Breccia

Here is a nice example from the Oakland hills of breccia, a kind of rock that consists of broken pieces of rock. There are several different ways to make a breccia, and not having a lab and a petrographic microscope I can’t specify what made this. But considering that the hills have been raised by vigorous tectonic action accompanied by earthquakes, it’s easy to imagine some seismic event opening a fracture, shattering the rock, and allowing mineral-charged fluids into the space. Other ways involve dissolving the rock until it collapses, slumping while the rock is young and poorly consolidated, and simply cementing together a pile of landslide debris or volcanic deposits.

Because breccia really signifies an activity rather than a material, geologists would rather think about brecciation when they see a breccia.

UPDATE: I used to have more breccia pictures on About.com, now preserved on archive.org.