by Gary Thompson, Geologist
photos by Dick Walton
Revised October 2011
Also see Vermillion Valley Geology for a detailed discussion of the interesting geology of the Pryors as seen from the south end of the Crooked Creek Valley.
Introduction to a Drive Back Through Time
Pryor Mountain Road from Highway 310 south of Bridger MT into the Pryor Mountains is a drive back through 400 million years of geologic history. The record of this history is in the sedimentary rock layers that are exposed along the road in cliffs, hillsides, and roadcuts.
From about 500 million years ago until about 70 million years ago (mya), this area was covered by ocean much of the time. Early on, from 500 mya to about 200 mya this area was on the western edge of ancestral North America. As sea level rose and fell, the ocean shoreline advanced to the east and receded to the west several times. From about 200 mya until about 70 mya, an inland sea, connected to the world ocean, lay here with land to the west as well as to the east. As sea level repeatedly rose and fell, the inland sea widened and narrowed. So this area was sometimes in deep water, sometimes in shallow water, and sometimes on dry land. This is reflected in the kinds of sediment (now sedimentary rock) that were accumulating here. The fossils in these rocks reveal repeated change in the life in the sea and on the land as organisms went extinct and new ones evolved.
Sediments Become Rocks
When this area was covered by the sea, sediments eroded from the adjacent land or precipitated from the sea water were gradually deposited on the sea floor leaving a record of the conditions at that time. The kind of sediments depended on several factors, such as the depth of the water, the distance out to sea, and the climate, as well as the topography of the land area from which sediment was eroded. When this area was above sea level, sediments were deposited here and there in rivers, swamps, and lakes; while some of the previously deposited record was eroded away to the sea elsewhere. The different sediment layers are like the pages of a geologic history book. As the layers built up, slowly seeping groundwater deposited mineral cement in the pores, and pressure compacted the deeper layers turning sediment into rock.
Rocks Rise from the Sea
About 70 mya the area of the Pryors began to gradually rise and tilt. The inland sea disappeared. The rock layers, originally deposited horizontal, were lifted more to the northeast than to the southwest. So the pages of the geologic history book were tilted down toward the southwest. The same rock layers which form the top of the Pryors slope downward and are about 5,000 feet under Highway 310. At the same time the land was rising and tilting, erosion worked to level the land, cutting across the tilted layers. The result is that as we drive east toward the Pryors we generally cross from layer to layer seeing older and older pages of the geologic history book. However, we do see some interruptions in this general sequence as we cross faults and folds that were created here and there where rock layers bent or cracked and shifted during the uplift and tilting.
We hope this Geologic Road Log will help you read those pages.
The Geology Tour
This Geology tour begins where Pryor Mountain Road leaves Highway 310 three miles south of Bridger MT. It follows the same route as the Driving Tour of the Pryors so those directions and map may be helpful. Note that the map colors show land ownership. Landowner’s permission is required to leave the road onto private or Crow land.
At some points in this road log, directions are denoted according to the face of a clock, with 12:00 in the direction the car is headed. 3:00 would be straight to the right, 9:00 straight to the left, and 6:00 straight back. Also, note that odometers vary, so your vehicle’s mileage could be slightly different from ours.
0.0 miles: Junction of Hwy. 310 and Pryor Mountain Road. The surrounding flat surface is a stream terrace along the Clark’s Fork of the Yellowstone River and a tributary, Bridger Creek. This surface is an old floodplain formed when the river flowed at a higher level during the Quaternary» period before it eroded down to its present level.
0.05 mile: Cross Bridger Creek. Notice how this small creek has eroded down, well below the terrace surface.
1.75 miles: End of paved road.
2.0 miles: Cross railroad tracks.
2.6 miles (just before right turn in road): Looking straight ahead up dip slope» on sandstone in lower Frontier Formation» (Cretaceous age). Hillside exposure of sandstone beds in the lower Frontier Formation» dipping to southwest at about 15 degrees in distance at 9:00. In this area the bedrock strata dip (are inclined downward) uniformly to the southwest, except where there are localized faults and folds (as for example, at Red Dome, which we will see ahead). Road turns right.
Click on photos below to enlarge.
2.8 miles: Good exposure of sandstone in the lower Frontier Formation on left surrounding home and yard. This layered yellowish tan rock resists erosion, and is what underlies and holds up the ridge above the home.
2.9 miles: Following along Bridger Creek on our right, we pass through a gap in the ridge. Bridger Creek has eroded through the tilted Frontier Formation to produce this gap in the ridge.
3.3 miles: Cliff on right on skyline is again sandstone in the lower Frontier Formation. The low flats along Bridger Creek are underlain by Quaternary alluvium» that covers the older Belle Fourche Formation» .
