The Tower Frequency: Geology
The Tower Frequency Podcast 1
The Geologic Story of Devils Tower.
- 13 minutes, 45 seconds
- Credit / Author:
- NPS/Piper Lewis
- Date created:
[start intro music fade in, then quiet]
Thank you for joining us on the Tower Frequency. My name is Piper Lewis, here at Devils Tower courtesy of GeoCorps and the Geological Society of America. Today we’re going to be gaining a deeper understanding of the Geology at Devils Tower National Monument.
Many people who come to America’s First National Monument are here to see the Tower. The Tower is a unique geologic structure, but it’s not the only important geologic feature of the park. Whether you’re here to be part of this powerful, spiritual place, to climb, or to get out in nature and enjoy the hiking trails, knowing a little more about geologic history of the park can enrich your visit. The Tower is actually one of the youngest geologic features in the park. The drive down from the tower is a trip back in time through the sedimentary layers. Today we’re going to start at the entrance to the park, all the way back in the Triassic.
One of the most noticeable geologic features at the park entrance, are the rust-red hills that line the Belle Fourche River Valley. Those are called Red beds. These red beds are the Spearfish Formation. The Spearfish Formation is the oldest visible geologic strata in the park, dating from the Triassic, about 225 million years ago. That’s older than the oldest dinosaurs! The Spearfish Formation is composed of sandstone and siltstone, with occasional thin layers of gypsum. Sandstone is composed larger particles like sand, whereas Siltstone is made of finer, smaller particles and includes clay. When these layers were forming the earth was united in one big supercontinent called Pangea. The Triassic began with a relatively wet and warm climate, though it grew hotter and dryer. The Triassic saw the first conifers joining ferns, ginko, and seed plants. There weren’t many dinosaurs in the Triassic but there were lots of reptiles, the ancestors of crocodiles and turtles. Geologists believe Devils Tower was a hot, dry, coastal climate in the Triassic, like the modern Persian Gulf. The gypsum layers indicate that the location the Spearfish formed had ephemeral lakes, or areas that were sometimes exposed to sea water, and sometimes dry. We know that because gypsum is an evaporite. This means it forms when saline water covers over an area, and then evaporates off, leaving gypsum salts behind. The most striking thing about the Spearfish Formation is that red color. The siltstones and sandstones are red because they’re high in iron oxide. This iron rich red color is found throughout the Triassic. The Triassic even got its name from red-beds. The name comes from ‘Trias’ a reference to the 3 distinct rock layers of red beds, marine limestone and a terrestrial mudstone/sandstone that were first identified from this period in Germany.
As we move closer to the tower, above the Spearfish Formation’s red beds, we find a smaller layer that’s not always visible from the road. This white band of sedimentary rock is the Gypsum Springs formation. Because gypsum is an evaporite it weathers really easily. This has caused sink holes in other parts of Wyoming. Since gypsum is laid down after an area has been covered in saline water, the Gypsum Springs formation and it’s thickness tells us that these rocks were deposited when the park was being flooded with sea water, then drying up, then flooded again. This fits nicely with our geologic time frame. The Gypsum Springs Formation is somewhere around 165 million years old. That’s about Mid Jurassic. Through the Late Triassic and Early Jurassic, that super continent, Pangea, was beginning to break apart. As the continents shifted, ocean water started covering what was once land. Eventually this leads to the Sundance Epicontiental Sea. During the Gypsum Springs time, this area of Wyoming was marine shoreline and tidal flats, as we can see from swim tracks of crocodiles and bipedal dinosaurs in the Gypsum Springs Formation in northern Wyoming. Scientists now believe this was a warm and relatively shallow sea. As time went on and the continents moved further apart, that sea grew deeper. The floor of that deeper sea is preserved in the Stockade Beaver Member which is part of the Sundance Formation, directly above Gypsum Springs.
The Sundance Formation also shares its name with that Epicontental Sundance Sea. The Sundance Sea was an arm of the Arctic Ocean and it covered this area of Wyoming in deep water. We can tell it was a deeper depositional environment because of the grainsize of the rock. The Stockade Beaver member is mostly a gray-green shale, meaning it’s mostly composed of clay and mud particles. These clay and mud particles can only settle out of a water column where there’s low wave action, imply a deeper depositional environment. Another cue to the depth of the water and the depositional environment are fossils. We find fossils preserved in the Stockade Beaver Member. At Devils Tower we mostly find evidence of belemnites, squid-like creatures that had partially internal calcified shells. They also had approximately ten limbs, ink sacks, hard beaks, and tail fins. Particularly well preserved belemnite specimens show they were likely powerful swimmers. In other locations where the Stockade Beaver Member and sediments from the Sundance Sea re exposed we find that this Western Interior Seaway was teaming with ammonites, oysters, fish, marine crocodiles, turtles, dolphin-like ichthyiosaurs, and megalneusaurus. Megalneusaurus-Rex translates to Great Swimming Lizard King. This massive predator would have been even larger than Liopluredon, the carnivorous marine reptile that dominated the seas covering Europe as an apex predator of the Mid to Late Jurassic. It’s theorized that megalneusaurus would have been up to 40 feet long and weight 20 tons, that’s only a little smaller than a semi-truck. Stomach contents of the megalneusaurus specimen found in Wyoming show it mostly ate belemnites.
