The Geology of Billings, Montana

Introduction to Billings’ Geologic Setting

Billings is located in south central Montana along the Yellowstone River. The city lies in the Northern Plains geographic province, situated between mountain ranges to the west and rolling plains to the east. Billings’ location adjacent to the Yellowstone River and at the foot of the Beartooth Mountains gives the area a diverse and dynamic geologic history.

Billings sits atop several thousand feet of sedimentary rock layers that reveal various past environments like ancient seas, swamps, and river floodplains. The oldest rock layers date back to the Paleozoic Era over 300 million years ago. More recent ice age glaciers, rivers, and lakes have also shaped Billings’ modern landscapes.

Paleozoic Era: Laying the Foundation

Cambrian Period (541-485 million years ago)

The earliest Paleozoic rock layers beneath Billings originate from the Cambrian Period when shallow inland seas covered the region. These marine sedimentary rocks include the Flathead Sandstone and Wolsey Shale formations consisting of sandstones, shales, and limestones.

Flathead Sandstone is visible along the rimrocks and valleys around Billings where its resistance to erosion produces cliffs. Fossils of trilobites and brachiopods can be found preserved in these early Cambrian rocks.

Mississippian Period (359-323 million years ago)

During the Mississippian the area returned to being an expansive shallow sea. The Madison Limestone and Mission Canyon Limestone formations consist of calcium carbonate marine deposits like limey muds that lithified into rock.

The Mission Canyon Limestone is exposed at places like Pompeys Pillar National Monument east of Billings. Its layering and abundant fossil crinoids display the Mississippian marine environment.

Pennsylvanian Period (323-298 million years ago)

As the Pennsylvanian Period began, the sea regressed and coastal plains and swamps covered the region. Extensive swamps hosted vegetation that became coal and peat bogs where plants gradually accumulated and transformed into petrified forests preserved in rock.

The Pennsylvanian Period is represented by the Amsden and Tyler Formations visible around modern day Billings. These formations contain sandstones, mudstones, and important economic deposits like coal beds.

Mesozoic Era: Expanding the Continental Identity

Jurassic Period (201-145 million years ago)

By the Jurassic Period around 200 million years ago, the area was situated well inland from any major bodies of water and becoming more arid. Wind-blown dunes dominated the landscape and later fused into the prominent cliff-forming sandstones seen around Billings like the Sawtooth Formation.

Massive petrified logs located within the Morrison Formation were deposited during the late Jurassic by river systems that transported fallen trees. The Morrison Formation is another critical unit exposed in the Billings vicinity that also has many dinosaur fossil sites.

Cretaceous Period (145-66 million years ago)

The early Cretaceous Period was marked by rising sea levels that inundated the landscape, connecting shallow seaways across parts of North America. Later in the Cretaceous, mountain building processes to the west raised the region upwards again.

Significant stratigraphic units in the Billings locale originating from the Cretaceous include the Thermopolis Shale, featuring black organic marine shales, as well as the Colorado Group’s Niobrara Formation and Carlile Shale members exposed prominently along the Yellowstone River. These rich marine mudstones and shales also host plentiful fossils.

Cenozoic Era: Shaping the Modern Scenery

Paleogene Period (66-23 million years ago)

The Laramide Orogeny mountain building event peaked in intensity during the Paleogene Period, influencing regional erosion and deposition patterns. One significant unit of the Fort Union Formation consists of mudstones, siltstones, sandstones and coal seams dating from Paleocene to Eocene times around 60 million years ago.

In various places surrounding Billings, the Fort Union Formation has productive coal seams that have been utilized as an energy source. It also hosts well-known fossil sites like plants and insects.

Neogene Period and Quaternary Period (past 23 million years)

The last few million years encompassing the Neogene and Quaternary Periods were dominated by intense glaciations that carved and deposited material over the Billings vicinity. Episodes of continental glaciation extended from the Beartooth Mountains at times.

Modern drainages have removed some sediments but also hosted recent river terraces and floodplain deposits along waterways like the Yellowstone River around Billings. The result is a complex surficial geology varying from bedrock exposures to loess hills and bluffs. Ongoing erosion of differentially resistant rock layers continues to sculpt badland terrain and striking rock formations.

