New Mexico Geological Society Annual Spring Meeting — Abstracts

The Gila Cliff Dwellings were constructed by the Mogollon people, in seven natural caves or alcoves, in Gila conglomerate about 800 years ago The caves are in a south-facing wall of Cliff Dweller Canyon, which is occupied by a small east-flowing tributary of the West Fork Gila River. The cliff dwellings are the main attraction at the Gila Cliff Dwellings National Monument. They are easily accessible at the main eastern portal to the Gila Wilderness, about 40 miles north of Silver City.

The caves that house the cliff dwellings are all at about the same stratigraphic horizon, along subtle weak zones in the conglomerate. The conglomerate consists largely of pebbles and boulders of Bloodgood Canyon Rhyolite Tuff and Bearwallow Mountain Andesite, derived from volcanic rocks in the adjacent Mogollon Mountains and deposited by rivers similar to the present day Gila.

The caves probably were initiated by lateral stream erosion and spring sapping about 500,000 years ago. At that time the stream in Cliff Dweller Canyon was at the level of the caves, approximately 65 meters above the present stream. Other alcoves now being carved by this stream , along the lower trail in Cliff Dweller Canyon, are probably forming by the same processes as the caves above. In contrast, however, the zone of weakness localizing these new alcoves is the contact of the conglomerate with the underlying andesitic lava flows.

Once the stream in Cliff Dweller Canyon had cut below the level of the future cliff dwellings, the caves continued to be enlarged mainly by roof collapse along planes of weakness such as joints and sedimentary bedding planes, aided by pressure-release exfoliation and gravity.

Pressure-release exfoliation not only contributed to the enlargement of the caves, but also modified the shape of the cave openings. The exfoliation process is perhaps best known for shaping large domes in intrusive granite terrains such as Yosemite National Park, and closer by, at Enchanted Rocks State Park on the Llano uplift, north of San Antonio, Texas. The unroofing of large granitic batholiths, by erosion of overlying rocks, creates stresses that are relieved by fractures oriented parallel to the land surface, or free surface, which produces large exfoliation slabs. Thus, on a much reduced scale, exfoliation is acting at all the free surfaces of the cliff dwelling caves, and has produced some striking examples of small scale pressure release exfoliation. The most defining aspect of pressure release exfoliation here is the way it ignores sedimentary bedding to conform to the cave openings. Also noteworthy, whether intentional or not, is the similarity of the masonry of the Mogollon builders to the natural architectural style of the exfoliation, particularly where the two are juxtaposed.