Geologic History

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Geologic History
The Geologic History of Zion Canyon begins where the history of the Grand Canyon ends. The top layer of the Grand Canyon is the bottom layer of Zion National Park. The arid climate and sparse vegetation of the area allows the exposure of large portions of bare rock and reveal the park’s geologic history. Zion was a relatively flat region 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 layers that accumulated caused the basin to sink, so that the top surface always remained near sea level. As the land rose and fell 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 then occurred when mineral-laden waters filtered through the compacted sediments. Iron oxide, calcium carbonate, and silica acted as cementing agents, and with pressure over long period of time these deposits transformed into stone. Seabeds became limestone; mud and clay became mudstones and shale; and desert sand became sandstone.
In the area from Zion to the Rocky Mountains, forces from deep within the earth started to push the surface up. This was a very slow vertical hoisting of huge blocks of the earth’s crust. Zion’s elevation rose from sea level to as high as 10,000 feet above sea level.
This uplift gave the streams greater cutting force in their decent to the sea. Zion’s location on the western edge of this uplift caused the streams to fall off the plateau, flowing rapidly down a steep gradient. A fast moving stream carries more sediment and large boulders then a slow moving one. These streams began eroding and cutting into the rock layers forming deep and narrow canyons. Since t...