Landscape and Topography
THE GEOLOGY OF THE CERRILLOS HILLS
by Stephen R. Maynard CONSULTING GEOLOGIST
The Cerrillos Hills is a group of low mountains that covers an area about 18 square miles [46.6 square kilometers] north of the village of Cerrillos, in Santa Fe County, New Mexico. The Cerrillos Hills' lowest elevation is 5600 ft [1,706.9 meters], along the Galisteo Creek. Most of the Cerrillos Hills is hilly terrain cut by locally steep-sided gulches and arroyos. The hilltops of most of the land range from 6000 to 6200 feet [1,828.8 to 1,889.8 meters] above sea level. Several prominent hills (Cerro Cosena, Grand Central Mountain, Cerro Bonanza, Lucera Hill, and Achavica Mountain) rise to summits over 6900 ft [2,103.1 meters]. The Santa Fe plateau, on the north side of the Cerrillos Hills, lies at about 6300 ft [1,920.2 meters] above sea level.
Galisteo Creek borders the southern flank of the Cerrillos Hills and flows east to west, to the Rio Grande. San Marcos Arroyo cuts through the southeastern part of the Cerrillos Hills and joins Galisteo Creek at the village of Cerrillos. Both streams flow intermittently, though Galisteo Creek usually has at least a trickle of water. Both streams are capable of flash floods as well. Other gulches and arroyos draining the Cerrillos Hills are ephemeral.
The Cerrillos Hills are covered by the Madrid, Picture Rock, Tetilla Peak, and Turquoise Hill U.S. Geological Survey 7.5-minute topographic maps.
REGIONAL GEOLOGIC HISTORY
The Cerrillos Hills Igneous Complex is considered by geologists to be part of the Ortiz Porphyry Belt, a 25-mile [40.2 kilometer] long, north-south trending group of igneous rocks that begins at South Mountain in the south, and ends at La Ciénega in the north. Throughout the Ortiz Porphyry Belt, the igneous rocks have invaded sedimentary rocks ranging from the Mississippian Period to the Tertiary Period. In the San Pedro Mountains, the Ortiz Mountains, and in the Cerrillos Hills, the igneous rocks have a direct relationship to the ore deposits found there.
In the Cerrillos Hills, igneous rocks are both plutonic (or intrusive) and volcanic (extrusive). The intrusive rocks can be divided into two main groups. The first group is about 34 million years old and is composed of quartz-bearing andesite- latite porphyry. The andesite-latite porphyry generally invaded the sedimentary rocks in sheets parallel to the sedimentary rocks' layering. The individual sheets are called sills. The sills were probably fed by vertical fractures. These vertical fractures filled with igneous rock are called dikes. A series of stacked sills connected by dikes is called a Christmas-tree laccolith. It is believed that such a laccolithic structure existed in the Cerrillos Hills, though its original form has been mostly destroyed by younger plutonic rocks, and later tilting and erosion.
Around 30 million years ago a series of quartz-poor latite and monzonite bodies (dikes and stocks) intruded the Cerrillos laccolith. These intrusions pushed the country rock up and tilted it away from the centers of the intrusive bodies. The monzonite stocks that form the highest hills in the Cerrillos Hills were formed at this time. A small, as yet unmined, copper deposit formed in the central part of the Cerrillos Hills at this time. Veins containing lead, zinc, and silver formed in northeast-trending fractures shortly afterwards (within a million years or so).
From about 30 million years to about 3 million years ago, a series of linked basins formed (the Rio Grande Rift) and filled with sediment. The Cerrillos Hills area was eroded and the broad plain of the Santa Fe Plateau formed. Geologists call this feature a peneplain. The peneplain appears to have extended over the Cerrillos Hills, with the highest hills, e.g. Cerro Cosena, Grand Central Mountain, Cerro Bonanza, Lucera Hill, and Achavica Mountain, left standing above its sloping surface. Similar peneplains are found over much of New Mexico. One of the most striking is the Ortiz Surface, which flanks the Ortiz Mountains on all sides and is easily viewed from the Cerrillos Hills.
During the period 2.8 million to 1.5 million years ago, during the time periods known as the Pliocene and Pleistocene Epochs, streams draining the Sangre de Cristo Mountains deposited sand and gravel on the Santa Fe Plateau. This deposit is known as the Ancha Formation. The Ancha Formation covers the Plateau with up to 40 ft. of sand and gravel. It can be seen in the eastern and northern parts of the Cerrillos Hills. Wolf Road, in the eastern part of the Cerrillos Hills, runs along a ridge capped with Ancha Formation gravel. A similar deposit called the Tuerto Gravel caps the Ortiz Surface that flanks the Ortiz Mountains.
As the Rio Grande Valley deepened in the last 1.5 million years, so did Galisteo Creek. Tributary arroyos and gulches cut down through the rock as well, first stripping away the Ancha Formation and Tuerto Gravel deposits, leaving them perched on mesa tops. Both Galisteo Creek and San Marcos Arroyo have significant terrace deposits that record times in recent geologic history when the streams were tens of feet higher than they are now and left stream-worn gravel on benches above their present courses.