Devils Pocket:

How a Nameless Montana Badland Became a Key to Reading Deep Time

An Anticline in the Big Snowy Mountains

In the southeastern reaches of the Big Snowy Mountains, in what is now Golden Valley County, Montana, a long ridge of Paleozoic rock rises from the plains and then abruptly fails. Geologists call the underlying structure an anticline, a broad upward fold in the earth’s crust, and this particular fold has a nose that plunges and is breached by erosion, so that a shallow, closed valley opens within the ridge itself. Ranchers and stockmen who worked this country in the early twentieth century gave the valley an unflattering name: Devils Pocket. It is not a town, not a battlefield, not the site of any recorded human drama. It is a fold in rock, hemmed by outcrops, difficult to reach and unremarkable to the untrained eye. Yet this obscure basin, roughly half a mile west of a ranch at the mouth of Stonehouse Canyon, would in time lend its name to a formally recognized rock unit that geologists still use to read 300 million years of Montana’s buried past.

The story of Devils Pocket is not a story of gold, outlaws, or catastrophe. It is the quieter story of how American geologists, working through oil booms, university theses, and government surveys across three decades, built a shared vocabulary for the rock beneath central Montana, and how a rancher’s derisive name for a badland became, almost by accident, a permanent fixture of the scientific record.

Early Surveys and the Quadrant Problem

The rocks that would eventually be called the Devils Pocket Formation were first bundled into a much larger, cruder unit. In the 1890s, government geologist Walter Weed and, separately, A. C. Peale had each described a package of strata between the Mississippian-age Madison Limestone below and Jurassic-age Ellis strata above. Peale’s term “Quadrant formation,” named for exposures near Yellowstone National Park, became the common shorthand for this entire interval across much of Montana. The problem was that this single name concealed enormous internal variety: sandstones, shales, limestones, and dolomites laid down under very different conditions over a very long span of time.

The first attempt to sort out this variety in the Big Snowy Mountains specifically came from geologist Frank Reeves, who conducted reconnaissance mapping of the range in the late 1920s for the United States Geological Survey. Reeves, working before detailed subdivisions had been widely accepted, elected to keep the entire package under the older Quadrant name rather than adopt the finer breakdown that a mining engineer named O. W. Freeman had proposed a few years earlier for the eastern part of the same range. Reeves’s caution was reasonable for a first pass at the geology, but it left the rocks of the Big Snowy Mountains without the more precise vocabulary that oil companies, increasingly active in central Montana in the 1920s, badly wanted.

The Big Snowy Group Takes Shape

That precision arrived through the work of geologist Harold W. Scott, who in 1935 formally proposed the term “Big Snowy Group” for the sequence of sandstone, shale, and limestone exposed on the north slope of the range, building on units earlier named by Weed in the Little Belt Mountains. Above the Big Snowy Group lay a younger sequence that most geologists continued to call, more loosely, the Amsden Formation, after a type locality far to the south in the Bighorn Mountains of Wyoming.

Through the 1940s and 1950s a succession of geologists working in the Big Snowy country—many of them associated with the Billings Geological Society—argued over exactly where to draw the boundary between the older Mississippian rocks and the younger Pennsylvanian rocks above them. This was not an idle academic quarrel. Oil companies exploring the Williston Basin needed a dependable way to correlate rock layers from one drill hole to the next across hundreds of miles, and inconsistent naming made that correlation unreliable. Indeed, the earliest hint that this rock sequence mattered economically had come decades before, when it was recognized as a probable source of petroleum feeding the sandstone reservoirs above it.

Gardner Names Devils Pocket

It was in this atmosphere of ongoing revision that geologist L. S. Gardner published his own comprehensive reassessment of the Big Snowy sequence in the American Association of Petroleum Geologists Bulletin in 1959. Gardner took a different view of the sequence than most of his contemporaries, treating the entire package, from sandstone at the base to limestone and dolomite near the top, as a single uninterrupted cycle of deposition rather than two cycles separated by an erosional gap. To support this reading, he expanded the Big Snowy Group upward to include three formations that other geologists preferred to keep separate, and for the uppermost of these—a distinctive sequence of dolomite, red mudstone, and sandstone—he needed a new name.

