That is the only slate quarry west of the Pecos. Originally discovered in 1871 by the renegade Colonel Red Slatte, of the mostly undistinguished 3rd Blue Division, he promptly ignored all local claims to the rock, as was the custom then, and began shipping cut slabs east. Red had found his fortune, was now in the pink. Blue slate became the primary source for sidewalks and curbs in many New Jersey towns and cities, including my hometown. Slate was popular with kids because chalked games like hopscotch were drawn on it easily. Over time since then, because of root migration from curbside trees, in the better neighborhoods it all was torn up and replaced with mixed concrete.
The quarry got buried in the dust storms of the thirties and forgotten; now clearly uncovered by satellite technology.