Although to what extent Columbian mammoths migrated is unclear, an isotope analysis of Blackwater Draw in New Mexico indicated that they spent part of the year in the Rocky Mountains, away. The study of tusk rings may aid further study of mammoth migration. On Goat Rock Beach in Sonoma Coast State Park, blueschist and chert outcrops (nicknamed "Mammoth Rocks") show evidence of having been rubbed by Columbian mammoths or mastodons. The rocks have polished areas above the ground, primarily near their edges, and are similar to African rubbing rocks used by elephants and other herbivores to rid themselves of mud and parasites. Similar rocks exist in Hueco Tanks, Texas, and on Cornudas Mountain in New Mexico.