Geologic Features


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 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.




Precambrian Era - (4500- 570 million years ago)

The Precambrian Era is the first part of Earth's history,  beginning with the formation of the Earth, about 4,500 million years ago and  ending with the advent of complex life forms 570 million years ago. In north-central  New Mexico the Precambrian Era is represented by 1,600- to 1,700-million years-old  metamorphic rocks and 1,400-million years-old granite in the Sandia and Sangre de  Cristo Mountains. Precambrian rocks are not exposed in the Cerrillos Hills, except  as isolated boulders of granite found as inclusions in the igneous rocks. These  inclusions were likely carried up several thousand feet by the molten rock to their  present positions.

There is no record of the Earth's history in New Mexico from about 1400 million  years ago to the Mississippian Period, about 330 million years ago. Geologists  surmise that at least for the latter part of that time, the land surface was above  sea level and was probably fairly level.

Paleozoic and Mesozoic Eras (570 to 65 million years ago)

The Paleozoic Era began 570 million years ago. As mentioned  before, there is no record of the early part of the Paleozoic Era in the Cerrillos  Hills area. During the Mississippian and Pennsylvanian Periods, 330 to 280 million  years ago, north-central New Mexico lay under a shallow sea. The limestone that  caps the Sandia Mountains and is found in the upper Pecos region of the Sangre  de Cristo Mountains was deposited during the Pennsylvanian Period.

During the Permian Period, at the end of the Paleozoic Era, and the Triassic  Period, at the beginning of the Mesozoic Era, 280 to 195 million years ago, the  seas receded from north-central New Mexico. Sandstone and mudstone was deposited  on broad flood plains. The earliest reptiles arose during the Permian Period and  dinosaurs appeared during the Triassic Period. Permian and Triassic rocks of the  region are typically reddish to maroon and crop out over a broad area of New Mexico.

The region stayed above sea level during the Jurassic Period and the early  Cretaceous Period (195 to 100 million years ago). Sand and mud, partly eroded from  volcanoes in southwestern Arizona, California, and Nevada, were deposited on river  flood plains during the Jurassic Period, as well as limestone and gypsum in a  shallow lake hundreds of miles wide. Morrison Formation sediments of the late  Jurassic Period are the oldest sedimentary rocks that crop out in the Cerrillos Hills.

There was no further deposition of sediments from the end of the Jurassic Period,  140 million years ago, until the beginning of the late Cretaceous Period, 100  million years ago. The land continued to be above sea level during this time.

The sea returned to north-central New Mexico in the late Cretaceous Period.  Sandstone and shale of the Dakota Formation, and black to gray shale, and lesser  sandstone and limestone of the 3,000-foot [914.4 meters] thick Mancos Shale, were  deposited in a shallow sea. This shallow sea extended from the Gulf of Mexico to  the Arctic Ocean and covered most of the Rocky Mountain and High Plains states  and provinces. Sandstone of the Dakota Formation, often displaying its characteristic  fossil worm burrows and traces, crops out in several locations around the Cerrillos Hills.  The black or gray shale of the Mancos Shale is widely exposed in the Cerrillos  Hills. The Mancos Shale forms most of the lowlands along County Road 52 (the  Waldo Road) west of Cerrillos village. Ammonite fossils are often found in the  Mancos Shale.

The sea retreated again from the Cerrillos Hills area in the late Cretaceous Period  and was replaced by an environment of sluggish streams and swamps separated from  the sea by a broad sandy beach. The sandstones and shales deposited during that  time are known as the Mesa Verde Group, which extends over much of the Four Corners  region. The large coal deposits of the Colorado Plateau, and the smaller ones of  the Madrid area, occur in Mesa Verde Group rocks. The Mesa Verde Group, with a  handful of small coal prospects, crops out in the southeastern part of the  Cerrillos Hills.

During the latest part of the Cretaceous Period and the beginning of the Tertiary  Period of the Cenozoic Era, western North America experienced a mountain-building  episode known as the Laramide Orogeny. (orogeny = mountain-building). The modern  Rocky Mountains developed during this time. The entire area was eroded, though the  Cerrillos Hills area was not tilted.

The end of the Cretaceous Period and the beginning of the Tertiary Period, about  65 million years ago, is marked by the disappearance of the dinosaurs and many  other species, and the emergence of mammals and flowering plants. It is believed  by many geologists that this extinction was caused by a catastrophic event,  probably by the collision of a large meteorite with the earth. Evidence for this  event cannot be found in the Cerrillos Hills area.

Cenozoic Era (65 million years ago to present)

During the earliest part of the Tertiary (65 million to about  35 million years ago) a broad intermontane basin extending from Lamy to Placitas  filled with sedimentary material derived from nearby mountains. These sedimentary  rocks are sandstone and mudstone of the Diamond Tail and Galisteo Formations.  Sandstone of the Diamond Tail Formation forms the buff-colored cliffs along NM-14  between Madrid and Cerrillos. The Diamond Tail and Galisteo Formations are  beautifully exposed on the eastern side of the Cerrillos Hills, where they have  been tilted to vertical. The vivid red mudstone between the sandstone layers belong  to the Galisteo Formation.

The Cerrillos Hills began to take their present form with the invasion of the  sedimentary rocks by molten rock (magma) 34 to 30 million years ago. The formation  of the Cerrillos Hills Igneous Complex resulted in four important features. The  first is that magma replaced much of the sedimentary rocks. Second, the remaining  sediments were pushed up around the edges of the intrusive complex. Third, the  deposits of silver, lead, zinc, and copper formed as the last stage of the  crystallization of the igneous rocks. Fourth, volcanic rocks, in the form of lavas  and pyroclastic flows, were erupted from a source thought to lie on the northeastern  side of the Cerrillos Hills. The volcanic rocks that erupted from the Cerrillos  Hills and the Ortiz Mountains make up the Espinaso Formation. The Espinaso Formation  is exposed in the northeastern part of the Cerrillos Hills and makes up the low  hills to the east of NM-14, north of Galisteo Creek.


Cerrillos Hills Igneous Complex

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 CiĆ©nega 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.

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