Mammoth rubbing rocks, Sonoma, California

Sonoma Coast State Beach in northern California is famous for its Mammoth Rubbing Rocks.  The rocks were polished smooth by the wooly hair of mammoths over 10,000 years ago.  Mammoths used the rocks as a scratching post, the same way that elephants rub against tree trunks today.

There is even a depression in the ground around these rocks that may be a relic mammoth wallow.

During glacial times the sea was 100 meters lower than today, putting the beach perhaps 20 kilometers further out to sea than it is now.   In the wetter climate of glacial periods, without the dry summers of modern times, the Sonoma coast was a wide grassland that supported herds of large herbivorous mammals, a "California Serengeti." Those animals included prehistoric species of bison, camels, ground sloths, horses and mammoths. All of these herbivores are known to rub themselves against large rocks or trees as part of their grooming behavior. 

Those vast herds of large animals are now gone, for reasons unknown, but the evidence of their happy existence for millennia is still with us.