The Land of the Park
The story of the geology of the Cerrillos Hills is the story of magma from deep within the earth 34 million years ago pushing its way upward and intruding into the older geology in this area. In the Cerrillos area, this intrusive material forced its way into the Mancos Shale and the Galisteo Formation, disrupting them, baking them, injecting them with mineral-rich fluids, and then erupting out at the surface to form a volcano. It is this process of intruding and pushing upward on older formations like the Galisteo Formation that created the spectacle of upturned geology that is the Garden of the Gods. It also this process of intrusion and eruption that created the Cerrillos Hills.
As a result of isotopic analysis of lead samples (the relative proportions of the various isotopes of lead) we now know that the Cerrillos deposits of galena were formed by mineralizing fluids associated with two distinct stocks (magma intrusions) and their associated faulting, creating a complicated swarm of cross-cutting ore-bearing veins. This would suggest that a minimum of two distinct volcanic events contributed to the building of the Cerrillos Hills.
Thirty-four to 30 million years ago the original Cerrillos Hills were formed as described, but over these last 30 million years the cumulative effects of weather and gravity have left only the erosional remnants of those ancient Tertiary volcanos. In the words of geologist Scott Renbarger, when we look at the Cerrillos Hills today we see the "frozen plumbing of an ancient volcanic complex". The missing volcano material has been in its turn deposited in low-lying areas - the local Tertiary-age deposition known as the Espinaso Formation. (Geology seems to be mostly the story of building up mountains and then eroding them away!)
The Ortiz Mountain complex to the south of the Cerrillos Hills was in its volcanic glory about 29 million years ago.
Around 20 million years ago this era of volcanism came to a close in New Mexico with a regional episode of crustal extension. The state was split by a great series of north-south offset depressions called the Rio Grande Rift. The crust adjacent to these depressions buoyed upward into steep-faced mountains such as Sandia, and differential offset created escarpments like La Bajada. The Rio Grande, following the growing depressions, came into existence at this time. The basins were filled with thick deposits of erosional material washed down from the flanking uplifts, the gravelly fill now known as the Santa Fe Formation, the host of the most popular aquifer where New Mexicans mine their water. In some areas at this time thick flows of dark basalt were also laid down.
There are in the Cerrillos Hills a few thin surficial stream deposits and pediment gravels called Quaternary Alluvium that date from about 2 million years ago, most notably represented by the gravelly Ancha Formation that covers the plain north and east of the hills.
Indian turquoise mining prior to 1000 A.D., to Spanish mines of the 17th century such as the Mino del Tiro, and to American mining of the 19th and 20th centuries. This is one of the oldest mining areas in North America. There is evidence of pre-Columbian vein workings and later smelter sites which are of immense historical and archaeological value and indicate sources of metallic and other substances used by past cultures. Except for turquoise (and sometimes including it), mining in the area has often been marginal and aimed at recovering localized concentrations of sulfides of zinc, lead, iron, and silver, deposits of copper ores, and, though dreams would have it otherwise, never much more than a trace of gold.
All abandoned mines are being surveyed for fencing or other closures (appropriate grating will be placed at known bat habitats.)
The nature of the mining to date has been mostly benign with respect to toxic substances, but there are two mill-site locations within the boundaries of the 19th century Cerrillos Mining District that are under the current scrutiny of the Environmental Protection Agency (concerns are for lead, cadmium, and arsenic.) Both of these EPA locations lie outside of the park.
The Cerrillos Mining District, of which the Cerrillos Hills State Park occupies a small part, was placed on the New Mexico Register of Cultural Properties in 1973.