(c) 2002 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Knockers are the rocky knobs on this central Californian hillside. The term is used informally by local geologists. They're too big to be boulders and too small to be hillocks, tors, inselbergs or any other kind of landform. The term is used in areas of mélange, a body of mixed rocks of all sizes and types.
A large part of western California is mapped as mélange of the Franciscan formation. It's peculiar rock, with almost no large-scale internal structure, largely made of serpentinite and other low-grade metamorphic rocks typical of high pressure and low temperature. It was a mystery until the theory of plate tectonics explained it as the material scraped off a subducting oceanic plate, deeply buried and then quickly raised back to the surface. You see hillsides with knockers like this throughout coastal California, from the Oregon border down to near San Luis Obispo.