This is rhyolite country

Imagine that it is 1860 — one hundred and fifty ago — and that you are standing at what is now the corner of Shattuck and University. If you looked north and east, towards the hills, what would you see? Not many buildings or people: there are only a few scattered farms about, and the name “Berkeley” hasn’t been thought up yet. You would see grass-covered hills and some oak trees near the streambeds. You would also see, jutting out from the ground like huge broken teeth, a number of large rock outcroppings. One of these rocks was particularly significant: the Rancho San Antonio boundary stone — the northern edge of the Peralta family land grant that stretched south to San Leandro.

The outcroppings are made of rhyolite, a volcanic rock which solidifies at the earth’s surface (in contrast to granite, which solidifies deep undeground). Our particular variety of rhyolite was named Northbrae rhyolite in 1914 by Andrew Larson, a U.C. geology professor who would later serve as chief geologist for the Golden Gate Bridge.

As Northbrae was being developed by Mason-McDuffie, owner Duncan McDuffie agreed to donate many of the outcroppings to the city to become neighborhood parks. Today we know them by names such as Indian Rock, Grotto Rock, Mortar Rock, and Great Stone Face.