3.8 miles: Driving along an east-west fault zone where the outcrops of the tilted Frontier and other formations on the right side of the road have shifted back to the west.
4.4 miles: To right at 2:00 on distant slope, thin sandstone cliff of Frontier Formation on skyline overlies long slopes of Belle Fourche Shale» and Mowry Shale.» . From 11:00 to 12:00 is dip slope on the Pryor Conglomerate Member» of the Kootenai Formation» (Cretaceous age), covered with scattered limber pines. At 10:00 is the red Chugwater Formation (Triassic age) exposed in the eroded core of Red Dome, at which we’ll get a closer look down the road. The strata dip away from the dome’s center, like an inverted set of nested bowls.
4.6 miles: Cross cattle guard.
5.0 miles: Pryor Conglomerate Member of Kootenai Formation on left and under road. Limber pines are common on the outcrops of the Pryor Conglomerate where water collects in the many pores and cracks. Fall River Sandstone» in low cliff beyond creek to right. Frontier, Belle Fourche, and Mowry are seen in the hills beyond.
5.6 miles: Sandstones and mudstones of the Morrison Formation» (Jurassic age) on left all the way to skyline at 9:00. The tan sandstones form ledges and cliffs, while reddish and gray mudstones erode into gentler slopes.
5.9 miles: From 8:00 to 11:00, exposures of Morrison, Swift » , Rierdon» , and Piper Formation» (all of Jurassic age); and Chugwater Formation» (Triassic age), all dipping to the southwest. At 3:00, across creek is hillside exposing sandstones of the upper Kootenai Formation which is on the other side of an east-west cutting fault that crosses under the creek here. The Kootenai has moved down relative to our side of the fault.
6.1 miles: The long ridge from 1:00 to 4:00 with a thin ledge of Frontier Formation at the very top is underlain by slopes of Belle Fourche, Mowry and Thermopolis Shale» (Cretaceous age).
6.6 miles (before left turn in road): Dip slope on Fall River Sandstone straight ahead.
6.7 miles (after left turn in road): Pryor Conglomerate at 11:00 to 1:00 on skyline straight ahead, with its scattered limber pines.
7.2 miles: Red mudstones in upper Kootenai Formation in hillside to left.
7.4 miles: Exposures of Pryor Conglomerate in “narrows” (guardrail on left) on both sides close to road.
Close up the conglomerate is distinct with a variety of pebbles many of which are dark chert. Chert is a hard very fine-grained sedimentary rock composed of tiny interlocking quartz. These particular dark chert pebbles are fragments eroded from older formations such as the Phosphoria Formation» , uplifted and exposed at the time in southwestern Montana and Idaho. These fragments were carried east by braided rivers, rounded, and deposited in the area of the Pryors.
7.7 miles: Red mudstone of Piper Formation in road cut on right; limestone of Piper on left.
7.9 miles: At 8:00 is the exposed center of Red Dome. The deep red siltstones and sandstones of the Chugwater Formation are exposed at the center of this eroded doming of strata. Note that the strata are dipping in all directions away from the center. Above the Chugwater is the paler, pink lower Piper Formation. Even small amounts of iron oxide can give rocks a reddish color. One source of the iron oxide is from deeply weathered tropical soils that existed at that time. There are some gray streaks in these reddish rocks that are impregnated with deposits of tar. The tar is a residue of crude oil that was once trapped under an impermeable layer of gypsum in the dome.
The Chugwater was deposited not far above sea level. After the Chugwater was deposited, uplift occurred and/or sea level dropped, and erosion carried away some of the rock record. Then, following subsidence of the land and/or a rise in sea level, the Piper was deposited, partly just above and partly just below sea level, on what was left of the Chugwater. This preserved an unconformity» representing a gap of about 70 million years.
8.3 miles: Limestone» beds on left dipping steeply towards road are densely fractured close to the large east-west fault running under the road. These beds have been bent down and fractured along the fault suggesting that the limestone, on the north side of the fault, has moved up relative to south side.
8.5 miles: Cross cattle guard. First view of the crest of Big Pryor Mountain on skyline ahead.
9.4 miles: Outcrop of Pryor Conglomerate on left; outcrop of Swift Formation below on right. East-west fault under road. Good view of Big Pryor Mountain ahead.
