Geology Spotlight: Eagle Formation and the The White Cliffs of the Missouri River
The geology of this region is fascinating and one of the most enriching parts of this adventure. Learning about the various geologic components and how this magnificent region was created over time is one of our focuses. Our guide service was founded by a passionate and knowledgeable geologist, Glenn Monahan, and this passion radiates throughout our multi-day canoe trips.
Arguably the most famous and renown section of the Upper Missouri River Breaks National Monument is the 46 mile stretch referred to as the ‘White Cliffs.’ This resource explores the geologic formation that encompasses the White Cliffs as well as a few fun features you will see as you paddle and hike within this section of river corridor.
The passage below is from our authored and published guidebook and history digest: Montana’s Upper Missouri National Monument and Wild and Scenic River. You can purchase this amazing resource on our website and utilize it on the river to bring the stories and landscape to life!
The White Cliffs formation across from Eagle Creek camp site.
The Eagle Formation (Eagle Sandstone) begins to make its appearance in the vicinity of Coal Banks Landing (river mile 41.5), located near the town of Virgelle, where the spectacular White Cliffs of the Missouri River begin. The striking and famed White Cliffs are composed of the The Eagle’s most prominent member, called the Virgelle Sandstone Member; it is composed primarily of sand particles with subsidiary pure white clay. Above this white sandstone is a layer of low grade coal and black shale.
The Eagle Formation can be easily identified by its characteristic creamy-white color, and its tendency to form striking cliffs, up to a couple of hundred feet high, in many places capped by the black coal/shale layer. The most captivating section stretches from river mile 53-77, otherwise referred to as The White Cliffs.
River mile 53, the White Cliffs descend into the water and allow paddlers to peacefully float by and even touch the cliffs. This is a very unique spot and we always take our time here.
Eagle Formation: Location of Outcrops
The first place that the Eagle begins to outcrop is in the Virgelle area (river mile 39.5, right), where it occurs as white vertical cliffs; but faulting then hides the Eagle Formation from prominent viewing for several miles downstream. At White Rocks (river mile 53, left), the Eagle Sandstone occurs prominently at river level, allowing paddlers to gain an excellent close-up view. From White Rocks to Slaughter River (river mile 77, left), the formation provides paddlers with a spectacular, spell-binding experience as it forms the dominant part of the scenery for 25 miles. Due to disturbances related to faulting, the sandstone cliffs are not contiguous, at times occurring at or near river level, and at other times occurring higher above the river. When the Eagle formation is not positioned at river level, the underlying Marias River Shale forms the banks. Finally, near Slaughter River, the Eagle - now noticeably thinner - makes it last major appearance, and soon dips below the water line, allowing the overlying Claggett Formation to dominate the banks. Downriver from Slaughter River, the Eagle makes occasional appearances at or above river level, where it has been lifted by faults, for example in the vicinity of The Wall dispersed campsite (river mile 81), and again at Sugarloaf Rock (river mile 101.5, right).
Eagle Formation: Environment of Deposition
The Eagle Formation was formed during a retreat of the coastline of the Western Interior Seaway, and was probably accompanied by uplifting of central Montana, and a shallowing of the sea. Along the coastline of the retreating sea would have existed a beach of white sand that was being worked by wave action. Some of the sand would have been deposited as off-shore sand bars. As the coastline retreated to the east, a continuous layer of white sand was left behind: later, these sand grains became cemented together to form sandstone.
Eagle Formation: Cementing Material
The cementing substance which holds the individual sand grains together in the Eagle Formation is called calcite, and it is the same material that sea shells are made of. Calcite is not a particularly strong cement - it is easily dissolved by rainwater, and the Eagle Formation actually erodes quite easily. However, other components within the Eagle, such as concretions, layers with different cementing materials and igneous intrusion combine to help resist erosion and are responsible for the array of unusual features which contribute to the beauty of the White Cliffs.
Example of a concretion found along the Neat Coulee hike behind Eagle Creek camp site.
