Some 700,000 years ago volcanic vents to the northwest, in the Long Valley area, erupted and spewed clouds of hot glowing rhyolitic ash and rock particles. Moving with the speed of an avalanche, the ash blanketed the landscape for great distances. Because it was so hot, the ash particles fused together to form the rock structure that makes up the Volcanic Tableland. The landscape became a broad, gently sloping sheet of ash material averaging several hundred feet in thickness. The solid ash deposit is commonly known as Bishop tuff.
Bishop tuff can be white through tan and pink to reddish brown.
Pick up a piece of tuff rock and notice its lightness. The small holes or pores you see on the surface of the rock allow water to pass through quickly. Commercially, tuff is used as pumice or landscaping stone. After the volcanic eruptions, faulting action deformed and warped this smooth ashen sheet into a rumpled, twisted landscape. Weather, wind erosion, and continued faulting are still changing this area today.