The Highwood Mountains and surrounding buttes, including Square Butte - a prominent outcrop near Geraldine were shaped through the erosion of the land surrounding lava that had been pushed up through fissures and faults and into underground pools called laccoliths. Square Butte north of Stanford began life as an intrusive laccolith formed about 75 million years ago which bulged upward and was more resistant than the surrounding sandstone to erosion, leaving a prominent vertical butte towering over the prairie.
The Highwood Mountains are also volcanic but rather than forming underground they are the result of a volcanic eruption about 50 million years ago. They began with an eruption of large volumes of ordinary light-colored magma and ended with the eruption of ‘shonkinite’, a peculiar, dark igneous rock that would be basalt if it were not greatly enriched in potassium. The shonkinite magma crystallized below the surface to form interesting intrusions. Shonkinite contains numerous blocky crystals of black augite set in a fine-grained matrix composed of minute crystals of augite and potassium feldspar. Large volumes of shonkinite exist in the Highwood Mountains but is rare elsewhere. The rock was named for Shonkin, in the Highwood Mountains, and is something of a Montana specialty, even though it does exist in a few other places around the world. ‘Shonkin’ from which ‘shonkinite’ comes is a Native American name for the Highwood Mountains.
Glacial Lake Great Falls and the Shonkin Sag
During The Bull Lake Ice Age (200,000 - 130,000 years ago), a great ice sheet spread southward and dammed the flow of the Missouri River, forming Glacial Lake Great Falls. The ice sheet experienced melting in the summers in addition to water added by the dammed-up Missouri, and the overflow water poured eastward through the Highwoods, over time eroding the large channel known as the Shonkin Sag, which was essentially a great, natural spillway for the glacial lake, and an alternative path for the Missouri River in warmer periods when the ice dam still blocked its other path. The Shonkin Sag is today home to Highwood, which sits in the Sag. A gravel road continues up the Sag to Shonkin, ending at Geraldine on Highway 80 - a scenic drive (in dry weather only!) on which you can witness this dramatic piece of geologic history as you drive around the north end of the Highwood Mountains.