Between fifty-five and thirty-four million years ago, volcanoes erupted in several areas of central Montana, and the upwelling magma that fed them was largely responsible for the Bears Paw Uplift and several other isolated mountain ranges there. As molten rock pushed upward, it sometimes arched the layered rocks above into magma domes called laccoliths, while in other places it rose vertically to form dikes that now appear as old fallen-down stone walls protruding through the ranchlands. Some of the magma crystallized into shonkinite, a rare rock named for exposures near Shonkin, Montana. Some laccoliths later eroded into large buttes, most prominently Square Butte northeast of Box Elder and Centennial Mountain northeast of Big Sandy. While the mountains were rising, an east-west rift developed near the crest, and rocks slid both north and south on Cretaceous marine shales, producing many faults in the adjacent Great Plains. The faults south of the mountains are especially prominent, while most of those to the north are covered by glacial deposits. The oldest exposed rocks in the range are about 350 million years old and consist of Madison limestone, and glacial ice during several glacial periods reached the lower slopes of the mountains except on the south. The origin of the range's name is uncertain, with one legend telling of a hunter trapped by a bear until lightning and thunder killed the animal and severed its paw, and other tales saying the name came from a mountain or spreading ridges that resembled a bear's paw.