The broad southwest side of Big Pryor represents the dip slope of the Madison Group limestone. Rising to the northeast the erosion-resistant Madison reaches over 8000 feet at the top of Big Pryor. The north face of Big Pryor Mountain, and of East Pryor Mountain beyond, is aligned with us along a deep east-west fault zone. From here, near Red Dome, this fault zone continues along a line all the way west to the neighborhood of Nye, Montana. Geologists call the long straight topographic feature, that reflects this large deep fault zone, the Nye-Bowler Lineament. As here at Red Dome, there are domed structure at other places along the fault zone of the Nye-Bowler Lineament. There is evidence for horizontal movement along this fault zone. That is, the rocks on the north side of the fault zone moved west relative to those on the south. A simple analogy used to explain the creation of elongate domes along fault zones can be demonstrated with a piece of cloth. With your hands placed a couple inches apart on a loose piece of cloth underlain by a firm surface, slide one hand forward and notice the folds that develop in the cloth. In more brittle rock, cracks (faults) would also form.
In the low area at 2:00 to 4:00 are several dip slopes (dipping to the southwest) on resistant layers in the Jurassic (Morrison, Swift, Rierdon, and Piper Formations).
9.6 miles: Cattle guard.
9.7 miles: Black Butte, a dome capped by Tensleep Sandstone» (Pennsylvanian age), at 2:00. Bowler Flat lays out before us.
11.5 to 11.8 miles: Tensleep Sandstone crops out on hillside to right.
12.2 miles: Road Junction at old town site of Bowler. Power transformers on right. Turn to left, continuing on Pryor Mountain Road. We are on Bowler Flat which is underlain by alluvium and represent in part the flood plain of the ancestral Shoshone River that flowed from Wyoming to the northeast through Pryor Gap which can be seen to the northeast. The old flat flood plain has been buried by younger alluvial fan deposits that washed in from the surrounding high areas.
15.0 miles: Enter Crow Reservation.
16.1 miles: Cattle guard.
16.3 miles: Tensleep Sandstone crops out in cliff on left.
16.6 miles: Cross road on old railroad grade. Castle Rocks appear in distance through Pryor Gap at 10:00. Castle Rocks are limestone cliffs of Madison Group» (Mississippian age). Reddish slope in Amsden Formation» (Pennsylvanian age) at 9:30. From 1:00 to 3:00 is the north face of Big Pryor Mountain. Limestone of the Madison Group at the top of Big Pryor Mountain, is bent and faulted down along Big Pryor’s north face to our level along Sage Creek. From Sage Creek northward, the Madison rises only slightly into the northern Pryor Mountains on the Crow Reservation. In much of this area, erosion has cut down to the Madison. Erosion is so impeded by this thick resistant Limestone that the rock determines the general topography.
17.5 miles: Enter shallow “canyon” with limestone cliffs of Madison Group at road level on either side of Sage Creek. Madison has many vertical and horizontal fractures which have been widened into caves in places by groundwater solution.
18.0, 18.3, and 20.7 miles: Cross cattle guards.
21.1 miles: Cross cattle guard. Recent rock fall from cliff of Madison at 9:30.
21.5 miles: Indian Spring on right.
22.0 miles: Cross cattle guard entering Custer National Forest.
22.6 miles: Road on left to Sage Creek Campground. Continue straight ahead on Pryor Mountain Road.
22.7 miles: Roadcut on left exposes Quaternary/Tertiary pediment gravel. We will see several roadcuts in these gravels and similar landslide deposits along the mountain front. These surficial deposits represent sediment in the process of eroding off the mountain and are draped over the much older bedrock.
23.2 miles: Beginning 0.25 mile of roadcuts on right in Amsden Formation. As we proceed diagonally up across the north face of Big Pryor Mountain and encounter forest, two things change. (1) we reverse our journey back through time and pass up through younger formations, and (2) rock exposures are fewer and harder to see.
23.5 miles: Cross cattle guard.
23.8 miles: Roadcut in Tensleep Sandstone on right.
24.1 miles: Cross cattle guard.
24.4 miles: Cross cattle guard. Here, for a short distance, the road follows a large east-west fault beneath us that cuts the north side of East Pryor and Big Pryor Mountains. Although we can’t see it here, Madison Group is on the right and Tensleep Sandstone on the left. The right (south) side has shifted up along the fault relative to the left (north) side, so that the younger Tensleep is against the older Madison across the fault.
25.3 miles: Quaternary/Tertiary landslide deposits in roadcuts on both sides.
26.3 miles: Good exposure of Chugwater Formation on hillside across valley at 9:00.
26.5 to 26.7 miles: Amsden Formation in roadcuts on right.
26.8 miles: Quaternary/Tertiary landslide deposit in roadcut on right.
26.9 miles: Gravel quarry on right.
27.2 miles: Roadcuts on right in Tensleep Sandstone on both sides of draw.
