West of Fort Peck Lake, in northeastern Montana, a cluster of domes and buttes rise above the Great Plains. Almost 200 miles east of the Rocky Mountain chain, this isolated uplift is known as the Little Rocky Mountains. While their namesake range was formed by compression within the North American craton, the Little Rockies developed as a dome of cooling magma pushed up through an overlying layer cake of Precambrian, Paleozoic and Mesozoic sediments.
This uplift occurred about 60 million years ago, in the early Tertiary Period. Erosion has since removed the Paleozoic and Mesozoic sediments from the center of the dome and has carved their uplifted edges into a maze of ridges and valleys. As one might expect, the sedimentary rocks ring the dome, decreasing in age from central areas toward the periphery; the primary exposures are of Cambrian sandstone and shale, Ordovician dolomite, Mississippian limestone (Madison limestone), Jurassic limestone and Cretaceous sandstones and shales.
Antoine Butte, 5740 feet, is the highest point in the Little Rocky Mountains, rising 2500 feet above the surrounding plains. The numerous streams that have sculpted the uplift drain northward to the Milk River or southward to the Missouri. Pine forest cloaks most of the Little Rockies, which produce a sky island amidst the drier grasslands, attracting a wide range of western wildlife; among the latter are mountain lions, bighorn sheep, mule deer, Clark's nutcrackers, Lewis' woodpeckers, pinyon jays, gray-crowned rosy finches and western tanagers.
