The Hoback River Canyon of Bridger-Teton National Forest in Wyoming is rich with geological history. Upon first glance at its mountainous landscape, it’s unclear what forces shaped the rocky slopes. However, with further investigation of the surrounding rocks and geologic formations, the evolutionary story of this area becomes clearer. Through field study, we surmise that there once was an ocean here that receded, a compression event that caused uplift and ductile deformation, and, finally, an expansion event was filled in by new sediment deposition.

 

          Along the Hoback River, there is a scenic pull off nicknamed Stinking Spring. Near the river’s edge is a porous rock formation spanning approximately 20 feet (fig. 1). Its subtle gray surface is riddled with rounded crevices, and some imprints of brachiopod fossils. Testing with hydrochloric acid resulted in fizzing, suggesting the presence of carbonates. With these findings, it is evident that this is a fossiliferous limestone. The traces of brachiopods suggest an age of around 340 million years old (Mississipian era) and marine depositional environment. Here in Jackson, Wyoming this deposit is referred to as Madison Limestone. Looking up the slope on the other side of the river, you can see nearly identical gray deposits. However, above the Madison Limestone rests the easily identifiable tan and pink banding of sandstone (fig. 2). The presence of a sandstone layer above pre-existing limestone suggests that the ocean once here receded. The resulting shallower waters would have allowed for the formation of coarser sedimentary rocks like the sandstone observed at the site’s peaks. Further, the exposure of these layers of sandstone and Madison Limestone suggests that in more recent geologic time, the Hoback River remained and carved its way through these deposits creating the Hoback Canyon.

 

          Similar sandstone formations can be seen at the Red Creek area, 43°16’56”N 110°34’44”W (fig. 3). At this site, three samples were collected from a talus slope facing northwest (fig. 4). First, a medium-grained, pinkish-red rock that felt sandy and jagged, evidently sandstone. Second, a fine-grained, soft gray rock with fine fracture lines that reacted with HCl, potentially limestone. Finally, a beautifully banded sandstone sample was found with similar colorations and textures to the first rock. Specimens like these were abundant on the talus slope. Looking up the cliffside, it is evident that these samples have fallen from the face of the exposed sandstone above. The presence of these sedimentary rocks prove that this area must be a younger extension of the marine depositional environment observed along the Hoback River. This particular geological formation is referred to as Tensleep Sandstone, having formed 300 million years ago during the Pennsylvanianian age.

 

          Looking southeast from the Red Creek site, an exposed anticline can be seen (fig. 5). Within the rich red of its cliff face evidence of a compression event can be seen. There are several different bands of deposits which feature a drag fold form towards the east. The bands, in order, within are known to be Phosphoria (255 ma), Tensleep (300ma), Amsden (320 ma), and Madison Limestone (340ma). The anticline, with its youngest rock on top, and eastward fold is indicative of a compression event from the west. We can infer that this resulted from a ripple effect as the Pacific Plate compressed and subducted under the western end of the North American plate. This event is known as the Sevier Orogeny, which occurred approximately 65 million years ago in this area. More evidence for this compression event is seen on the other side of the creek. Facing northwest, you can see brick-red rocks jutting down, nearly vertical (fig. 6). Upon inspection of its talus slope, ripple marks were found on sandstone samples suggesting that they are a high energy deposit from a watery setting, likely the Hoback River (fig. 7). These deposits are known as the Chugwater Formation which is known to be 240 ma and rich in oxidized iron-rich sandstone. Seen above the younger Chugwater Formation is more of the older Tensleep Sandstone at a 30° angle. This discontinuity and folding is indicative of a compressive thrust fault (Hoback Thrust fault) that occurred during the Sevier Orogeny which pushed the Tensleep up and over the Chugwater Formation.

 

          From the summit of Mt. Anne you can see clear across the canyon to the surrounding peaks including Cream Puff and Beaver Mountain. On the westerly face of these ranges, there’s a noticeable 60° slope; this uniformity suggests that the action of a normal fault is present. The rock observed underfoot at Mt. Anne is Madison Limestone, from the same marine deposit previously discussed. Beaver Mountain, observed to the east of Mt. Anne summit, is also composed of Madison Limestone, but is at a much higher elevation. This is evidence of the downward movement of the limestone along a normal fault line. Further, while at the site named Stinking Spring, the smell of sulfur was a clear indicator that there is an active fault where the water was circulating underground and bringing back up with its sulfur gas. The fault line is visible along the ridgeline of Cream Puff, called the Hoback Normal fault, and became active 10 million years ago.

 

          In studying the Camp Davis formation (fig. 8), to the west of Mt. Anne, a variety of rock samples were found. They were of varying sizes and many conglomerate rocks were found. The soil felt gritty between my fingers, much like silt. When testing with HCl, a rock sample fizzed indicating the presence of carbonates or calcite cementing. Based on these observations, it is clear that the Camp Davis formation is part of an alluvial fan. It spans across the canyon under Lion’s Paw and was created by erosive forces some 5 million years ago.