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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 layer was deposited across the Colorado
Plateau some 300 million years ago when a sea
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.
The
Formation 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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