National Park is sitting on top of a salt bed named the Paradox formation. It is responsible for the formations at Arches (pedestals, arches, domes, spires, cavities, anticlines, pinnacles, balanced rocks, fins). The salt bed was covered with residue from the sea that flowed into the Colorado Plateau 300 million years ago (the Pennsylvanian period, Paleozoic era, Phanerozoic eon). When the sea evaporated and the salt bed was covered with these residues, all that was left was the debris compressed to form rocks. In some places, this was one mile thick. The salt bed is very unstable under the pressure of the sandstone. This made the rock shift and created domes. The dome sections dropped into cavities. The Moab Fault is the result of a 2,500 ft displacement and it was a normal fault. Water washed away the "cement" that was holding the sandstone together creating solitary fins. The wind washed away the left over debris. In the winter, water would freeze and expand in the cracks creating a flaking of the rock. With the pressure of the cold, the fins broke off and the weak and damaged fins collapsed. The strong survived. This process took 125 million years. Faults have caused the displacement of the Moab Fault and the Salt Valley. Most of the rocks are salmon-colored Entrada Sandstone, which took 20 million years to create, 180 million years ago. The arches form mostly of this and Navajo Sandstone. Folds and warps in the layers show the movement of the salt bed and deposits.
    Erosion is the big creator of arches: water, extreme cold and hot temperatures, and underground movement.
     Natural bridges are different from arches because the water from the stream that they sit over, has worn them away. Pothole arches are formed when water collects and eventually cuts through the rock below. They help organisms to survive long periods of time when they go through dehydration. These pothole arches can be hurt be human impact.
    In the late Jurassic period, 135 million years ago, most salt movement had stopped. Sixty to ten million years ago, the erosion began to take place. In the last 10 millions years, more than 5,000 feet of vertical rock have been eroded. The oldest rocks are from the Pennsylvanian Period. Check out the maps part of this site to learn more.
    "This is the geologic story of Arches- probably. The evidence is largely circumstantial" (www.arches.national_park.com).