A series of rubbled hills runs alongside Highway 89 about thirty miles north of Flagstaff. They look fairly unremarkable, but really, they’re remnants of mines that once produced pozzolan.
Pozzolans are a class of silica- and aluminum-rich materials commonly derived from explosive volcanic deposits. The early Romans knew of their value, incorporating them into aqueducts and buildings. The word pozzolan comes from a town in Italy.