City of Rocks

City of Rocks. Photo by Mark Erickson.
City of Rocks. Photo by Mark Erickson.

The landscape of Southern New Mexico, West Texas, and northern Mexico has not always looked like it does today. In fact, beginning some 45 million years ago, parts of the region literally exploded, dramatically altering the shape of things. Time after time, volcanoes in the area erupted, spewing forth immense quantities of thick lava and clouds of boulder-to-dust-sized rock fragments. Torrential rains caused mudflows of volcanic debris to surge off the hillsides, drowning valleys and basins in mucky layers of debris. Lava oozed into horizontal and vertical cracks in the older layers, doming up whole areas, forming peaks, and hardening in rooster-comb-like dikes.

After several million years of relative calm, the area exploded again with volcanic activity. This time, fiery clouds of burning ash blew out of dozens of vents, burying hundreds of square miles of the landscape. The ash layers were so hot that the particles fused together when they settled, forming thick layers of solid rock called tuff. The many vents poured forth more hot ash layers, and finally, layers of basalt and other molten lava atop the earlier tuffs. At the same time, earthquakes split the landscape along a series of north-south-trending faults, pushing some sections of the earth’s crust up, and dropping others down. Some pieces broke and tilted crazily, others stayed more or less intact. Today’s many, small, north-south-trending mountain ranges, including the Franklins, the Organs and the San Andres were formed by this earth movement.

One particular layer of tuff deposited during these times of cataclysmic change now forms one of the region’s many geologic oddities: City of Rocks State Park, about 30 miles north of Deming, New Mexico. City of Rocks is named for the dense cluster of house-sized rocks that sits in a bowl-shaped basin. The rocks, eroded along natural joints into queer giant forms, are part of one of the early tuff formations from the long-vanished volcanic vents. Faulting pushed the area upwards, allowing erosion to strip away the thick layers of rock that once lay above this tuff layer. With the weight of the overlying rock layers removed, the nubbly tuff cracked into regular joints. The freeze-thawing action of water, prying action of plant roots, and the abrasive action of wind act in concert to shape a once-solid rock layer into today’s blocky sculptures.

City of Rocks is a popular recreation destination, attracting picnickers, campers, and other visitors. This fascinating area of rock sculptures has long attracted humans. When you visit City of Rocks, watch for pottery fragments, arrowheads, and grinding holes in the rocks left by earlier people. Of course, leave these artifacts untouched for others to see when visiting City of Rocks, a visible reminder of our area’s past.