The Invisible Line:

How Montana Got Its Northern Border in the Summer of 1874

ORIGINS OF AN UNMARKED BOUNDARY

For more than half a century after diplomats in London and Washington agreed that the northern edge of American territory west of the Lake of the Woods would run along the 49th parallel of latitude, that boundary existed mostly on paper. The Anglo-American Convention of 1818 had drawn the line with a ruler across maps that were, in the empty stretches west of Minnesota, largely guesswork. No one had walked it, measured it, or marked it on the ground. Two limited efforts tried to locate the line near the Red River, one under Stephen Long in 1823 and another under Major Samuel Woods in 1849, but neither extended the survey westward into what would become Montana Territory. As fur trade, homesteading, and rail construction pushed settlement north and west through the 1860s, an unmarked border became a practical liability rather than a diplomatic abstraction, and both governments finally agreed to fund a joint commission to survey and permanently mark the line from the Lake of the Woods to the crest of the Rocky Mountains.

ASSEMBLING THE COMMISSION

The Northern Boundary Commission organized in 1872 under American commissioner Archibald Campbell, with Major William J. Twining of the Army Corps of Engineers serving as chief astronomer, the officer who would carry primary responsibility for the fieldwork. A British commission under D. R. Cameron worked in parallel, the two teams operating under an agreement that split the labor along the parallel itself: American surveyors would fix astronomical stations and map a belt of territory five miles wide south of the line, while their British counterparts did the same to the north. The commissions spent the winter of 1872–73 organizing at Pembina in Dakota Territory before beginning fieldwork the following spring, completing 408 miles of survey between the Red River and points west by the close of the 1873 season, along with the connecting topography east to Rainy River during the following winter.

THE MONTANA SEASON OF 1874

The work remaining after 1873 was formidable: 358 miles of unsurveyed boundary between longitude 106°12’ and longitude 114°05’, reaching from the Dakota plains across nearly the full width of Montana Territory to the Continental Divide. The steamboat Fontenelle left Bismarck in the spring of 1874 carrying seventy-five men, one hundred fifty animals, and thirty wagons, along with forage and supplies for the survey crew under Major Twining’s command, while the steamboat Far West carried the military escort assigned to protect the party. That escort, drawn from five companies of infantry and ten companies of cavalry, was placed under the command of Major Marcus Reno, and while only 357 miles of boundary remained to be surveyed, most of that distance passed through what contemporaries described as hostile Indian country. Reno’s presence on the 1874 survey is a detail easily overshadowed by what came two years later on the Little Bighorn, but in the summer of 1874 he was simply the officer tasked with keeping a scientific party alive while it worked its way toward the Rockies.

Twining’s own account of the season, filed with the State Department that December, describes the logistics in blunt terms. He organized the parties in St. Paul on the first of June and traveled by rail and river to Fort Buford, where the advance working parties reached the boundary line on the first of July. Because the season was short and the distance still to be covered after the survey’s completion was immense, the work had to finish by early September, and the parties were, in Twining’s words, pushed to the utmost limit of their endurance. By September 1, the eight astronomical stations assigned to the American commission had been fixed, and the line had been connected with the terminal station of the earlier Northwest Boundary Survey at the summit of the Rocky Mountains, the survey that had marked the parallel from the Pacific coast inland during the late 1850s and early 1860s. The commission then returned from Fort Benton to Bismarck, a journey recorded in official reports as twelve hundred miles by river, in open boats in eighteen days, and the men were discharged on October 5. In four months the expedition had covered thirty-seven hundred miles of travel, nine hundred by land and twelve hundred by water, while surveying and marking 358 miles of boundary.

ACROSS THE PLAINS TO THE MOUNTAINS

Twining’s report also preserves a working surveyor’s impression of the country he crossed, and it is worth reading for what it says about Montana Territory on the eve of major white settlement. The land east of the Milk River crossing he found an open plain without timber, generally passable for wagon trains except near Frenchman’s Creek and the Milk River itself, where badlands forced wide detours. Between longitude 106° and the Milk River he judged the country unattractive, with light rainfall, scarce water in summer, and alkaline soil supporting mostly sagebrush and cactus. From the Sweet Grass Hills west to the Rockies, however, the character of the land changed entirely: rainfall appeared ample, and the belt along the mountain front combined striking scenery with, in his words, unsurpassed fertility. He noted, too, that northwestern Montana still supported immense herds of buffalo, observing in his field notes that their numbers seemed to be increasing locally in 1874—though broader historical records confirm that northern buffalo herds were actually in rapid ecological decline across the decade. He identified the region as the country of the Blackfeet and Piegan, contested ground shared with the North Assiniboine, the Gros Ventre of the prairie, and the River Crow, with occasional Sioux war parties ranging as far northwest as the Sweet Grass Hills, and he noted that with the exception of the Sioux, these tribes appeared peaceably disposed toward the commission. That single line, tucked at the close of an official report otherwise devoted to mileage and monuments, is a rare and valuable piece of testimony about the intertribal geography of north-central Montana in the years immediately before the region’s remaining open range was closed by reservation boundaries and white settlement.