Gardner turned to the landscape itself. He designated the type section in a dry drainage called Road Canyon, about half a mile west of the ranch house at the mouth of Stonehouse Canyon in Golden Valley County. Gardner named the Devils Pocket Formation from the anticlinal valley where the strata measured 141 to 143 feet thick. Within two years, the catalog of central Montana formation names compiled for the Billings Geological Society’s guidebook had already absorbed Gardner’s new term, crediting the name to Gardner and citing its full thickness range across the district.

Reading the Rock, Dating the Past

Naming a rock unit is only the beginning. Geologists still had to determine when the Devils Pocket sediments had actually been deposited, and this required painstaking fossil work. Paleontologist William Easton, in a 1962 Geological Survey study, identified fusulinids—tiny single-celled marine organisms whose shell shapes changed predictably over geologic time—that placed the Devils Pocket squarely in the middle portion of the Pennsylvanian period. Later workers refined this further, concluding that the formation’s fusulinid fauna corresponded to the Atokan through early Desmoinesian stages, meaning the rock had formed roughly 310 to 305 million years ago.

The most thorough reconciliation of all this competing evidence came in 1967, when geologists Edwin Maughan and Albert Roberts published a lengthy Geological Survey study explicitly aimed at resolving the tangled nomenclature. They retained Gardner’s Devils Pocket name for the uppermost formation while restoring some of the boundaries Gardner had redrawn, arguing on the basis of a demonstrable erosional break that the Big Snowy Group and the overlying Amsden Group did in fact represent two separate depositional episodes rather than one continuous cycle. Their work traced the Devils Pocket’s dolomite and sandstone across dozens of measured sections, establishing it as a regionally traceable marker bed useful to petroleum geologists working the subsurface of the Williston Basin.

A Rancher’s Name in the Geologic Record

Devils Pocket today occupies a permanent, if narrow, place in American geologic nomenclature. It appears in the United States Geological Survey’s National Geologic Map Database (Geolex), the federal government’s standing registry of formally recognized rock-unit names, still keyed to Gardner’s original type section and still bearing the name of the anticlinal valley from which it was drawn. The Montana Bureau of Mines and Geology has continued to reference the unit in its own regional mapping of the Big Snowy Mountains into the twenty-first century.

What makes the history of Devils Pocket worth telling is not drama but persistence: the patient, unglamorous accumulation of measured sections, fossil identifications, and disputed boundaries by which geologists gradually converted a stockman’s derisive name for an inhospitable badland into a tool for understanding what Montana looked like some 300 million years before any rancher rode across it. The rock does not care what it is called. But the name persists because the men who mapped it, argued over it, and eventually agreed on it needed some word for the place, and the plain speech of the land itself—Devils Pocket—was the word close at hand.

Works Cited

Easton, William H. Carboniferous Formations and Faunas of Central Montana. U.S. Geological Survey, Professional Paper 348, 1962.

Gardner, L. S. "Revision of Big Snowy Group in Central Montana." American Association of Petroleum Geologists Bulletin, vol. 43, no. 2, 1959, pp. 329–349.

Hadley, H. D., and P. J. Lewis. "Catalog of Formation Names for Central Montana and Adjacent Areas." Judith Mountains, Central Montana, edited by D. I. Foster, Billings Geological Society Guidebook, no. 7, 1956, pp. 141–150.

Maughan, Edwin K., and Albert E. Roberts. Big Snowy and Amsden Groups and the Mississippian-Pennsylvanian Boundary in Montana. U.S. Geological Survey, Professional Paper 554-B, 1967.

Reeves, Frank. "Geology of the Big Snowy Mountains, Montana." Shorter Contributions to General Geology, 1930, U.S. Geological Survey, Professional Paper 165-D, 1931, pp. 135–149.

Scott, Harold W. "Some Carboniferous Stratigraphy in Montana." The Journal of Geology, vol. 43, no. 8, pt. 2, 1935, pp. 1011–1032.

United States Geological Survey. "Devils Pocket." National Geologic Map Database, Geolex.