Tuff - welded volcanic ash

Deep deposits of volcanic ash that are welded by weight of ejecta above them and their own heat. The rock is very lightweight, like pumice, but is formed by a different process. Red colors indicate various amounts of oxidized iron.
This is in the Jemez Mountains of north-central New Mexico, USA, which are one large volcanic area. Back in the 1980s, when "stone-washed" denim became fashionable, a lot of tuff was mined from private lands in the Jemez Mountains, much to the displeasure of the locals: why ruin our mountains just for some new-fangled fashion?
The tuff is soft enough that elsewhere in the Jemez, particularly at Bandelier Nat'l Monument and its outlying Tsankawi Pueblo, the ancient natives carved small, individual dwellings back into the tuff.
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