Geologic Tour of New Mexico

This recreation area in the Lincoln National Forest has spectacular cliffs and features a year-round spring-fed stream, waterfalls (the largest of which has a 150-foot drop), and many refreshing clear-water wading pools. There are picnic areas and several hiking trails, including T–68 and T–68A that lead to the spring source of the waterfalls. The Apache name for the area was “gostahanagunti,” which means hidden gulch, and the presence of flakes of worked chert, grinding holes (bedrock mortars), and nearby pictographs testify to long-standing Native American use of this wildlife-rich oasis in the desert.

The Valles Caldera National Preserve is one of the most geologically unique and significant areas in North America. The preserve encompasses much of the Valles caldera, a huge volcanic crater that formed 1.2 million years ago during an enormous volcanic eruption that spread ash over large parts of New Mexico. The caldera is located near the summit of the Jemez Mountains, a large volcanic complex in north-central New Mexico. The Valles caldera exhibits world-class examples of the landforms produced by a very large, explosive volcano, and the preservation and exposure of geological features within the Valles is spectacular. Much of what geologists know about large-scale explosive volcanism began with detailed studies of the rocks in the Valles caldera and Jemez Mountains, and the area continues to draw geologists from around the world. Since the eruption 1.2 million years ago, there has been uplift of the crater floor, followed by the eruption of smaller, younger volcanoes called “domes” within the crater left by the large eruption. Since then, the caldera has from time to time been home to a series of large lakes. This dynamic geological history is responsible for the beautiful and unique landscape that we see in the region today.

Cerro Pedernal, one of the most recognized landmarks in north-central New Mexico, is located in the northern Jemez Mountains . Cerro Pedernal lies in the transition zone between the Colorado Plateau and the Rio Grande rift. Typical, relatively flat-lying, Colorado Plateau Mesozoic stratigraphy is exposed at the base of the mountain, while younger Cenozoic basin fill sediments underlie the shoulders of the peak. The andesite and basalt flows capping Cerro Pedernal, which give the mountain its distinctive flat top, were erupted from the northern Jemez volcanic field about 8 million years ago. The lava flows and underlying rocks on Cerro Pedernal and on mesas to the southeast have since been faulted and down-dropped to the southeast by as much as 1870 feet (570 m) during Rio Grande rift extension in the last 8 million years.

Sevilleta National Wildlife Refuge (NWR) is located in central New Mexico about 15 miles north of Socorro. The refuge spans the Rio Grande valley from the Sierra Ladrones on the west to the Los Pinos Mountains on the east, an area of approximately 30 miles by 15 miles, encompassing 230,000 acres. The area was designated a Spanish land grant, Sevilleta de la Joya, in 1819. The land grant was sold in 1928 to Socorro County and then, in 1936, to General Thomas D. Campbell, under whom it became a ranch for sheep and cattle. The Campbell Family Foundation donated the land to the Nature Conservancy in 1973, which then passed it on to the U.S. Fish & Wildlife Service to become a national wildlife refuge dedicated to preservation and enhancement of the integrity and natural character of the land. The transfer stipulated that the land should undergo natural processes of succession, including floods and fires, without human interference. Thus, most of the refuge has very limited public access, but the Mesa View Trail west of the Visitor Center is open for hiking at least five days per week.

Fort Union was established in 1851 to provide a much-needed military presence to travelers on the Santa Fe Trail. The trail had been active since 1821, when much of what is now New Mexico became U.S. territory in 1848, at the end of the Mexican War. Fort Union’s spectacular location on the western edge of the Great Plains was a strategic one, sited as it was near the junction of the Mountain Branch of the Santa Fe Trail with the Cimarron Cutoff. Between 1851 and 1891 Fort Union contained the largest American military presence in the Southwest; over 1,600 troops were stationed there in 1861. In 1878 the railroad came to New Mexico over Raton Pass, and by 1891 Fort Union had been abandoned. Visitors to the national monument can see some impressive historic ruins, including the foundations and adobe walls of many of the original buildings, and remnants of the deeply-rutted Santa Fe Trail.

Fenton Lake State Park is in the Jemez Mountains on the west side of the Valles and Toledo calderas, large collapse features that formed during voluminous volcanic eruptions 1.6 and 1.25 million years ago. The landscape around Fenton Lake is characterized by broad, grass-covered valley bottoms that lie between dissected orange-brown to white mesas that are bound by imposing cliffs. The cliffs are formed by the 1.6 to 1.25 million year old outflow sheets of Bandelier Tuff that erupted from the calderas. The south-flowing Rio Cebolla (Spanish word for onion), which feeds Fenton Lake, cut one of the broad valleys after 1.25 million years ago.

Angel Peak, a 7000-foot pinnacle capped by sandstone, is a prominent landmark near Bloomfield in northwestern New Mexico. It is in the northern part of the San Juan Basin, a large structural depression that formed starting about 75 million years ago during compressional Laramide deformation. The San Juan Basin was surrounded by mountainous uplifts to the north (now buried by the 35 to 20 Ma San Juan volcanic field in southwestern Colorado) and to the east that also formed by Laramide-related compression. The sediments that can be seen at Angel Peak National Recreation area were eroded off of the old Laramide highlands and deposited in the basin 50 to 65 million years ago. More recently, over the course of the last 5 million years (or less), the westward-draining San Juan River, a tributary to the Colorado River, has eroded the rocks of the San Juan Basin. Drainages feeding into the San Juan River have carved the scenic landscape we see today.

The Salinas Pueblo Missions National Monument is located in the westernmost part of the Great Plains province, in Torrance County near Mountainair. The monument consists of three small and widely separated units — Abó, Gran Quivira, and Quarai.

When the Spanish arrived in this area in the late 1500s they found multiple inhabited pueblos, and they referred to this area as the Salinas District. However, because of drought, famine, disease, and Apache raiding, by the late 1600s the entire Salinas District was depopulated of both Native Americans and Spaniards. What remains today are reminders of this earliest contact between Pueblo Indians and Spanish colonists: the ruins of three mission churches and a partially excavated pueblo that Juan de Oñate called Las Humanas or, as it is now known, Gran Quivira Pueblo.

City of Rocks State Park is truly a geologic monument; it is formed by large sculptured rock columns (pinnacles) or boulders rising as high as 40 ft and separated by paths or lanes resembling city streets. About 34.9 million years ago a large volcano erupted, forming the rocks in an instant (geologically speaking); then erosion over millions of years slowly formed the sculptured columns that now provide a natural playground for children and adults alike. City of Rocks State Park was established in May 1952 to preserve this geologic wonder.

Percha Dam and Caballo Lake State Parks, located south of the town of Truth or Consequences, offer striking views of the Caballo Mountains and Red Hills, prominent uplifts in the southern Rio Grande rift. The Percha and Caballo dams were built for the Bureau of Reclamation’s Rio Grande Project in 1918 and 1936–38, respectively. Percha Dam, although only 18.5 feet tall, diverts water into the Rincon Valley, irrigating farmland where much of New Mexico’s famous green chile is grown, along with many other crops. Two miles upstream, the 96-foot-tall and 4,590-feet-wide earth-fill Caballo Dam stores water released from hydroelectric- power generation at Elephant Butte Dam and regulates delivery of that water to downstream users during irrigation season.