This geocache is part of an informal “series” where I am trying to put a cache out on different types of rock formation outcrops in the Sandia area. This geocache is a Traditional type, but has a completely informal and entirely optional “mission”, should you choose to accept it. The outcrop that is the subject of this cache is a rock type that I have yet to encounter along any Sandia trails; however, the outcrop is located too close to a couple of other pre-existing caches (one of which is not mine) to allow for a Traditional cache to be placed on it. The location of the physical cache is not far from the actual outcrop.
This cache is located along a portion of the “Old La Luz” Trail that extends between the Tramway Trail and Forest Service Road 333. Not far west of the “Kidnapped” cache (GC8VZZK), a keen observer might notice some anomalous, coarsely crystalline, heavy, dark hand-specimen-sized rocks along the trail and just off the trail on the uphill side. If it’s hot and sunny, and if you grab and hold onto one of these rocks, you might just burn your hand, they soak up so much heat! Hikers in the Sandias are used to seeing abundant and widespread pink or white granite, and gray limestone, as well as red and tan sandstones, and even sparkly metamorphic rocks and light-to-white colored (Pegmatite) dikes in the Indian Hills and Rincon Ridge. But these black rocks are not common here, from what I’ve observed so far.
UPDATE on 02/25/2023: The most impressive black mafic dike I've found so far is located started at Sandpig's "The Thing" and extending north for about 0.17 miles. See attached picture!
Optional “mission” to the true outcrop
Once you sign the log, you are welcome to accept the optional “mission” of actually finding the true outcrop. To reach the outcrop location you will be required to carefully hike a short way uphill. This puts the outcrop location somewhat mid-way between “Kidnapped” (GC8VZZK) and “Sandia Ski Jump Cache – The Landing Area” (GC2K6PM). I would recommend wearing long pants, hiking boots and gloves, and use your hiking poles too. You can manage to reach the cache without damaging any desert plants, because they are not growing too thickly along this slope, but you will still want to be careful to avoid the various cacti. As always, watch for snakes in warm weather, and maybe do this cache in a cool early morning.
There are three “Stages” that are part of the “mission”, and Waypoints are provided for each one.
Stage 1
To start to your “mission”, head east from the cache location toward “Kidnapped”. Keep your eyes on the trail to see if you can observe any of these anomalous black rocks. You do not need to hike too far before you should start seeing them. Also look just off the trail on the uphill side, where you should see some rather large black rocks and even boulders. A good destination to reach is Waypoint Stage 1 (N 35 12.769; W 106 28.984). By the time you have reached this waypoint, you should have encountered lots of black rocks along the trail, and many just off to the side and above the trail. Note that many of the black rocks on the trail are small enough they could be kicked around by hikers, so perhaps at some point many of them will have been kicked off. But at the time of posting there were still plenty right on the trail.
Rocks along trails can originate in one of two main ways, including weathering in place and rolling downhill from above. Of course, another possibility is that man has brought materials in to form a trail substrate. Considering the topography and the widespread granitic terrain around this local area, a logical conclusion would be that the rocks tumbled down from uphill.
Also of interest are the white dikes you may notice simply hiking to the physical cache. If you parked your car in the parking areas along Forest Service Road 333 at the Old La Luz trail head there, your hike along the trail to the physical cache location will take you across some white Pegmatite dykes. These dikes are labeled “felsic” on the Geologic Map, which means they contain light-colored minerals, such as quartz and potassium feldspar. “Pegmatite” is a physical descriptor, meaning the individual crystals can become very large. These Pegmatite dikes permeate Rincon Ridge (aka Dragon Spine), and go through the Indian Hills, and some smaller versions of them can be seen along the Old La Luz trail right below your feet (see some of the attached pictures). The Pegmatite dikes are very old, and they trend mainly east-west to southeast-northwest. They contrast with the Sandia Granite, which is generally more pink and tan with finer crystal size.
Stage 2
Depart the trail and start heading carefully uphill, keeping an eye out for more black rocks. A good destination to aim for would be Waypoint Stage 2 (N 35 12.838; W 106 28.951). Thought question: as you head uphill, what do you notice about the size and abundance of black rocks resting on the ground? Your observations should indicate that you are getting closer and closer to the source of these rocks. Be careful to note that sometimes there are enough black lichens on a granite boulder face to make the granite appear dark from a distance. However, if you break open some of the black rocks to expose a fresh surface, you should observe that these rocks are fairly black throughout, and have different character from the granite. If you do break open a rock, note, you should be wearing safety goggles, or at least protect your eyes in some fashion.
