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. | 
