On my return to Utah from Texas in July of 2013, I drove through Southern New Mexico and visited the City of Rocks State Park.  The park is located 45 km northest of the town of Deming, in the scenic Mimbres Valley of the Chihuahuan Desert, in southwestern New Mexico.

     City of Rocks State Park is a small, only 2.7 square kilometers, but utterly fascinating geological wonder composed of large, naturally-sculptured rock columns and pinnacles 12 m high.

     When approached from a distance, the giant rocks appear like a city on the horizon–(a dense conglomeration rising our of a mesa), pictured above.

     Most of the rocks are sufficiently spaced to allow me to walk among them, or alternatively to find shelter from the burning summer sun.  However, fortunately for me, the sun was ready to go down for the evening.

     These rocks are 35 million years old, and are volcanic in origin.  They were originally part of a very large flow of hot volcanic ash, or tuff, which surged over the area and then, because of its high temperature, became welded into a solid stratum of rock.

     Over time rain and wind has eroded the softer substrate, leaving rows of these harder rocks behind.  Picture above, they resemble huge molars, sticking out of the desert floor, some crooked, some straight.  Many stand alone.  Other are grouped together, and some appear to be leaning on each other for support.

     The ash-flow tuff at City of Rocks is part of the Kneeling Nun Tuff.  In many areas, the ash fragments in the layer were hot enough to compact and weld together, forming the solid lava-like rock exposed.  Cooling and contraction of the tuff formed a pattern of vertical cracks called columnar jointing.  The “streets” in City of Rocks are formed by orthogonal fractures. More than one mechanism formed the fractures that trend north-northeast, east-northeast, and northwest.

     The ash-flow tuff that forms City of Rocks is of rhyolite composition. A close look will reveal phenocrysts or mineral grains of Biotite, Hornblende, Quartz and K-feldspar,  surrounded by a finer groundmass or matrix. 

     I noticed the “City” has a number of bird-species living among its rocks, nesting in cavities and crevasses. 

     The City of Rocks has also attracted human inhabitants, beginning in 750 AD with the Mimbres Natives, who lived here until 1250 AD.  The Mimbres found the rocks useful for grinding grain, and many of the rocks still have the crosses that were carved into them by Spanish conquistadors who passed through the area in their explorations of the southwest.