The Stockade Beaver member is at the base of the Sundance Formation, but it’s hard to know precisely where it stops. The Sundance Sea slowly retreated, and as the water level fell, the area around Devils Tower slowly shifted from deep water to being a barrier island or a beach. This slow, gradient of shift is seen in the rock record as the Stockade Beaver gives way to the Hulett Sandstone.
The Hulett Sandstone was laid down in the Late Jurassic, about 145-150 million ago. This evenly sorted, yellow-white sandstone shows this was an area of high wave action. This former beach doesn’t weather easily and can be seen in the yellowed cliffs about 70 feet tall encircling the tower. Above the Hulett Sandstone there are a few other sedimentary deposits but these rocks are mostly obscured, covered by the talus field of boulders from the tower, invisible to visitors. The next prominent feature above the Sundance Formation is almost 90 million younger! That formation is the focal point of the park, Devils Tower.
The tower was formed about 60 million years ago during the Laramide Orogeny. That’s the mountain building event that created the Rockie Mountains. The Faralon plate was subducting off the West coast at a pretty shallow arc, and so there was lots of magma coming up to the surface, forming volcanoes. Devils Tower is made of an igneous rock called Phonolite Porphyry. Igneous means it formed from a magma or lava. The first part of the name, Phonolite, refers to the composition. This rock is mostly composed of feldspars. Another neat fact about Phonolite is that if you strike it hard with a hammer it will ring. The second part of its name Porphyry refers to the texture. There are 3 kinds of igneous textures: Aphanitic means you can’t see any crystals, it’s all one indistinguishable rock mass; Phaneritic means crystals are visible to the eye. A Porphyry is a combination of those two. There are large feldspar phenocrysts which you can see with the naked eye set into a background matrix which does not have distinguishable crystals.
There are several theories about how precisely Devils Tower formed:
The original proposition by Carpenter in 1888 was that Devils Tower was a volcanic plug, or the “neck” of an extinct volcano. That remained filled with magma following an eruption. The columnar jointing was formed as the magma cooled and contracted. Even though there is no evidence of volcanic activity in the surrounding area it is possible that material could have eroded away.
A seond theory was brought up in 1907 by Darton and Ohara who concluded that Devils Tower must have been a Laccolith, a large mushroom-shaped mass of intrusive igneous rocks that never reached the surface. This theory points to phonolite porphyry fragments found on and in stream terrace deposits as evidence, but the vent which is present in Laccoliths has never been found.
In 1956 Robinson proposed devils tower to be a stock a small intrusive body of magma, which cooled underground and later was exposed by erosion. Robinson pointed to the relatively small amount of debris, the mineral composition and texture, which is more typical of shallow igneous rocks, and the fact that no evidence of extrusive igneous activity has been found surrounding Devils Tower as support for his theory.
To this day, geologists still debate which of these theories is accurate. None of the theories completely explains the evidence found here at Devils Tower and all of the theories are lacking some lst aspect of proof. For today none of the theories are wrong, but also, none of them are completely right. To truly know the answer, more of the tower would have to be eroded so we could have a better understanding of what’s below the base.
We do know that the Tower cooled slowly beneath the surface, under pressure. As it cooled the rock contracted. When it contracted it cracked along weak points those cracks connected and the cooling mass began to form the columns we see today. Not all the columns are hexagonal, some have three sides, or seven sides, but many are a perfect six.
Today the Tower stands proudly above the landscape due to erosion. The Belle Fourche River runs through the park. While the river isn’t that impressive today, the Belle Fourche hasn’t always been dammed. When the tower was first being eroded that river did a lot of the work. The melting glaciers from the last ice age fed the Belle Fourche into a river able to scour the Tower from 90 million years of sediment. The most important recent erosional force on the tower is ice wedging. Water trickles down into cracks in the rock and freezes. When water freezes it expands, and that pushes those cracks open wider allowing even more water to fill that gap. The last column fell 10,000 ago, a fact we’ve gleaned from studying lichen on the rocks, but boulders still do come down off the tower every year. Today, this unique and compelling geologic feature draws thousands of people, to hike the Tower Trial around its base, to climb its many faces, and to worship in its presence.
Thank you for joining me on this tour through time at Devils Tower National Monument. As always, my name is Piper Lewis, podcasting The Tower Frequency for you courtesy of GeoCorps and the National Parks Service. For more information on Devils Tower please see our website www.nps.gov/deto. Our theme music was composed by Bensound, and can be found at bensound.com. Thank you for listening!