Evidence of the Pleistocene “Ice Ages” can be found along the Sonoma Coast north of San Francisco today.  On a bone-chilling cold and windy day a few decades ago, two scientists were checking out an area north of Bodega Bay near Goat Rock. To take a break away from the blasting onshore winds that frequently occur in the area, they took shelter on the leeward side of one of the onshore sea stacks, or tall rock outcrops. Happening to look up, to their surprise they saw highly polished rocks that extended far above their heads. Examining the rough and fractured outcrops more closely, they found other polished rocks, also smoothed from a few feet above the ground surface to up to 14 feet or so high. Typically, these occur in specific places along the edges of intersecting rock walls, or at overhangs, and on the leeward sides of rocks away from the wind.


There are many hypothetical geologic explanations for polished rock – including abrasion from ice, wind and water; frost action; chemical dissolution and precipitation; and others. None of these explanations fit the polish on the outcrops near Goat Rock.  There had to be another explanation. Animals.

In a common grooming behavior, many large mammals like to rub themselves against rocks, trees, and other solid objects to relieve itches and remove parasites. Polished areas below about 6 feet high can be attributed to the cattle and sheep pastured in the Goat Rock area during the past century. The polished areas extending much higher could not have been reached by these domestic animals. After the outcrops were examined by numerous scientists, and several alternative hypotheses were discussed, only one source for this polish was clear. Mammoths. Mammoths rubbing their coarse fur against the rock.

Pleistocene mammoths (Mammuthus columbi) are known to have occupied this coastal region – and these gigantic animals stood as high as 14 to 16 feet tall at the shoulder. While rubbing their woolly coats against the rock, possibly over a time span of thousands of years, they smoothed the highest rocks into the gleaming surfaces that can be seen today. Check out the photo of my son Eric, a bit over 6 feet tall, standing next to a Columbia mammoth model at the La Brea Tar Pits museum in Los Angeles. Scary!

Mammoths, along with many other large mammal species, became extinct around the globe roughly 10,000 years ago. The reasons for this are controversial, but some combination of over-hunting by expanding populations of humans is likely a major contributor, along with climate change and habitat loss. IF mammoths still roamed Sonoma County today, and I happened to encounter a herd – or even just one — of these enormous creatures, I’d run!