Rocks interact with animals of all kinds. Obviously, lizards and voles and snakes and woodchucks live on rocks and/or dig under them. Humans paint on rocks and move them around and blow them up. Today, however, I’m going to talk about animals that scratch themselves against rocks — rather, rocks that animals have rubbed for thousands of years.
Last week John Christian, a sharp-eyed and inveterate walker of our hills, showed me this outcrop next to the Little Farm in the Tilden Nature Area.
Not much to look at unless you get up close. When you do, you’ll see that it’s covered with moss and lichens, except for some oddly smooth bare spots on the outermost surfaces.
Some of these are smooth enough to gleam in the sun.
These features are well known in buffalo country, elephant country and other places around the world. Large herbivores deal with the mites and lice and other irritants in their skin by rubbing themselves against anything scritchy they can find, preferably after a nice wallow in high-quality mud or at least a good roll in the dust. These marks, in a word, are sandpapered onto the rocks.
This outcrop appears to have gotten its smooth spots from that cause. But the cover of lichen and moss shows it hasn’t been used in a very long time. Today, deer have plenty of trees to use, but historically — and prehistorically — most of coastal California was treeless because the Indians kept it that way with regular fires. However, deer aren’t tall enough to make most of these marks. Now during the ice ages, though, this was a treeless cold savanna that supported herds of elk and mammoths and ground sloths and horses and bison and camels. Could those extinct animals really have buffed these boulders?
The best case for that is on the Sonoma coast just south of Jenner. I wrote a piece about the “mammoth rocks” there, which you’ll have to pick through on the Internet Archive because the folks who paid for it threw it away. The archaeologist who discovered the site has also written it up. John and I both know that site, but I never thought to look around our own hills whereas he did. When we hiked a little farther down Wildcat Canyon and he showed me a polished boulder of the same blueschist found on the Sonoma coast, I had a shiver of recognition.
Berkeley is justly famed for the rock parks in its boulder-strewn northern hills. They, like the Little Farm outcrop, feature the Northbrae Rhyolite, a particularly tough volcanic rock that you have to climb to appreciate. Even the tiniest fingerholds are as solid as steel.
At Indian Rock, generations of climbers haven’t buffed anything smooth. Nevertheless you’ll see two kinds of smooth spots. There are slickensides, formed where rocks slide against each other.
And then there’s this wide, vertical rock face that looks like it might be very inviting to an itchy mammoth or ground sloth. And it’s polished.
A few other Berkeley rocks display the same kind of evidence, if you look closely. But I had to see if Oakland can boast it too. Thinking like a mammoth, I visited my favorite Oakland blueschist outcrop in Knowland Park to reconnoiter. It looked very mammoth-friendly, including a good site for a wallow in the headwaters of Upper Elmhurst Creek.
But no such luck. Every surface of the outcrop was rough and rugged as can be. The same with this notable serpentinite knocker farther down the stream valley.
I blame Oakland’s rocks. We don’t seem to have anything as tough as the Northbrae Rhyolite, capable of retaining a polish for tens of thousands of years. But I’ll keep my eyes open; you never know.
Oakland Geology 
22 March 2017 at 8:21 pm
A wonderful post, Andrew!
17 March 2017 at 3:53 pm
Years ago, i found a rock in the woods of Knowland Park which has very shiny, smooth marks exactly like the ones you are showing which were polished by large mammals. it’s not huge but approximately chest or chin-high. i’ll try to find it and get some pics.