#125 - Bighorn Canyon in Southeastern Montana

Photo courtesy of Visit Southeast Montana

The following was written by Rob Thomas, Ph.D., geology professor at UM Western and coauthor of the second edition of Roadside Geology of Montana. He wrote the following as a post on the Montana Geology Facebook page. Watch for Rob's new Montana Rocks book to come out soon.

Grand Canyonesque . . .
Like most people, geologists are impressed by the scenery in the Grand Canyon in Arizona, but they are awed by the thick pile of undeformed Paleozoic sedimentary rocks. Such rocks in western Montana tend to be buggered up by compression, intruded by granitic rocks and torn apart by extension, making them difficult to study. East of the mountains, tectonic influences dissipate, but so do exposures of Paleozoic rocks, found tilted up around igneous intrusions and gently folded in ranges cored by Laramide compressional faults, like the Pryor and Bighorn Mountains south of Hardin. The Bighorn River passes through these mountains, eroding into the rising land and exposing up to 1500 feet of mostly undisturbed Paleozoic sedimentary rocks in the steep walls of one of the grandest canyons in Montana.

A couple choices . . .
There are two access points to the Bighorn Canyon National Recreation Area, Highway 313 south of Hardin in the north and Highway 36 from Lovell, Wyoming in the south. In the north, the river cuts through a gentle fold in the sedimentary rocks called “the monocline”, which formed over a rising basement block around 70 million years ago. It is best reached by boat from the Ok-A-Beh Marina upriver to the monocline, where the river bites into Cambrian through Pennsylvanian sedimentary rocks deposited by rising and falling tropical oceans. From the south entrance, the road follows close to the rim of the canyon with unparalleled views into the steep canyon cut into Mississippian Madison Group limestone. This tropical marine limestone is loaded with fossils of corals, crinoids and brachiopods that lived in the warm ocean waters that covered the area about 350 million years ago.

Red limestone? . . .
The best canyon views are from Devil’s Canyon Overlook and Sullivan’s Knob Trail, where you can make the canyon walls echo! The upper part of the Madison Group limestone here has a distinctive dissolution zone, with karst features like caves and collapse breccias. It likely formed during a period of sea-level drop and exposure of the limestone to slightly acidic water. Some geologists argue that the red color of the overlying Amsden Formation is from soil formed during millions of years of weathering and dissolution of the exposed Madison Group limestone, concentrating insoluble, oxidized (red) clays. Perhaps the red color comes from oxidation of iron-bearing sediment deposited during Amsden time or staining of the sediments by iron-rich groundwater long after its deposition. In any case, surface runoff from the Amsden trickles down the light-colored cliff walls of Madison limestone, leaving behind red streaks as it stains the rock.

Fossil meander? . . .
Although the folds and faults of the Pryor and Big Horn Mountains formed during Laramide compression, the Bighorn River canyon is much younger. Notice that the river cuts across these structures and has the sinuous shape of a low gradient, meandering stream. This is clear evidence that it once flowed over a flat surface, likely in loose sediment that covered over those structures. The plains have recently been on the rise due to crustal extension in the western United States, so as the region has slowly expanded up, the old and sinuous river has cut downward into the hard rock below, while still maintaining its sinuous shape. The river no longer meanders back and forth as it once did, because it’s now stuck in the hard rocks, what’s called a fossil meander.

Below: Geology diagram from Roadside Geology of Montana, Hyndman and Thomas, 2020. Click on image to enlarge.