The Fins along Park Avenue.  Arches National Park lies atop an underground salt bed called the “Paradox Formation” which is responsible for the arches, spires, balanced rocks, fins and eroded monoliths common throughout the park. Thousands of feet thick in places, the Paradox Formation was deposited over 300 million years ago when seas flowed into the region and eventually evaporated. Over millions of years, the salt bed was covered with the residue of floods and winds as the oceans returned and evaporated again and again. Much of this debris was cemented into rock. At one time this overlying layer of rock may have been more than a mile thick.