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City
of Rocks State Park
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The Butterfield Overland Mail Route began in 1858 and passed south of the park. It ceased to operate in 1861 because of the Civil War.
The railroad reached Deming in 1881 bypassing City of Rocks, although settlers coming into the area camped and picnicked at the site for decades, just as people do today.
The rocks forming the City of Rocks, the Kneeling Nun Tuff, were produced by a very large volcanic eruption that occurred 34.9 million years ago. The Kneeling Nun Tuff was formed by a violent eruption of volcanic pumice, ash, and hot gas in an eruption 1000 times greater than the May 18, 1980 eruption of Mount St. Helens. The entire eruption of the Kneeling Nun Tuff would have taken from months to years. The history and style of eruption can be determined by studying the rock seen at City of Rocks, and elsewhere. The initial stages of the Kneeling Nun eruption are likely to have produced large volumes of pumice that rained out of the sky, blanketing the surrounding countryside. This stage is referred to as the plinian eruptive phase. The magma that produced the eruption was probably located between 6 and 15 kilometers below the earth’s surface. A pipe, or conduit, formed between the magma chamber and the earth’s surface, allowing the magma to escape through a volcanic vent.
During cooling of the Kneeling Nun Tuff, cooling cracks, or joints, would have formed. These joints form perpendicular to the cooling surfaces, so are oriented perpendicular to the ground surface. Through time, the cooling joints and other cracks are widened by a number of erosional processes. Freeze- thaw action and wind are two major factors, and vegetation growing into cracks, and acid produced by vegetation may be important as well. As material in the cracks becomes loosened by freeze- thaw and vegetation, finersized material is stripped away by the wind, causing the cracks to become enlarged. The length of time required to form the current City of Rocks is not well known. However the multiple flares observed in the pillars suggests that several major periods of weathering have occurred and that the pillars may have formed in a series of episodes, rather than by a slow, continous process. Continued erosion and modification of the pillar shape continues today and, in several million years, the City of Rocks may be eroded to a flat plane, while other “City of Rocks” may have been formed in other similar volcanic deposits in the area. To the north- northeast, the prominant peak is Mimbres Peak on Whitehorse Mountain, composed entirely of volcanic rocks. Possibly visible on the distant skyline beyond is the crest of the Black Range, one of the major mountain ranges in New Mexico. Nearby, to the northeast, is Table Mountain with an upper and lower cliff of welded rhyolite tuff and slopes of softer bedded rhyolite tuffs, all part of the Sugarlump Formation. To the east is Round Mountain, composed of andesite and latite flows, breccias, and tuffs.
Just to the right of Cooke’s Peak, and only a little more than 4 miles away, is the intrusive rhyolite dome of Taylor Mountain at the western foot of which lies the channel of the Mimbres River. On the horizon to the southeast are the jagged peaks of the Florida Mountains, jutting abruptly upward from the flat surface of the Deming Plain. Nearer, to the south- southeast, is the basaltcapped volcanic peak of Black Mountain, beyond and to the right of which is Red Mountain. Volcanic peaks cluster about Grandmother Mountain to the south- southwest and farther to the southwest where the highest is Soldier’s Farewell Hill. Nearly due west on the horizon is the broad crest of the Big Burro Mountains. The pale- colored band of mill tailings from the Hurley mill of Kennecott Copper Corporation is to the west- northwest. In the middle distance to the northwest are the volcanic Cobre Mountains, which overlook the Chino mine on the north side. Beyond, on the distant skyline, is the Pinos Altos Range, also composed of volcanic rocks.
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