Arches
National Park
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.
Salt
under pressure is unstable, and the salt bed below Arches began to flow under
the weight of the overlying sandstones. This movement caused the surface rock to
buckle and shift, thrusting some sections upward into domes, dropping others
into surrounding cavities, and causing vertical cracks which would later
contribute to the development of arches.
As
the subsurface movement of salt shaped the surface, erosion stripped away the
younger rock layers. Water seeped into cracks and joints, washing away loose
debris and eroding the "cement" that held the sandstone together, leaving a
series of free-standing fins. During colder periods, ice formed, its expansion
putting pressure on the rock, breaking off bits and pieces, and sometimes
creating openings. Many damaged fins collapsed. Others, with the right degree of
hardness and balance, have survived as the world famous formations of Arches
National Park.
Faults deep in the Earth also contributed to the instability on the surface. The
result of one such 2,500-foot displacement is called the Moab Fault and is
visible from the Arches Visitor Center. Salt Valley was also formed by such a
displacement. Except for isolated remnants, the major rock formations visible in
the park today are the salmon-colored Entrada Sandstone, in which most of the
arches form, and the tan-colored Navajo Sandstone.
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