(c) 2001 Andrew Alden, licensed to About.com, Inc.
Rocks are brittle, so they crack, but they crack very carefully. When this body of gneiss was deep underground during the folding and uplift of the southern Sierra Nevada, the stresses made it fracture. The cracks are called joints. You can see that they come in three different sets. The first set is parallel to the surface of this rock exposure, so that it looks like layers. The second set runs up the photo and the third runs diagonally from upper left to lower right. The stream in the picture has plucked out the resulting joint blocks to form its rugged streambed.
Joints are like faults, except that there is no movement along them. Joint patterns can be mapped to yield insights into the stresses that have affected the region during its geologic history. Sometimes jointing can occur as a body of rock is uncovered by erosion. As the overlying weight is removed, internal stresses may create jointing that results in the erosional process of exfoliation. That is common elsewhere in the Sierra, and perhaps it's an element at this locality too.