Rockbridge is one of the largest natural bridges in Wisconsin, measuring 20 feet wide and 10 feet high. Archaeological investigations have revealed that its sandstone overhang was once used as a shelter by Native Americans. Through the efforts of local citizens, Rockbridge was made a public park in the early 1920s. The West Branch of Pine River flows under Rockbridge and joins the Pine River a few hundred feet to the east. Long ago, however, the West Branch of the Pine River flowed south in its own river valley on the other side of this sandstone ridge. Over time, the West Branch of the Pine River and the Pine River shifted back and forth, gradually scouring away their sandstone river bottoms grain by grain. Eventually the rivers undercut their valleys along the east and west sides of the sandstone ridge, creating the cliff overhang and natural bridge opening here. After passing under Rockbridge, the West Branch of the Pine River merges with the Pine River and flows south toward the Wisconsin River.