Eagle Formation: Concretions
Structures called concretions protect the Eagle from rapid erosion. Concretions are the result of concentrations of an iron-rich cement, called hematite, which replaces the calcite cement and is much more erosion resistant. Typically, this red-brown colored cement aggregates in spherical pods within the sandstone, some as large at eight feet in diameter. The hematite cement is more erosion resistant compared to the bulk of the white, calcite-cemented sandstone, and in places where the concretion lies on the surface, it forms a protective cap above the more easily eroded sandstone. This cap protects the sandstone from the erosive effects of rain, allowing it to remain standing while all of the other white sandstone around it erodes away. The result is the formation of exquisite pedestal rocks, columns, towers and other unique features of the White Cliffs. The capping concretions, being iron-rich, can be identified by their rusty, red-brown color, which contrasts sharply with the underlying white sandstone. Concretions are also responsible for the hole-studded, pock-marked appearance of some Eagle Sandstone cliffs. This results from concretions which have loosened and fallen out of the cliff faces, leaving holes in the sandstone; these holes are frequently inhabited by rock pigeons, swallows and other birds. While hiking in the White Cliffs, keep an eye open for concretions lying on the ground, often resembling cannon balls.
The White Sandstone is very easily eroded. The deposits of multi-colored rocks are not original features to this area. These transplants were transported south from the movement of glaciers, which also completely changed the trajectory and flow of the Missouri River.
Eagle Formation: Iron-Cemented Layers
At some locations, particularly at the top of the white Eagle Sandstone, entire layers of sand particles are cemented together with hematite rather than with the easily dissolved calcite cement. Since the hematite is much more difficult for rain water to dissolve, this layer is instrumental in protecting many of the cliffs, spires, towers, walls and pedestal rocks. Hematite is the same iron-rich mineral that is responsible for the concretions mentioned above. As with concretions, the hematite cement layers in the Eagle can be identified by their rusty-brown color, which contrasts sharply with the characteristic white sandstone; they occur as layers, typically a few to several inches thick.
Impressive example of a pedestal rock with the hematite layer remaining while the White Sandstone erodes underneath i
Example of a hole and arch formed by a tiny pinhole that grew as the wind passed through over time. This can be seen on the Neat Coulee hike.
Eagle Formation: Arches and Holes
If a vertical slab (a fin) of Eagle sandstone wears in such a way as to form a small hole, wind becomes able to blow through the hole, which greatly accelerates the removal of additional sand particles, resulting in the formation of an arch or a hole, such as Hole in the Wall (river mile 64).
Eagle Formation: Igneous Intrusions
Other erosion-resistant materials were introduced into the Eagle Sandstone during the period of intrusive igneous activity. Like the concretions, the igneous rock is more resistant to erosion compared to the Eagle Sandstone, and it, too, acts to protect the sandstone from erosion. Many cliffs and buttresses of Eagle Sandstone are supported by igneous rocks which have been injected into fractures in the Eagle, a dramatic example of which is Hole in the Wall (river mile 64). The wall in which this feature occurs is thin, erosion-resistant intrusion of igneous rock that is sandwiched between the two sheets of sandstone, which the tougher igneous rock is supporting. This interaction between the igneous and sedimentary rock can be easily observed by climbing to the top of this ridge via the trail on the upriver side.
The always impressive Hole in the Wall. If you have time, a prominent trail leads up to this feature. Fair warning, though, the final climb to the top involves a little bit of rock scrambling for those we are up for it!
On the contrary, there are impressive igneous intrusions prominent throughout this section where the sandstone has been completely eroded away and the features stand along. The most striking examples are The Grand Natural Wall (river mile 56.5 ) and Citadel Rock (river mile 62).
The very impressive Citadel Rock.
The Grand Natural Wall extending to the water’s edge.
Have we piqued your interest in the fascinating geology of the Upper Missouri River? You can learn more by purchasing our history digest and guidebook (click here.) Or, better yet, you can join on of our guided river trips and see it for yourself! To learn more you can visit our Guided Trips page or Contact Us for more information. We hope to guide you down this magnificent river.