Article by Justin Tweet, Paleontologist, American Geosciences Institute
Devils Tower National Monument, the first U.S. national monument, is justly famous for its namesake feature. The 867-foot-tall Tower, a gray igneous monolith with a characteristic fluted appearance, can be seen for miles in northeastern Wyoming and has long inspired and intrigued viewers. The Tower does not exist in a geologic vacuum, though; it formed in a landscape of much older sedimentary rocks, some of which preserve shallow marine fossils from a time when northeastern Wyoming was much different. Historically, geologists have focused on the Tower, but have sometimes observed fossils nearby. In 2019, the paleontology program of the NPS’s Geologic Resources Division made the first paleontological inventory of the monument in response to new reports of fossils. A public version of this report can be found on IRMA.
Long before the Tower formed, northeastern Wyoming was much closer to sea level, and sea level was higher. For hundreds of millions of years, shallow seas advanced and retreated across low-lying areas of western North America, leaving behind thick deposits of sedimentary rocks. These cycles ended in the Late Cretaceous, around 70 million years ago, when tectonic activity caused the North American plate to warp up in the western interior, beginning the growth of the modern Rockies. The Black Hills, which includes the Tower, is part of this warping, and the Tower dates to igneous activity that occurred about 49 million years ago.
Visitors to the monument enter on a road that passes beneath brightly colored red and yellow bluffs and cliffs south of the Tower. These rocks belong to most of the exposed geological units that underlie and surround the igneous Tower. The red rocks belong to the Spearfish Formation, which dates to approximately 250 million years ago and was deposited in various settings in and around a coastline. Above the Spearfish Formation is a poorly exposed whitish interval made up of the easily eroded rocks of the Gypsum Spring Formation and Stockade Beaver Shale Member of the Sundance Formation. These two units were also deposited near and in a shallow sea, but much more recently than the Spearfish Formation at roughly 170 to 165 million years ago. The sea is known as the Sundance Sea. The yellow cliffs and bluffs are part of the Hulett Sandstone Member of the Sundance Formation, a shoreface sandstone. Visitors can get a close look at these four geologic units by hiking the Red Rocks Trail, which crosses each unit as it winds around the monument.
Two other geologic units are found in the monument, but are poorly exposed. The Lak Member of the Sundance Formation is the yellowish unit exposed along the road near the visitor center, and the Redwater Shale Member of the Sundance is a gray unit mostly made of limestone. It is generally covered by vegetation or talus from the Tower, but remnants are visible in higher elevations of the monument (not including the Tower). It is a marine unit deposited about 163 million years ago. Other younger formations were once present here, such as the famous Morrison Formation of the Late Jurassic and the Lakota Formation of the Early Cretaceous, but they have been lost to the same erosion that has exposed and sculpted the Tower. Scattered fragments of hard white quartzite are remnants of the Lakota Formation. There are also isolated pieces of an unusual type of rock formed from rock fragments in an igneous matrix, sometimes called “agglomerate”. This type of rock is connected to the formation of the Tower, and provides glimpses of the bedrock beneath the surface.
The marine waters that covered northeastern Wyoming in the Jurassic were populated by abundant invertebrates and vertebrates, and fossils of some of them can be seen within the monument. The Sundance Sea supported a fauna that was similar in many ways to what we might see in shallow seas today (oysters, scallops, crabs, lobsters, sea lilies, sea urchins, and sharks), but very different in others (ammonite and belemnite cephalopods, ichthyosaurs, and plesiosaurs). The most abundant types of fossils in the monument are bivalve shells and belemnite guards.
Bivalve shells are locally found in crowded accumulations in rocks. The shells belong to several species of different sizes and shapes, but most of them are smaller than an inch across. In many cases the original shell material has been lost, leaving behind a natural mold or cast of the exterior of the shell, or a natural cast of the interior (a kind of fossil called a “steinkern”). Belemnite guards are pointed conical objects up to a few inches long. They were internal structural supports for belemnite cephalopods, which were otherwise similar to many modern squids in appearance. The guards are frequently fractured in half along the long axis and broken into smaller pieces. Those that are fractured in half often show a smaller triangular space at the wide end, which is a socket where a conical structure called the phragmocone was located.
Other types of fossils are less common. A researcher in the 1950s described microfossils of algae. Some rocks have what appear to be simple vertical burrows. A small chip had parallel rusty markings resembling conifer foliage, but it was too small to be certain. Other pieces of plants have been reported from the fragments of Cretaceous quartzite and sandstone. Small pieces of bone are occasionally seen in the bivalve-bearing rocks and probably belong to fish or small marine reptiles. A bone was found in the 1930s by monument staff, but its current whereabouts are unknown. Features resembling dinosaur tracks have been found in a block of sandstone, but they may just be weathering features. Finally, much more ancient fossils have been reported from the “agglomerate”, such as brachiopods (“lamp shells”) from the Carboniferous, probably originating from the same kind of limestone that hosts the caves of Jewel Cave National Monument and Wind Cave National Park.
The fossils of Devils Tower National Monument help to provide a more complete picture of the natural history of the Tower and its surroundings, and are part of our national paleontological heritage. If you should see a fossil while visiting the monument, feel free to take photos, but remember to not remove or disturb it; share your find with a Park Ranger instead.
Series: Park Paleontology News - Vol. 11, No. 2, Fall 2019
Last updated: October 4, 2019