Medicine Rocks State Park began as a sea of sand dunes formed about 61 million years ago during the Tertiary Era, when a large freshwater river deposited fine-grained sands along its shores and coastal winds blew the sand into dunes that eventually compacted into soft sandstone. That soft rock was especially susceptible to wind erosion, which hollowed out holes in the rock faces, and the rocks exhibit crossbedding, with thin layers of sandstone lying at angles with thick sandstone beds. Over millions of years, wind sculpted the soft sandstone into many strange and bizarre shapes. Native Americans recognized the distinctiveness of the area early and used it as a vision quest site, meeting place, shelter, and lookout to spot enemies and bison. A Lakota named Charging Bear called the rocks a place "where the spirits stayed and medicine men played." Non-Indian settlers also recognized Medicine Rocks as a place to gather and enjoy one of Montana's geologic wonders.