From the National Park Service
Website:
Zion National Park is located along the
edge of a region known as 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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