1. Geology of Caballo and Apache Gap quadrangles, Sierra County, New Mexico [electronic resource] Yale Public content 2005. 2007-09-28T13:14:45Z. 2011-03-29T00:05:33Z. Seager, William R. New Mexico Bureau of Geology and Mineral Resources. English. Public. New Mexico, Sierra County, Caballo Quadrangle, and Apache Gap Quadrangle.