Zoned welded tuff from the climactic eruption of Mt. Mazama, which formed Crater Lake. The tuff is darker (lower-silica) on top and lighter (higher-silica) below. This is interpreted to represent an inverted stratigraphy of the magma chamber; the lighter, higher-silica magma was higher in the chamber (owing to lower density) and was erupted first. Trivia: the steep-sided conical erosion forms are called "hoodoos".