The intrusion (dike) which makes the Snake Dike route on Half Dome. The dike is created when molten lava squeezes into the cracks of older, solid rock. In the case of Snake Dike, the volcanic intrusion is harder than the surrounding granite, so it erodes more slowly. This results in a projecting ridge up the side of Half Dome.    Yosemite -- 7/8/2006