ENCOUNTERS ALONG THE LINE

The threat Twining alluded to was not abstract. Near the Three Buttes, later remembered as a long stop on the survey’s route, the party drew the curiosity of a band of Hunkpapa Sioux under the warrior Long Dog, the same man who two years later would take part in the destruction of Custer’s command at the Little Bighorn and who would later trouble the North-West Mounted Police after the Sioux took refuge in the Wood Mountain country of Canada. Reno’s cavalry, present specifically to guard against this kind of encounter, escorted the survey without the violence that many on the expedition had reason to fear. The episode illustrates a broader truth about the Northern Boundary Survey: it was not simply a technical exercise in astronomy and triangulation but a military undertaking staged across land whose Indigenous inhabitants had not been consulted about the line being drawn through it, and whose presence shaped nearly every operational decision the commission made that summer.

SCIENCE ALONGSIDE THE SURVEY LINE

The commission’s work was not confined to latitude and longitude. Its naturalist, Elliott Coues, a U.S. Army surgeon and ornithologist attached to the expedition, spent the 1873 and 1874 seasons cataloging the bird life along the route. Coues’s resulting field notes describe a line surveyed by the commission that extended 850 miles from the Red River of the North to the Rocky Mountains along the 49th parallel, and they stand as one of the earliest systematic scientific records of the wildlife of the high plains straddling the Dakota-Montana boundary. That such a record survives at all is a reminder that the boundary commissions, whatever their primary diplomatic purpose, doubled as vehicles for the era’s broader push to inventory the natural resources and geography of the American West. The expedition also attracted at least one artist: William de la Montagne Cary, whose 1874 oil sketch of the survey party under Major Twining is held today by the Gilcrease Museum in Tulsa, offering a rare visual record of the commission’s work on the Montana plains.

COMPLETING THE LINE AND MARKING IT PERMANENTLY

Reaching the Rockies in September 1874 completed the survey portion of the project, but it did not finish the work of marking the boundary on the ground. Along the stretch surveyed in 1873, ninety miles of markers had been placed only temporarily, pending resolution of a technical dispute over the precise astronomical definition of the 49th parallel, and permanent monuments still had to be set in their place. The building of monuments along the border continued for another full year after the 1874 survey season closed, with crews placing iron posts, stone cairns, and earth mounds at intervals along the line through 1875. The Montana Historical Society’s archival holdings from this same period, including contemporary maps of trading posts along the Milk River reservation, help situate the survey within the broader commercial and administrative landscape the boundary was cutting across, a landscape still organized around fur trade posts and unceded tribal land rather than the fenced, surveyed townships that would follow within a generation.

LEGACY

The Northern Boundary Survey of 1874 gave Montana Territory, and eventually the state of Montana, its northern edge in a form more durable than treaty language alone could provide. It did so through unglamorous, exhausting labor: astronomical stations fixed every twenty to forty miles, plane-table topography carried out under military escort, and hundreds of miles of open plain crossed by wagon and pack train in a single compressed season. The line the commission surveyed has needed remarkably little correction in the century and a half since, a fact that speaks to the discipline of Twining’s astronomical work as much as to the emptiness of the country he crossed. It also marks one of the last moments in which the region’s Indigenous nations, whose territory the line bisected without their consent, were dealt with primarily as a security risk to be managed by cavalry rather than as parties with a stake in where the border fell. The survey’s own records, preserved in State Department correspondence, scientific field notes, and regional archives, remain the clearest window available into how that boundary, so unremarkable to look at on a modern map, was actually drawn.

WORKS CITED

Davis, Jim. “Northern Boundary Survey.” Dakota Datebook Archive, Prairie Public, 25 May 2022, https://news.prairiepublic.org/show/dakota-datebook-archive/2022-05-25/northern-boundary-survey. Accessed 19 Aug. 2026.

“Map Showing Location of Trading Posts in 1874 on Milk River Reservation – Judith Basin.” Montana History Portal, Montana Historical Society Library and Archives, https://www.mtmemory.org/nodes/view/87792. Accessed 19 Aug. 2026.

“Northern Boundary Survey under Major Twining.” Gilcrease Museum, by William de la Montagne Cary, 1874, https://collections.gilcrease.org/object/017. Accessed 19 Aug. 2026.

“RCMP Heritage Stories.” Boundary Trail National Heritage Region, http://www.bthr.ca/boundary-trail-archives/boundary-archives/. Accessed 19 Aug. 2026.

United States Geological Survey. Coues, Elliott. Field Notes on Birds Observed in Dakota and Montana along the Forty-Ninth Parallel during the Seasons of 1873 and 1874. 1878, https://www.usgs.gov/index.php/publications/field-notes-birds-observed-dakota-and-montana-along-forty-ninth-parallel-during. Accessed 19 Aug. 2026.

United States Department of State, Office of the Historian. “Mr. Campbell to Mr. Fish, Enclosing Report of Major W. J. Twining, Chief Astronomer.” Papers Relating to the Foreign Relations of the United States, 1874, https://history.state.gov/historicaldocuments/frus1874/d2. Accessed 19 Aug. 2026.