One of the most remarkable windows into the natural history of northern New Mexico is accessible to all of us in the Cerrillos Hills State Park, about a 30 minute drive from Santa Fe, and around an hour’s drive from Albuquerque. And with over 1000 years of mining history hidden in these austere cerrillos, the park is a window into the long cultural history of the American Southwest, as well.
There are so many natural features to explore in the park that I’m sure I’ll be coming back many times to touch on them. Although its geology is complex, and still under investigation by State geologists, even a short walk along its dusty trails will reward you with examples of basic physical geology concepts, the sort of things students meet in their very first classes in Earth Science.
Natural history walks in the park begin in the parking area, which you can see in the lower left hand of this Google Earth image, and wind up and over the Jane Calvin Sanchez trail, which heads off sharply to the right, across the road.
With over 5 miles of trails to explore, there is plenty you can investigate on your own. There are also things you can see from the village of Cerrillos, just south of the park, as well as from the drive down from the village of Madrid, and – if it hasn’t been raining – from the unpaved Waldo Canyon Road which skirts the hills on the south and eventually brings you to Interstate 25 between Santa Fe and Albuquerque.
One of the most basic concepts in physical geology is the contact. A contact is the surface separating a distinctive body of rock from its neighbors. Contacts are what geologists trace on their maps when they are working out the natural history of an area. The contacts between rock bodies form in different ways, and in Cerrillos Hills State Park three important kinds can be seen within a short walk of the parking area.
A few steps down into the dry wash just below the park’s signs brings you to an eroded bank where grey, sharply tilted beds of shale are abruptly overlain by a flat-lying, crudely bedded alluvial gravel, cemented into conglomerate. The shale forms one distinctive body of rock, the conglomerate another, and the surface between them is the contact. Here is a picture of a similar contact, seen from the village of Cerrillos:
This contact is an example of a very significant type of contact called an unconformity. This is an unusual word and it will take a few lines of explanation to unpack its meaning.
Among sedimentary rocks, uniform depositional conditions result in packages of parallel, or conformable, strata. The grey shale, for example, consolidated from mud that accumulated slowly on the floor of an inland sea, and its innumerable thin layers are all more or less parallel to one another.
Very broadly speaking, accumulations of sediment like this marine mud start and stop depending upon whether the area is above or below water. Once a region rises above sea level erosion dominates and sedimentation stops. The Cerrillos Hills sit far above sea level and erosion is active, but here and there streams have left deposits of alluvial gravels in channels cut into the older shale. These gravels are in unconformable contact with the shale. Like most unconformities, this contact is a buried erosional surface, and like all unconformities, it represents a break in the continuity of the geologic record. Metaphorically speaking, unconformities split the geologic record into chapters. Hence their significance to natural history.
A short walk from the wash up along the Jane Calvin Sanchez Trail brings you to a second kind of contact, the intrusive contact. These contacts reveal the Big Story in the Cerrillos Hills, because, as we will see in other posts, the Cerrillos Hills are a gigantic blister of subvolcanic activity, shot through and through with bodies of formerly molten rock. These magmas burrowed their way through the shale, intruded upon one another, fed a small volcanic complex that is now completely eroded away, and set up the (long dead) hot spring activity that brought the metal ores that brought the generations of miners and gem hunters here.
Just behind the pleasantly smiling ranger you can see a prominent outcropping of rough rock, with a juniper-covered slope resting on its left side. This change marks the contact between a dike of hard igneous rock and the softer shale into which it was intruded.
The Cerrillos Hills are full of these intrusive contacts. Here is a closer view of the contact between the Mancos Shale and an injection of andesite, one of the first magmas to invade the shale. The molten andesite was hot enough to bake the shale into a hard rind of hornfels, a type of metamorphic rock.
By the way, this kind of metamorphism is called – wait for it – contact metamorphism.
The Cerrillos Hills are pitted all over with prospects dug into promising areas where a miner might find a lode of silver-bearing galena, or lead-zinc sulfides (with a bit of copper thrown in), or crusts of gem-quality turquoise. Several of these holes are visited by the network of trails in the park, all sadly fenced in for your protection. The first one you encounter on the Jane Calvin Sanchez Trail gives us a look at a third kind of contact, the fault contact.
In the photograph above, notice the angled contact, just to the right of the old juniper-pole cribwork, where a body of fractured and bleached andesite (left side) rests against some tilted and distinctly bedded, but not baked shale (right side). This contact represents a shear zone in the rocks along which two different rock types have been dislocated and shifted into contact. In other words, a small fault.
An entire system of northeasterly-trending shear zones transects the Cerrillos Hills. These zones channeled the hydrothermal activity set up by larger, later, and hotter igneous intrusions, which in turn focused the mineralization which has, in its turn, attracted people here for over 1000 years. Below is a Google Earth image looking straight down on the Christian Lode. Notice the bleached zone angling through the pit and striking off to the northeast, a reflection of the shear zone below:
I think this is a pretty good harvest of examples for such a small investment of walking. And we’ve barely scratched the surface here, so to speak. The Cerrillos Hills are a showplace of igneous activity rarely exposed in such an accessible way, so we will be back to have a look at its other geological treasures in future posts.