Before it solidifies into stone, granite is a viscous pudding of feldspar, quartz, and (usually) hornblende and/or mica. These minerals vary in hardness, and their distribution within a given granite formation can be far from homogeneous. The upshot is that a granite cliff, slab, or boulder seldom erodes uniformly. More durable areas of the rock surface will more stubbornly resist weathering, sometimes creating fantastic, mushroomlike protuberances—typically three to twelve inches across, and projecting a similar distance from the adjacent plane—as the surrounding stone erodes away. These weird protrusions are known as chickenheads or knobs. Snow Creek Wall in the Wenatchee Mountains of Washington State and several of the cliffs overlooking the western entrance to Yosemite Valley are renowned for their abundance of chickenheads

Jon Krakauer