Between 76 and 73 million years ago, during a geologically brief interval of about 3 million years, volcanoes erupted blocks of congealed lava and volcanic ash interspersed with lava flows in this part of the Rocky Mountains. The volcanoes that produced this material were later eroded away. Mount Chisholm is a plug formed when magma intruded the surrounding volcanic rocks, and because this plug is a hard variety of basalt, it eroded more slowly than the softer rocks around it. Narrow ridges a mile northeast of the overlook are dikes formed when magma filled north-south fractures in fragmental volcanic rock; after cooling, the hard rock eroded more slowly than the surrounding material and remained as ridges. The Missouri River gradually cut a valley through this volcanic field to create the present landscape. About 20,000 years ago, the Continental Ice Sheet advanced southward from Canada and formed a dam against the north edge of the Highwood Mountains about 70 miles northeast of here, impounding the Missouri River and creating Glacial Lake Great Falls. This ice-dammed lake flooded the Missouri River valley about 80 miles upstream from the overlook to the vicinity of Townsend, about 90 miles south of here. Around 15,000 years ago, the ice sheet melted enough for the Missouri River to return to its former valley, draining Glacial Lake Great Falls. Mount Chisholm, named for a local settler, is the rocky promontory directly west of the overlook. The Lewis and Clark Expedition passed through this canyon in July 1805, and Meriwether Lewis noted its steep cliffs, mistakenly thinking they were granite. At this overlook, Glacial Lake Great Falls was more than 450 feet deep. The Hardy Bridge, built in 1931 by Great Falls contractors Angus McGuire and Evarts Blakeslee on the old highway below the overlook, spanned the Missouri River and, named for the nearby community, served as an important part of U.S. Highway 91 until it was bypassed by the Interstate in 1968. The bridge later figured prominently in the 1987 film The Untouchables, where it stood in for a fictional United States-Canadian border crossing and the scene of a pitched battle between U.S. Treasury agents, Canadian Mounties, and rumrunners employed by Al Capone. During Prohibition in the 1920s, men regularly crossed into Montana from Canada, smuggling illegal whiskey and other spirits into the United States.