Pictograph Cave

Pictograph Caves State Park is a 23-acre area of three caves (Pictograph, Middle, and Ghost caves) located 5 miles south of Billings. Excavation of the three caves began in 1937, and they were the site of some of Montana's first professional archeological studies. Pictographs are still visible in Pictograph Cave, which is the largest of the three caves. The pictographs are thought to be between 200 and 2,100 years old. However their interpretations are still debated over. The oldest pictograph is that of a turtle, radio-carbon dated to be approximately 2,100 years old.
-from Wikipedia