The Formation of Zion
More than 250 million years ago, the stunning geologic features of Zion were formed. Once the area was covered by a low body of water; eventually huge rivers carved their way through the landscape. Later it was left one of the largest deserts on the earth. This desert’s sand dunes became what are now the breathtaking 2,000 foot cliffs of Zion National Park. The park now houses what are some of the most scenic canyon views in the country. In just a 229 square mile radius there resides enormous pine and juniper covered plateaus, narrow sandstone canyons, the windy Virgin River, and many seeps, springs, and waterfalls.
The Colorado Plateau
Zion National Park is located along the edge of the Colorado Plateau, a large, uplifted region which extends from Central Utah to Northern Arizona, and including part of Colorado and New Mexico. Over a period of millions of years, rock layers in this region were uplifted, tilted, and eroded, exposing a series of colorful cliffs called the Grand Staircase. This “staircase” presents a spectacular record of Earth’s history from nearly 2 billion years ago up to the most recent geologic period. The rock layers of Grand Canyon, Zion and Bryce Canyons record the sequential geologic events of this region with remarkable clarity.
Zion was a relatively flat basin near sea level 240 million years ago. Nearby mountains eroded sand, gravel, and mud, and streams carried these materials into the basin, where they were deposited in layers. The weight of these layers caused the basin to sink, and the top surface remained near sea level. As the land rose and fell with climate change, the environment fluctuated from coastal plains to shallow seas to a desert of windblown sand. This process, called sedimentation, continued until over 10,000 feet of material accumulated.
The mineral-filled waters slowly filtered through the compacted sediment layers. Working as cementing agents, iron oxide, calcium carbonate, and silica transformed layers into stone over extensive periods of time. Ancient seabeds turned to limestone, mud and clay became mudstones and shale, and desert sand transformed into sandstone. As each layer originated from a distinct source, so each is now different in thickness, color, mineral content, and overall appearance.
Slowly, forces deep within the earth pushed the surface up in a process called uplift. This was a vertical push that forced huge blocks of the crust upward. Thus, Zion’s elevation rose from near sea level to as high as 10,000 feet above sea level. This uplift is still occurring: in 1992 a 5.8 magnitude earthquake caused a landslide visible just outside the south entrance of the park in Springdale.
Uplift gave streams greater cutting force as they paved their way to the sea. The location of Zion along the western edge of the uplift caused the streams to tumble rapidly off of the plateau. As they cut into the rock layers, they carried sediment and large boulders with them, 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 carving its path. A landslide once dammed the Virgin River, forming a lake. As sediment settled to the bottom of the still waters, the river breached and the lake drained. What was left was a flat-bottomed valley. This change can be witnessed from the scenic drive south of the Zion Lodge near the Sentinel Slide. The slide was again active in 1995, damaging the road. Flash floods have also played a key role in forming the park. These occur when sudden thunderstorms dump water on exposed rock, and with little soil to absorb the moisture, water runs downhill, gathering volume as it moves. These often spontaneous floods 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.