Square Butte and Shonkin Sag

Between the communities of Square Butte and Geraldine, two prominent geologic features dominate the skyline. About 50 million years ago, the Highwood Mountains had a sizeable volcano that erupted a rare rock called shonkinite, a dark rock similar to basalt. About 75 million years ago, magma injected eastward between layers of sandstone laid down along the shores of a broad inland sea extending to the southeast. Locally, that magma bulged up to form a huge blister or “laccolith” that solidified to form the immense flat-topped Square Butte to the south. The vertical cracks that fed magma to the laccolith radiated out from the center of the volcano and are called dikes. Square Butte is one of many laccoliths scattered from the Adel Mountain Volcanics south of Great Falls to the Bears Paw Mountains near Havre. During the Bull Lake Ice Age between 70,000 and 130,000 years ago, the continental ice sheet spreading south from Canada blocked the Missouri River, backing up its water to form Glacial Lake Great Falls. The glacial meltwater poured eastward through the Highwoods to erode Shonkin Sag, the large valley extending west, north of Square Butte. Shonkin Sag is one of the most famous abandoned meltwater channels in the world.