The Marin Headlands features “excellent exposures” of pillow basalt, Sloan wrote, which is essentially “ancient ocean crust” composed of basaltic lava flows and so-called pillow lava.

After visiting a couple of less satisfying turf labyrinths in Oakland and Fairfax, I found myself pursuing a rumor in the Marin Headlands. Supposedly there was a labyrinth here, on a point above ...