Geology of the Beartooth Mountains

Geología de las montañas Beartooth

Avalanche Lake The Beartooth Mountains are a result of nearly 4 billion years of Earth processes occurring. This history is discussed here in terms of the geologic timeline. Following is a brief description of the Beartooths geologic history:

Pre-Cambrian:

The Precambrian is the period of time from the origin of the earth (4.6 billion years ago) to the beginning of the Cambrian Period (540 million years ago). A great variety of rocks found in the Beartooth Mountains are Precambrian in age.

4 to 3.4 billion years ago, igneous rocks crystallized to form some of the oldest rocks in the world. These rocks were eroded, deposited, and incorporated into younger sedimentary rocks.

3.0 to 2.5 billion years ago, ancient sediments and crystallized igneous rocks were subjected to heat and pressure, forming the granites and gneisses in the rock of the main block of the Beartooth Mountains. These rocks were then subjected to heat and pressure to form metamorphic rocks, such as the Hellroaring Quartzite which is found on the Hellroaring Plateau. Zircon minerals (photo) in the Hellroaring Quartzite provide a way to date rocks using Uranium-Lead dating methods. Since the age of the earth is generally accepted as 4.6 billion years, these Beartooth rocks represent some of the first rocks formed after the cooling of the initially molten earth. These ancient rocks are called the Precambrian “basement” because they form the foundation for the much younger sedimentary rocks that were deposited on top.

At about 2.8 billion years ago, the granites and gneisses were intruded by many small igneous rock bodies, to form sills, stocks, and dikes. At about 2.7 billion years ago, the Stillwater Complex was emplaced. The Stillwater Complex is a succession of ultramafic to mafic rocks derived from a large complex magma body deep in the Earth’s crust. The Stillwater Complex hosts valuable minerals, including platinum, palladium, gold, silver, chromium, and iron that have been actively mined since the late 1800’s.

Following emplacement of the igneous rock bodies, a long period of erosion created nearly flat, low lying topography. This erosion removed much of the rock record, creating a gap in the geologic record for more than 2 billion years of geologic history. This gap is referred to as an unconformity.

Cambrian period:

540 million years ago, this part of Montana and Wyoming was along the western edge of the Proto-North American continent. The Flathead sandstone, the first sedimentary rock to be deposited on top of the older granite and gneiss basement rock, represents an ancient beach on the edge of the continent. Additional shales and limestone units were deposited atop the Flathead sandstone.

Ordivician, Silurian, and Devonian periods:

500 to 360 million years ago, sea level changed, depositing a sequence of sedimentary rocks, including the Ordivician Bighorn dolomite. Above the Bighorn dolomite is an unconformity. There are no Silurian rocks in the Beartooth Mountains. In the Devonian the sea returned to deposit the red rock of the Devonian Beartooth Butte formation, which contains numerous plant and animal fossils, and the gray and brown limestone and dolomite of the Jefferson formation.

Mississippian, Pennsylvanian, and Permian periods:

360 to 250 million years ago, shallow seas repeatedly flooded and retreated across the region, depositing sediments that later form sandstone, shale, limestone, and siltstone. Fossils of ancient marine life are visible throughout these rock units.

Triassic, Jurassic, and Cretaceous periods:

250 to 70 million years ago, the North American continent grew and sea level changed. The area of the current Beartooths was no longer located on the western coast. Instead, from around 200 – 70 million years ago a shallow inland sea, called the Western Interior Seaway flooded the continent, leaving this area, once again, underwater. More sedimentary rocks were deposited that also reflected changes in sea level of the Western Interior Seaway. By 70 million years ago, the sea was retreating because the land was starting to rise. In the period from 530 – 70 million years ago, approximately 11,500 feet of sediment was deposited in the area of the current Beartooths. Specific rock units and thicknesses are displayed in the stratigraphic column.

Tertiary period (65 to 2 million years ago):

Around 65 – 57 million years ago, the pre-Cambrian granitic and gneissic rocks were pushed upwards along reverse faults during a period of extensive mountain building known as the Laramide Orogeny. The Laramide uplift caused the originally horizontal overlying sedimentary rocks to be folded and faulted to the nearly vertical position of the limestone palisades visible today along the Beartooth front. As the mountains were being uplifted, the sedimentary rocks that were on top of the plateau were eroded and deposited into the adjoining basins. Beartooth Butte (discussed above) is evidence that there once were sedimentary rocks on top of the Beartooth Mountains. Because the inland sea was shrinking, the environment in the basins was much like a modern day swamp. As sediment was eroded from the mountains, it buried organic matter in the swamp. This organic matter eventually decayed and became the coal beds of the Fort Union formation that underlies the city of Red Lodge and much of eastern Montana.

Quaternary Period (2 million years ago to present):

The landforms currently observed in the Beartooth Mountains are largely due to alpine glaciers, which first formed about 1.6 million years ago (grasshopper glacier in 1910). As a glacier flows down a valley it erodes the bedrock to create visually striking land forms such as cirques, arêtes, horns, hanging valleys, and U-shaped valleys. At the terminus or end of the glacier, depositional landforms are created, such as moraines, kettle lakes, kames, eskers, and outwash plains. Many erosional features can be observed from Vista Point overlook along the Beartooth Highway. Many of the depositional features can be found at the head of the valley as you near the town of Red Lodge on the Beartooth highway. Click here for additional information about Beartooth glaciers.

Today, the Beartooth mountains continue to erode and deposit material in the adjacent basins. However, the mountains still maintain such a high elevation due to the recent presence of the Yellowstone hotspot to the south. This increase in heat and pressure under the crust causes the mountains to still maintain their elevation while actively being eroded.