Key Geologic Structures and Landforms in Billings

Rimrocks

The most visually prominent geologic feature surrounding Billings are the imposing sandstone cliff rims bordering the community, known locally as rimrocks. These form protective barriers above the city and also display rocky outcrops with diverse designs and colors reflecting the different geological formations.

The Sawtooth Formation with its distinctive jagged appearance comprises sections of the eastern rimrocks. Bands within the mission Canyon and Amsden Formations also compose dramatic walls to the north and west.

Pompeys Pillar National Monument

East of Billings near the Yellowstone River is a notable area showcasing eroded remnants of the sandstones and shales of the Amsden Formation capped by Madison Limestone that preserves trace fossils on top of the butte.

These resistant rock layers form the central rock pillar protruding over 150 feet upwards. The site has additional historic significance associated with William Clark’s engraved name left behind in 1806. The layers visible around Pompeys Pillar provide a useful record of local geology.

Canyon Creek Drainage

One example of local erosional patterns shaped into badland formations occurs southeast of Billings at this canyon creek drainage situated between the Beartooth Foothills and a raised plateau of the Fort Union Formation capped by volcanic remnants.

The easily erodible mudstones and poorly cemented sandstones of the Willwood Formation spanning the Paleocene to Eocene epochs are exposed at Canyon Creek. Ongoing water flows carve the distinctive spires, hoodoos and ravines displayed.

Beartooth Uplift

The impressive Beartooth Mountains closely flanking Billings to the southwest represent part of the immense regional structural dome that uplifted Precambrian era basement crust upwards. This feature named the Beartooth Uplift initiated during the Laramide Orogeny.

Subsequently, erosion has exposed the ancient granitic and metamorphic cores of the mountains, starkly visible from Billings, as overlying sedimentary layers got stripped off the rising terrain. The sheer elevation contrast where the plains meet the steep Beartooths illustrates well this significant geological structure.

Modern Hazards and Resources

Earthquakes

Being located directly adjacent to the active Beartooth Uplift at the dynamic edge of the North American tectonic plate, Billings lies within the Intermountain Seismic Belt, a zone of concentrated earthquake activity.

While most seismic events near Billings are too small to detect, larger magnitude earthquakes can shake the city, like those centering closer to Yellowstone. For example, the 1959 Hebgen Lake Quake northwest of Yellowstone measured 7.3 magnitude and caused property damage in Billings while being felt across much of Montana.

Groundwater

The variety of sedimentary formations make much of the bedrock surrounding Billings porous and permeable, facilitating groundwater storage, especially within sandstones. This groundwater resides in aquifers utilized broadly across the valley. One of the most productive aquifers taps into sandstones and fractured shales of the Cretaceous Colorado Group. Wise conservation and protection of these below ground water resources will be crucial long-term.

Coal and Oil

Lastly, the oil fields extracting petroleum deposits from Cretaceous and Jurassic sandstones and the coal mines tapping into rich reserves like in the Fort Union Formation demonstrate how Billings geology harbors valuable natural energy resources that will likely continue playing vital economic roles in the future.

Conclusion

In summary, Billings has an extensive geologic history spanning over 300 million years displayed across the variegated sedimentary layers, rugged mountains, and carved river valleys comprising the local terrain.

The details within those rock formations and landscape features recount the diverse paleoenvironments, climatic shifts, structural events, and erosional forces that have ultimately given shape to the modern city of Billings, Montana.

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  • Start out going northwest on Main St toward N 28th St. Take the 1st right onto N 28th St. Turn right onto Grand Ave, then quickly turn left to merge onto I-90 W toward Sheridan. Follow I-90 W for approximately 70 miles, then take exit 442 for Blue Creek Rd toward Billings. Turn right onto Blue Creek Rd and the destination will be on your right after about 0.3 miles.
  • Head west on Main St toward N 29th St. Turn right onto N 29th St, followed by a slight left to merge onto I-90 W. Stay on I-90 W, driving for roughly 70 miles, before taking the Blue Creek Rd exit on the right. Take the ramp onto Blue Creek Rd and go straight for 0.3 miles until you arrive at 3910 Blue Creek Rd on your right.
  • Start by getting on I-90 W from Main St. Drive on the interstate for about 70 miles then take exit 442 to merge onto Blue Creek Rd. As soon as you exit, turn right to continue on Blue Creek Rd. 3910 Blue Creek Rd will be around 0.3 miles down the road on your right side. The travel time should be just over an hour from downtown Billings.