Tuff

Tuff (from the Italian tufo) is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is compacted into a solid rock in a process called consolidation. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered tuffaceous.

• http://en.wikipedia.org/wiki/Tuff
• Page 186 - Schmidt, K. L., et al., 2016, Mesozoic tectonics west of the accretionary boundary in west-central Idaho: A road log along U.S. Highway 95 between Moscow and New Meadows, Idaho: Geol. Soc. Am., Field Guide 41, p. 175-209.