Zion National Park is a work long under construction. Geologists have traced 250 million years of history in the park's rock ramparts, and frozen in them is a fascinating fossil record.
Zion National Park encompasses some of the most scenic canyon country in the United States. Within its 229 square miles are high plateaus, a maze of narrow, deep, sandstone canyons eroded by the Virgin River and its tributaries. Zion also has 2,000-foot Navajo Sandstone cliffs, pine- and juniper-clad slopes, and seeps, springs, and waterfalls supporting lush and colorful hanging gardens.
The geologic formations of Zion-formed over approximately 250 million years- record periods of time when this area was covered by a shallow sea; when huge, sluggish rivers, bordered by swamplands, meandered across the landscape; and when a vast desert-perhaps the largest on the planet-covered the region. The sand dunes of this desert are now Zion's famous sculpted and colorful 2,000 foot cliffs.
Most of the sedimentation occurred during the age of dinosaurs (Mesozoic), while the uplift began in the Cenozoic, long after the sedimentary formations were laid down.
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.
The Fossil Record
Though today the park is dominated by soaring sandstone walls, once upon a time this area was inundated by a salty, shallow sea where brachiopods, gastropods, bivalves, ammonoids, nautiloids, and crinoids lived along the silty bottom. Large reptiles lounged and foraged along inland drainages, park scientists tells us.
You can get an idea of this 240-million-year-old setting in Zion National Park’s museum collection. There the fossils and survey data represent a smorgasbord of paleo-environments, including shallow marine, coastal, desert sand dunes, rivers, and lakes. Ranging in age from Permian through Holocene, fossil plants, animals, and tracks are available to scientists, educators, and park managers as a window to past life.
Zion’s petrified wood collections record terrestrial plant evidence from the 220-million-year-old Chinle Formation's paleo-environment. Other Chinle fossils include bone fragments, fish and reptile teeth, coprolites (fossilized poop), plant material, and invertebrate burrows. Chinle Formation terrestrial-vertebrate body fossils include phytosaur and ornithischian (crocodile-like reptiles) remains.
Perhaps the collection’s most impressive fossil evidence are dinosaur track imprints and casts. Survey data for dozens of Moenave and Kayenta Formation fossils preserved in-situ are available, including Eubrontes and Grallator trackways, and other track types and swim tracks associated with theropod (“three-toed”) dinosaurs.
A journey through Zion’s fossil collection provides an avenue for further paleontological research and opportunities for interpretation and visitor education. If evolution had taken another path, we might be walking along the Par’usTrail today with a herd of thousand-pound Dilophosaurus carnivores thundering toward us. How about meeting in the museum instead?