Mafic igneous rocks are relatively rich in elements magnesium and iron. Mafic minerals are generally dark in color, and typical ones are olivine, pyroxene, amphibole and biotite (Wikipedia). A common mafic rock is basalt, such as forms the Albuquerque Volcanoes. Basalt is a volcanic (extrusive) igneous rock, having cooled relatively quickly from lavas that flowed onto the surface, and the individual crystals are too small to be seen by the naked eye. Generally, igneous dikes are intrusive, having cooled relatively slowly enough to allow time for crystals that can be seen by the naked eye to grow. Can you see crystals in the black rocks? (Yes!)
At this stage you should be close to completing your “mission”! In what direction do you think you need to continue? (Uphill just a little further!)
Stage 3 (Outcrop)
Hopefully you can reach the outcrop of the origin of the black rocks that have tumbled downhill. Waypoint Stage 3 (N 35 12.838; W 106 28.939) will help you assure you are at the right spot. Be sure to scout around the outcrop, and also hike further uphill just a little way. If you are one of the few who have taken on this “mission”, please take a picture of the outcrop and load it with your online log. Pretend you are a geologist and include something in your photo (a walking stick or a backpack for example) that provides a sense of scale, so the size of the portion of outcrop you select to photograph can be fully appreciated.
An outcrop is important to a geologist, who maps the surface distribution of different rock formations (geologists also interpret the subsurface distribution of rock formations, using bore hole penetrations and well logs, for example). “Loose” rocks on the ground (sometimes called “float”, among other terms), unless they have weathered ‘in place”, just do not contribute to this objective, although they provide valuable clues to the geologist doing the mapping.
This outcrop of black rock is interpreted to be a mafic dike. By “outcrop” what is meant is that this surface location is the original location of the rock, and is not simply made up of rocks that have tumbled downhill. To be frank, this dike is not a “textbook” example, in that, at least at the surface, it is relatively small and not laterally extensive, but it is certainly cool nevertheless, considering this type of rock is not common in the area. The Geologic Map in the References (and in particular the paper version, not the downloadable version) shows quite a few mafic dikes in the western side of the Crest, all with a similar roughly north-south trend, and they extend for thousands of feet and even miles. The Map does not show a mafic dike at this particular outcrop location, either because it is too small to bother mapping at the scale of the Map and/or the geologists doing the mapping did not know of its presence. Note that these mafic dykes are Tertiary in age, much younger than the Pegmatite dikes.
At this point you have completed your “mission”! But some thought questions remain:
Does the dike stick up above the surrounding granite, or does the granite stick up above the dike?
Did the dike intrude the granite, or the other way around? Conversely, which is younger – the dike or the granite?
What is the rough orientation of the dike? Is this consistent with the other mafic dikes in the area, which have a general north-south trend?
When you hiked a little further uphill above the mafic dike outcrop, did you observe any black rocks (which we now interpret to be pieces of mafic dike) on the ground?
Despite the Geologic Map showing quite a few mafic dikes west of the Crest, the CO has yet to encounter any of these others along trails yet. That is another “mission”.
References/Resources
Bauer, Paul W., Richard P. Lozinsky, Carol J. Condie and L. Greer Price, 2003, Albuquerque A Guide to Its Geology and Culture, New Mexico Bureau of Geology and Mineral Resources.
Connell, Sean, 2006, New Mexico Bureau of Geology & Mineral Resources, Open-File Report 496: Preliminary Geologic Map of the Albuquerque – Rio Grande Metropolitan Area & Vicinity, Bernalillo & Sandoval Counties, New Mexico.
Hedlund, D. C., 1985, Geologic Map of the Sandia Mountain Wilderness, Bernalillo and Sandoval Counties, New Mexico, U.S. Geological Survey.
Julyan, Robert and Mary Stuever, editors, 2005, Field Guide to the Sandia Mountains, University of New Mexico Press, Albuquerque.
Rocks and Minerals, Smithsonian Nature Guide, 2012, Dorling Kindersley.
Schumann, Walter, 1993, Handbook of Rocks, Minerals & Gemstones, Houghton Mifflin Company.