The Ice Ages

By editor

Glasgow, Valley County, Montana

The story of northern Montana is written in stone and ice, in the slow and relentless advance of glaciers that reshaped the land over epochs beyond human memory. About 190,000 years ago, long before any recorded history, colossal sheets of ice crept down from the Arctic, pressing onto the northern Great Plains of Montana. These glaciers, part of the Illinoian glaciation, originated far to the north and northeast, moving southward with immense weight and cold. Though the ice here was thinner than in lands further north, where the Laurentide Ice Sheet reached its thickest, the impact upon the region was profound.

The glacier’s southern margin lay just north of what would become the Milk River valley, a boundary where the ice’s mass thinned and eventually halted. As the glacier advanced, it blocked the Missouri River’s original channel. Rivers, unlike ice, seek paths of least resistance, and so the Missouri was forced to carve a new route around the ice’s edge. This diversion created the Milk River channel between Havre and Wolf Point--a route the Missouri has maintained for the past 130,000 years since the ice melted away.

The retreat of the Illinoian glacier came roughly 130,000 years ago. As the ice melted, it left behind a landscape utterly transformed. The terrain, once swept clean and compressed beneath the ice’s immense pressure, now bore the marks of glaciation in the form of glacial till -- a mixture of clay, sand, gravel, and boulders deposited as the ice receded. Among these deposits lie the erratics, enormous boulders transported hundreds of miles by the ice’s slow movement. Some of these monstrous stones originated in northern Manitoba, carried over the plains, while others came from rock formations west of Lake Winnipeg. These erratics, often granite or other hard igneous rocks, appear incongruous amid the softer sedimentary bedrock of Montana’s plains.

Between these scattered boulders, shallow ponds dot the landscape--remnants of kettle lakes formed when blocks of ice buried in sediment melted, leaving depressions that filled with water. These ponds, though modest in size, preserve a record of the glacial past and provide habitat for a variety of aquatic species. The gravel beds deposited by outwash streams, those rivers of meltwater that flowed from the glacier’s edge, form another distinctive feature of the region’s post-glacial topography.

The Ice Age story did not end with the Illinoian glaciation. About 25,000 years ago, the Wisconsin glaciation brought another sweep of ice into northeastern Montana. This was the last major glacial advance of the Pleistocene epoch, and its influence remains fresh in the geological record. Yet, unlike the earlier glacier, the Wisconsin ice sheet did not cover all of northern Montana. The highlands northwest of Glasgow--formed by older uplifted rock--diverted the glaciers to the east and west, leaving a large ice-free zone between Glasgow and Hinsdale. This refugium provided sanctuary for megafauna during the coldest times.

During the interglacial period that separated the Illinoian and Wisconsin advances, the area was home to a remarkable array of Pleistocene animals. Mammuthus primigenius, the woolly mammoth, roamed alongside Bison antiquus, a giant bison species larger than modern bison, and Equus ferus, the prehistoric wild horse. Fossils of these species have been uncovered across the northern plains, evidence of a steppe environment that supported grazing herds and their predators. The paleontologist Richard Foster Flint, who studied glacial geology extensively, described the landscape as “a vast grassland punctuated by waterholes and sheltering the great beasts of the Ice Age.”

The Wisconsin ice sheet began its retreat from Montana roughly 11,000 to 8,000 years ago, marking the end of the last Ice Age and the dawn of the Holocene epoch. The melting ice released vast quantities of water, contributing to the reestablishment of rivers and lakes and shaping the modern drainage systems of the region. It was during this time that the Milk River and Missouri River found their current courses, carving valleys through the glacial deposits and underlying bedrock.

The geological history of this region held practical consequences for the people who would come much later. By the early 20th century, a road known as the Theodore Roosevelt International Highway, or U.S. Highway 2, was promoted as “the most wonderful Highway in America.” Yet, in Montana, this route was little more than a patchwork of rutted country roads that turned to gumbo mud when wet, a stark contrast to the grand vision of a continuous transcontinental highway. The highway passed through the glaciated terrain of northern Montana, linking communities that had grown up on the plains shaped by the ice sheets.

Captain Meriwether Lewis, during the famed Lewis and Clark Expedition of 1804-1806, recorded observations of the Missouri River’s shifting channels, noting how the river’s course was influenced by the landscape. He wrote, “The river here is very crooked and the banks are low and sandy… it appears to have changed its course often.” His words, though from a time before detailed geological understanding, hint at the ancient ice-driven rerouting of the river. The Missouri’s detour around the glacial ice of the Illinoian epoch shaped the hydrology that Lewis and Clark encountered.

Walking the plains today, one can observe the scratch marks etched into bedrock by the glacier’s abrasive movement. These striations, along with the scattered erratics, tell a story of immense forces at work--slow motion avalanches of ice that ground and polished the earth. The cold air, once thick with the chill of glacial winds, has long since warmed, but the scars remain.

Northern Montana’s Ice Age legacy is a complex layering of geology, hydrology, and biology. The glaciers carved valleys, dammed rivers, and dropped rocks that now serve as clues to scientific inquiry. The animals that survived between the ice sheets, the rivers diverted by frozen walls, and the gravel plains left behind form a natural archive. As the naturalist John Wesley Powell once said, “The fields of geology have a vast and enduring interest, for they teach us the story of our world’s past and the forces that have shaped it.”

In this land shaped by immense ice and time, the present is inseparable from the ancient past. The glaciers that once encased Montana’s northern plains were agents of transformation, reshaping the earth on a scale and timespan that challenge human imagination.

See also

• The Ice Ages at Glasgow, Valley County
• The Great Inland Seaway at Park City, Stillwater County
• Paleocene Mammals and Albert Silberling at Harlowton, Wheatland County