Forming a topographic wall across northeastern Arizona and northwestern New Mexico, the Chuska Mountains are the remnants of an elongated, sandstone plateau. Almost sixty miles long, the range angles from the NNW to the SSE and sits atop a broader warp of the underlying Jurassic strata, known as the Defiance Plateau, which extends southeastward from the canyon country of Utah.
The sandstone of the Chuska mountains was deposited as windblown dunes but its age remains controversial; initially thought to have accumulated during the Oligocene, some 30-40 million years ago, most geologists now believe that the Chuska Sandstone is late Cretaceous in age (about 65-70 million years old). Heavily dissected by streams, the Chuska plateau ranges in elevation from 8000-9000 feet, with higher pediments, protected by a cap of basalt, rising to 9800 feet or more; these pediments are most numerous in the northern (Arizona) portion of the range and the volcanism that produced their caps occurred during the Pliocene (2-10 million years ago).
Today, the Chuska Mountains and associated ranges provide a high, forested corridor between the San Juans of southwest Colorado and the ranges of the Mogollon Rim, to the southwest; as a result, their varied fauna include elk, mule deer, black bear and mountain lions. To either side of the Chuskas, the terrain falls away to semiarid shrublands of the San Juan watershed; precipitation is greatest on the west side of the range and the deep, scenic canyons of this region include Canyon de Chelly National Monument, the site of artifacts from early American cultures. Now lying within the Navajo Nation, the high spine of the Chuskas provides almost 70% of the Nation's water supply.