A Pleistocene Wonderland

By editor

Cut Bank, Glacier County, Montana, 12,000 years ago

If you had stood on the northern Great Plains of Montana 12,000 years ago, you would have recognized the Rocky Mountains to the west and the Sweetgrass Hills on the horizon to the north. But the animals grazing on the hummocky grassland before you would have been largely unfamiliar.

Great herds of horses, pronghorn antelope, elk, camels, and giant bison moved across the plains. Among them wandered blond-haired Shasta ground sloths -- nine feet long, walking on their knuckles -- and shaggy musk oxen. Columbian mammoths with long, curving tusks roamed in small groups. Dire wolves and short-faced bears followed the herds in search of easy meals. The short-faced bear, at ten feet in length and more than 2,000 pounds, dwarfed today's grizzly in size and ferocity.

The Pleistocene epoch, which lasted from approximately 2.6 million years ago to 11,700 years ago, was the age of great ice sheets and great mammals. During the last glacial maximum, approximately 20,000 years ago, ice covered most of Canada and extended well into the northern United States. The Laurentide Ice Sheet, which covered most of eastern North America, and the Cordilleran Ice Sheet, which covered the mountains of the West, together buried much of Montana under thousands of feet of ice. The land that is now the Cut Bank area was near the edge of the ice, in a zone of periglacial tundra where the ground was permanently frozen and the vegetation was sparse.

As the ice retreated, beginning about 15,000 years ago, the landscape changed rapidly. The meltwater carved new river channels, deposited new sediments, and created the lakes and wetlands that attracted the great herds. The animals that had survived the glacial maximum by retreating south or finding ice-free refugia moved north into the newly available land. The mammoths and mastodons, the ground sloths and camels, the horses and giant bison: all of them expanded their ranges as the ice retreated.

The first people arrived in Montana during this same period of rapid change. The Clovis people, named for the distinctive fluted spear points first found near Clovis, New Mexico, were hunting mammoths and other large game in Montana by at least 13,000 years ago. Their campsites and kill sites have been found throughout the state. At the Anzick site in Park County, Montana, archaeologists found the remains of a Clovis child buried with more than 100 stone and bone tools, including spear points and foreshafts. The child was buried approximately 12,600 years ago, making the Anzick site the oldest known burial in the Western Hemisphere.

The great mammals did not survive the end of the Pleistocene. Between 13,000 and 10,000 years ago, thirty-five genera of large mammals went extinct in North America, including the mammoths, mastodons, ground sloths, horses, camels, short-faced bears, and dire wolves. The cause of this extinction is debated: some scientists emphasize the role of human hunting, others emphasize the role of climate change, and most believe that both factors were involved. What is not debated is the result. The Pleistocene megafauna of North America are gone, and the grasslands of Montana are occupied by a much reduced fauna: bison (nearly exterminated in the nineteenth century, now recovering), pronghorn, elk, deer, wolves (exterminated in the twentieth century, now recovering), and grizzly bears (reduced to a fraction of their former range).

The Sweetgrass Hills, visible from the Cut Bank area on clear days, are a cluster of isolated buttes rising from the plains. They are composed of igneous rock intruded into the sedimentary layers of the plains about 50 million years ago. During the Pleistocene, they would have been landmarks for the animals and people moving across the open grassland, visible from great distances and useful for navigation. They are still visible from great distances. The animals that once used them as landmarks are mostly gone.

See also

• A Pleistocene Wonderland at Cut Bank, Glacier County
• Early Horse History at Miles City, Custer County
• The Ice Ages at Shelby, Toole County

The horses that roamed the Pleistocene plains of Montana were not the horses that Lewis and Clark encountered in 1805. Those horses -- the ones ridden by the Shoshone and the Blackfeet and the Crow -- were descendants of horses brought to North America by the Spanish in the sixteenth century. The Pleistocene horses were a different species, smaller and more varied, and they had been in North America for millions of years before they went extinct at the end of the Pleistocene. The horses that returned with the Spanish were returning to a continent their ancestors had left. The Crow and the Shoshone and the Blackfeet who adopted the horse in the seventeenth and eighteenth centuries were adopting an animal that had been part of the North American landscape for millions of years before it vanished and returned.