All about Sandstone.
A quick glance at Zion reveals that the geologic story is about sandstone. The most obvious formation, the Navajo, is an orange to white sandstone that forms huge cliffs. Standing 2200 feet tall in Zion, the Navajo was formed by sand dunes about 180 million years ago (m.y.a.). The Navajo's tendency to form huge cliffs is largely responsible for Zion's stunning, high-wall scenery.
Zion sits on the southern end of the Colorado Plateau, an area stretching from central Utah and western Colorado down to northern New Mexico and Arizona. It is a vast landscape of desert sandstone with rock layers generally laying flat.
Let's start from the beginning, and look at the rocks as they were deposited.
The sedimentary rocks of Zion were laid down as loose sediment - sand, clay and silt - between 210 and 160 m.y.a., during the Jurassic period, when dinosaurs were the dominant species. After being deposited, the sediments were covered with more sediments and pushed down into the earth, where with pressure and elevated temperature, the loose sediments were pressed and baked into rock. During the last 80 million years, forces deep within the earth uplifted the Colorado Plateau, the overlying rocks and sediments were eroded off and canyons were formed.
While the geology is evident throughout the area, a good place to view the layers talked about here is from the bridge across Pine Creek, looking up at the East Temple.
Formations of Zion
Geologists talk in terms of rock FORMATIONS - a group of rock layers laid down in a specific order over a short period of time - and LAYERS or MEMBERS - a specific rock layer. Formations consist of several layers or members much like a paragraph consists of several sentences.
Moenave & Kayenta Formations
210 to 190 million years ago
In geology, the beginning is at the bottom, where the oldest rocks are. In Zion, the crumbly rocks leading up to the base of the Navajo are the Moenave and Kayenta layers. Both are deep red, look like piles of boulders or talus, and form steep slopes that are not quite cliffs. There are short cliff bands in each.
The Moenave Formation (400 to 570 feet thick) is at the bottom, and has two members. It starts with a crumbly siltstone - the Dinosaur Member - that was deposited in a rainy, equatorial environment by slow streams and ponds, like a Louisiana bayou. Above the Dinosaur is the Springdale Member - a solid, blocky sandstone that forms a 30m (100 foot) sheer cliff. These sands were left behind by a faster flowing river that left the sand, but swept the lighter clays and silts out to sea. The Springdale Member forms the charming gorge and waterfalls above the bridge in lower Pine Creek, in addition to the cliffband just above the town of Springdale.
The Kayenta Formation (500 to 700 feet thick) starts immediately above the Springdale Sandstone with more of the crumbly red siltstone that forms steep, loose slopes. Near the top, just below the Navajo, the Kayenta is a solid, red sandstone that forms long ledges and benches. The floor of the Subway and the red waterfalls below the Subway are Kayenta.
The Kayenta formed from silt and sand left behind by streams at the southern edge of a great desert. Rainy summers and dry winters left behind a mix of sediments.
190 to 180 million years ago
The Navajo Formation consists of one rock layer, Navajo Sandstone. While the Navajo is a dominant rock layer across the Colorado Plateau, it is thickest in Zion National Park - 2200 feet (700 m) thick. Navajo is a cliff-forming sandstone, but soft and easily cut by flowing water. These somewhat contradictory characteristics result in the formation of dramatic canyons.
A vast area of sand dunes that stretched, at one time, from central Wyoming to the southeastern point of California left behind the sand that became Navajo Sandstone. The climate was much the same as the current Sahara desert, and the sand accumulated in a slowly sinking basin. The sand blew back and forth, rounding the crystals and allowing impurities such as clay and silt to blow away. The rock shows traces of the dunes that formed it as CROSS-BEDDING - short horizontal layers of diagonally banded rock. The diagonal bands are layers left behind by the lee faces of the dunes. The cross-bedding is seen best on the upper layers of the Navajo beside the road near the East Entrance.
Sandstone is held together by impurities such as clay and lime that are deposited with the sand. Navajo lacks these impurities and thus is poorly held together. The rock is weak and often crumbly, but still forms huge cliffs. The rock near the top is especially pure, white and weak, partly because it was deposited as pure sand, and partly because rainwater, over tens of thousands of years, has dissolved what little binding material was available and carried it down through the rock to the lower layers.
