Tuff and lava rock, Tuff Canyon; Big Bend National Park, TX. Tuff consists of volcanic ash with silica a major component. The tuff, which is Oligocene in age (the Oligocene occurred 24-34 million years ago), lies on top of the older Bee Mountain Basalt (erupted 34-30 million years ago. Tuff can be in the form of a friable ash as in the foreground or solidified into rock-like material as seen in the background. The cactus is the blind pear, Opuntia rufida.