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Zion
National Park is a showcase of geology. Geologic
processes have played an important role in shaping
Zion. The arid climate and sparse vegetation
allow the exposure of large expanses of bare
rock and reveal the park’s geologic history.
Zion is located along the
edge of a region called the Colorado Plateau.
The rock layers have been uplifted, tilted,
and eroded, forming a feature called the Grand
Staircase, a series of colorful cliffs stretching
between Bryce Canyon and the Grand Canyon. The
bottom layer of rock at Bryce Canyon is the
top layer at Zion, and the bottom layer at Zion
is the top layer at the Grand Canyon.
Sedimentation
Zion was a relatively flat
basin near sea level 240 million years ago.
As sands, gravels, and muds eroded from surrounding
mountains, streams carried these materials into
the basin and deposited them in layers. The
sheer weight of these accumulated layers caused
the basin to sink, so that the top surface always
remained near sea level. As the land rose and
fell and as the climate changed, the depositional
environment fluctuated from shallow seas to
coastal plains to a desert of massive windblown
sand. This process of sedimentation continued
until over 10,000 feet of material accumulated.
Lithification
Mineral-laden waters slowly
filtered through the compacted sediments. Iron
oxide, calcium carbonate, and silica acted as
cementing agents, and with pressure from overlying
layers over long periods of time, transformed
the deposits into stone. Ancient seabeds became
limestone; mud and clay became mudstones and
shale; and desert sand became sandstone. Each
layer originated from a distinct source and
so differs in thickness, mineral content, color,
and eroded appearance.
Uplift
In an area from Zion to the Rocky Mountains,
forces deep within the earth started to push
the surface up. This was not chaotic uplift,
but very slow vertical hoisting of huge blocks
of the crust. Zion’s elevation rose from
near sea level to as high as 10,000 feet above
sea level.
•Uplift
is still occurring. In 1992 a magnitude 5.8
earthquake caused a landslide visible just outside
the south entrance of the park in Springdale.
Erosion
This uplift gave the streams greater cutting
force in their descent to the sea. Zion’s
location on the western edge of this uplift
caused the streams to tumble off the plateau,
flowing rapidly down a steep gradient. A fast-moving
stream carries more sediment and larger boulders
than a slow-moving river. These streams began
eroding and cutting into the rock layers, forming
deep and narrow canyons. Since the uplift began,
the North Fork of the Virgin River has carried
away several thousand feet of rock that once
lay above the highest layers visible today.
•The
Virgin River is still excavating. Upstream from
the Temple of Sinawava the river cuts through
Navajo Sandstone, creating a slot canyon. At
the Temple, the river has reached the softer
Kayenta Formation below. Water erodes the shale,
undermining the overlaying sandstone and causing
it to collapse, widening the canyon.
Geology
in Action
A landslide once dammed the Virgin River forming
a lake. Sediments settled out of the quiet waters,
covering the lake bottom. When the river breached
the dam and the lake drained, it left behind
a flat-bottomed valley. This change in the character
of the canyon can be seen from the scenic drive
south of the Zion Lodge near the Sentinel Slide.
This slide was active again in 1995, damaging
the road.
•Flash
floods occur when sudden thunderstorms dump
water on exposed rock. With little soil to absorb
the rain, water runs downhill, gathering volume
as it goes. These floods often occur without
warning and can increase water flow by over
100 times. In 1998 a flash flood increased the
volume of the Virgin River from 200 cubic feet
per second to 4,500 cubic feet per second, again
damaging the scenic drive at the Sentinel Slide
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