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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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