Temple Cap and Carmel Formations
180 to 160 million years ago
At the top of the stack, above the Main Canyon and visible as flat caps on the higher peaks, the Temple Cap Formation was deposited atop the sand of the Navajo desert by streams carrying red mud, leaving a couple of feet of bright red clay and silt. Just above this, desert conditions resumed, though a little wetter, and 100 feet of orange sand was deposited, now forming towers and short mesas atop some of the peaks. Soon after, a shallow sea like the Mediterranean flooded the area, and a thin layer of very hard limestone was deposited. This is the lowest layer of the Carmel Formation.
More Rocks 160 to 20 million years ago
More rocks were deposited on top of this, but in the Zion area they have mostly been eroded off. Some of these rocks can be found on the east side of the park, and are responsible for the terrible sticky clays that make the North Fork Road impassable after rain.
In some places, the laying down of the rocks was not quite so orderly as the description above might imply. The contact between the Navajo and Kayenta was, in places, much the like the shuffling of cards. This is seen clearly in The Subway by the many spring layers apparent between the first obstacle and the Red Ledges. A 90-foot thick layer of Navajo Sandstone (called the Lambs Point Tongue) is visible halfway down the Kayenta in Pine Creek Canyon. This forms a vertical cliff that is rappelled when exiting Spry Canyon.
What would water do?
Knowing about rocks allows us to understand how the rock eroded to the dramatic scenery we find in Zion today. Let us imagine ourselves as a raindrop falling on the very top of the West Temple, to see how the various layers interact with the erosive power of water.
Falling on the flat limestone layer atop the West Temple, raindrops don't do much. The limestone is very resistant to weathering, and is responsible for the very flat tops of the Temples. Softer rock above broke up and was carried away - but the thin layer of limestone remains.
For the sake of our story, let us imagine the rain collecting and finding its way to the edge of the limestone and dripping onto the Temple Cap sandstone beneath. The Temple Cap sandstone is strong but porous, allowing the water to penetrate the surface. The Temple Cap is like the Navajo, in that it likes to form vertical and near-vertical cliffs, by shedding massive blocks rather than by wearing down grain by grain and forming lower-angled slopes.
Our imagined raindrop flows down the cliff and soaks through the rock until it finds the bottom of the sandstone, where it encounters a thin layer, the bottom of the Temple Cap, a non-porous layer of siltstone that stops downward progress. Again, the moisture runs slowly across the surface until if finds the edge. Undulations in the siltstone concentrate the flow of water like small streams in the porous rock above. These are seen all over the park where they reach the edge, producing springs and seeps at the top of the Navajo. The West Rim Spring just off the West Rim Trail is such a seep, but there are many other examples easily seen when climbing the West Rim Trail toward Behunin Pass.
Our raindrop has now arrived at the top of the Navajo sandstone, dripping onto the soft, white rock and quickly soaking in. Many hundreds of years are required for our little raindrop to find its way through 2000 feet (600m) of porous rock to somewhere near the bottom of the Navajo. Where the rock becomes less porous, our raindrop moves sideways to find a path to continue its downward journey, often finding a zone of weakness. Eventually, it reaches the non-porous Kayenta layer and moves directly sideways looking for a way out. Joined with other raindrops, the moisture follows weaknesses in the rock caused by previous faulting and natural variation, and finds a place to seep out of the canyon walls. These are the springs and seeps found throughout the park at the bottom of the Navajo, from Pine Creek to Weeping Rock to The Subway.
Below this, our raindrops become a swift-flowing stream of naturally-filtered, fresh, clean water.
Zion's canyons form along faults in the Navajo Sandstone that run north-south and east-west. Beginning 80 million years ago, the Colorado Plateau was uplifted and compressed when North America ran into the Pacific Ocean plate. Over 50 million years, east-west compression formed the Rocky Mountains and Sierra Nevada, and the Colorado Plateau was lifted 7000 to 10,000 feet. The Navajo Sandstone stayed mostly intact, but fractured or faulted in places in response to the compressive forces. These fractures run mostly north-south.
