This is a brecciated mudstone from a northern California beach. It started as a simple rock, but some time after it consolidated, something—probably motion along a fault—shattered it and cemented it together again. The matrix between the clasts appears to be the same substance as the clasts, although there are also a few small veins of silica from the process of brecciation.
Mudstone Breccia
Picture Gallery of Sedimentary Rocks
This breccia specimen is a sedimentary rock—specifically a cherty mudstone—that was brecciated by the action of faulting. (more below)
Image (c) 2001 Andrew Alden, licensed to About.com (fair use policy)
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