Lake Musselshell was a former glacial lake in Montana that occupied the lower, north-flowing Musselshell River basin during the Pleistocene. It was one of several proglacial lakes created when lobes of the Laurentide Ice Sheet advanced into eastern and central Montana and blocked major drainages during the late Wisconsin glaciation. The basin, underlain by easily eroded Cretaceous rocks and shaped by folds, faults, and joints, preserves important evidence of this history. Nine terraces and more than 100 glacial boulders remain, with the boulders resting on the terraces, showing the terraces are older. The site records major Ice Age drainage changes in Montana and the Laurentide ice’s impact on the Missouri River basin.