More recently, in the last 20 million years, the forces reversed and a great expansion has been taking place, as California is stretched away from Utah. The result is the Great Basin, which starts just west of Zion with the Hurricane Fault, running along I-15. This expansive stress added more fractures to the Navajo Sandstone, many running east-west.
When water falls onto the Navajo, it soaks into the rock and seeks the easiest path. The water seeps to the fracture zones, where the rock is slightly broken up. As more water flows through a fracture zone, the binding minerals are carried away and the rock becomes weaker. This first forms a shallow canyon that attracts more water at the surface, causing more erosion and a deeper canyon. In this way, canyons form along the lines of the N-S and E-W fault lines.
Much of Zion is characterized by HEADWARD EROSION. Pine Creek is a good example. The big canyon at the bottom is working its way upcanyon by eroding the rock away at the base of the Navajo. The canyon above concentrates the water, which seeps through the Navajo to the top of the Kayenta, where it runs horizontally to burst out of the canyon wall as a spring. The flow of water carries away the binding minerals, making the rock weak, and the rock erodes away at the base of the Navajo, forming spectacular alcoves. Eventually, the undercut rock breaks away in blocks, and the large canyon progresses, headward.
When did the canyons form?
The present canyon system started to take form about 15 million years ago, at first shallow then developing into the canyons we know and love today in the last 3 to 5 million years. Much of the carving was probably done during the wet period at the end of the last ice age, 100,000 to 2 million years ago.
Rock Slides and Hot Lava!
Due to the expansion starting 20 m.y.a., the crust became thin in places and hot lava was able to push its way to the surface. Rather than forming volcanoes, this type of lava flowed out of rifts and small cones. These can be seen along the Kolob Terrace Road leading to Lava Point. On at least 20 occasions, lava flowed from rifts and created flows a couple miles long and a couple hundred feet thick. These flows date from 1.5 million years ago to about 200,000 years ago. Lava flowed into the already-formed canyons of the Left and Right Forks, damming them, forming lakes. In the Left Fork, the flow came over the edge near the current trailhead, forming a dam that backed water up as far as the Subway. The lake filled with sediment and the stream overflowed the dam, cutting a new channel in the soft sediments. Sediments from the lake can still be seen at the top of the lava when hiking out of the Subway.
Massive landslides formed other lakes in Zion. The most famous is Sentinel Lake, formed by a giant slide 4000 years ago. This slide came off The Sentinel (at right), just north of the Streaked Wall, and dammed the canyon above Canyon Junction, backing water up past Angels Landing. The floor of Zion Canyon is wide and flat from Zion Lodge to the Temple of Sinawava - this is the lake bed of Sentinel Lake. The lake was at least 350 feet deep, and filled with sediment over about 400 years. Eventually the lake filled and the river was reborn, piercing the dam and cutting sharply through the sediments to form the V-shaped canyon between the Court of the Patriarchs and Canyon Junction. Continued slumping of lakebed sediments continues to this day. A slide in 1995 dammed the river, which overflowed the road. The river is insistent and soon swept the sediments away.
In Zion today, many of the flat-bottomed valleys (Hop Valley, South Fork Taylor Creek, Left Fork, Right Fork and the Main Canyon) are due to lakes formed by landslides and lava flows. Two slide-lakes currently exist in Zion - the Mystery Canyon seasonal lake, and Chasm Lake hidden in the backcountry between Kolob Terrace and LaVerkin Creek.
What makes Zion special?
Why is Zion different than other areas on the Colorado Plateau? Zion in located on the southern edge of the Markagunt Plateau - a section of the Colorado Plateau that extends from Springdale 60 miles north and east past Cedar Breaks. This particular chunk of rock has been uplifted and tilted in the last 10 million years. Rivers and streams were already established heading southward down the Plateau, and they had to cut deeply to keep up with the uplift. The highlands created by the uplift attract more rain and snow than the surrounding desert, a lot of which ends up draining through Zion. So, combine a rapid uplift, an established drainage system and plenty of water to provide cutting power - and we end up with a dramatic concentration of incredible, deep canyons: Zion National Park.
Images on this page courtesy Wikimedia Commons and the National Park Service.