27.5 miles: Road cut in Chugwater Formation on right.
27.8 miles: Roadcut in Tensleep Sandstone on right.
28.2 miles: Roadcut in Phosphoria Formation on right.
28.6 miles: Roadcut in Chugwater Formation on right.
28.8 miles: Roadcut in Phosphoria Formation on right.
29.2 to 29.3 miles: Roadcuts in Chugwater Formation on right.
29.5 miles: Cross cattle guard.
29.7 miles: Tie Flat Road on right.
30.2 miles: Roadcut in Tensleep Sandstone on left.
30.3 miles: Junction with Crooked Creek Road. From here, East Pryor Mountain lies to the east. It represents another southwest-tilted block similar to Big Pryor Mountain, capped by limestone of the Madison Group. From this saddle between Big Pryor and East Pryor, Crooked Creek flows to the south having cut a canyon with vertical walls in the limestone. Among the trees along the east front of Big Pryor Mountain, dolostone» of the Jefferson Formation» and Bighorn Dolomite» are exposed under the Madison Group. Here, these formations are dipping almost vertically where they have been bent and faulted down along another deep fault that runs north-south separating Big Pryor from East Pryor. This deep fault zone continues north, across the Nye-Bowler Lineament, separating the northern Pryors on the Crow Reservation into two blocks as well.
The Vermillion Valley is 14.8 miles south on Crooked Creek Road. See Vermillion Valley Geology for a detailed discussion of the interesting geologic history and structure of this area.
Out of sight on the east face of East Pryor Mountain are exposures of the oldest rocks in the Pryors. There sandstone, shale, and limestone of Cambrian age underlie the Bighorn Dolomite and overlie metamorphic rocks of precambrian age over 2.5 billion years old.Sources»
There are many technical publications about Pryors geology available in academic libraries. (Some references are listed on the geologic map below.) One excellent source is the geologic map and report that David Lopez has compiled from his work and that of earlier workers in the area including the Pryors. This is the Geologic Map of the Bridger (6 MB) 30’ x 60’ Quadrangle, Montana; centered on the Pryors and published in 2000 by the Montana Bureau of Mines and Geology in cooperation with the U.S. Geologic Survey and the U.S. Department of Energy. This map is beautiful to look at and, if you can decipher it using the legend and technical symbols, is the best source for understanding the overall geology of the Pryor Mountains.
A dip slope is a planar hillslope that is everywhere underlain by a dipping, erosion-resistant rock layer.
The upper member of the Kootenai comprises reddish, purplish, and greenish shale interbedded with tan sandstone and thin gray limestone. The total thickness of the Kootenai is 200 to 250 feet. The sediments of the Kootenai were deposited in rivers and lakes during the early Cretaceous Period. Kootenai fossils include plants, freshwater mollusks, and dinosaurs. The dinosaur Deinonychus, a 10 foot long predator, was discovered in the Kootenai just a few miles from Pryor Mountain Road in 1964, and has been important in establishing the close relationship between dinosaurs and birds. The Kootenai Formation was named in 1885 after the Kootanie Tribe for rocks exposed in southern Alberta. In Wyoming the Kootenai is called the Cloverly Formation.
An unconformity is a surface that separates a sedimentary rock layer from underlying rock and represents time during which no sediment was deposited, or, sediment or rock that had been formed was removed by erosion before the overlying layer was deposited. It’s a gap in the rock record. In most cases an unconformity represents a period of erosion.
North America geologists recognize three types of unconformities. (1), A nonconformity is an unconformity separating plutonic or metamorphic rock below from sedimentary rock above. (2), An angular unconformity is a surface separating the eroded edges of tilted strata below from horizontal strata above. (3), A disconformity is an unconfomity separating parallel strata. Generally, nonconfomities represent the longest gaps, angular unconformities represent shorter gaps, and disconformities represent the even shorter gaps. All the unconformities encountered along Pryor Mountain Road appear as disconformities. On the east face of East Pryor Mountain a nonconformity separates metamorphic rock from overlying sedimentary rock, representing a gap of about 2 billion years.
Lopez, D.A., 2000, Geologic map of the Bridger 30′ x 60′ quadrangle, Montana: Montana Bureau of Mines and Geology: Geologic Map 58, 1 sheet(s), 1:100,000
Wilmarth, M. Grace, 1957. Lexicon of geologic names of the United States: U.S. Geological Survey Bulletin 896, parts 1 and 2.
Wilson, Druid, Sando, William J., and Kopf, Rudolph W., 1957. Geologic names of North America introduced in 1936-1955: U.S. Geological Survey Bulletin 1056-A.