CONGRATULATIONS TO BUDABELI FOR FTF!
When was the last time you were hiking on a trail that crossed a streambed and you purposely looked closely at the types of rocks up and down it? This Earth Cache (EC) asks you to do just that. The subject of this EC is simply about trying to identify different kinds of rocks in the alluvial deposits of streambeds in the Placitas area trails, and thinking about where these rocks came from. The intent is not about trying to describe the origin of each rock type (such as how limestone rocks formed) or to interpret all the different types of unconsolidated alluvial deposits themselves. Don’t worry about getting all your answers on identifying rocks right!
The recommended parking is the first National Forest parking lot a short distance south from NM165 on FR445.
Other EC’s in the Sandias/ABQ area provide some “big picture” background on the genesis of the Albuquerque Basin, uplift of the Sandia Mountains, formation of the Great Unconformity and distribution of unconsolidated sediments across the Basin itself; these EC’s are relevant to the subject of this EC:
The Great Sandia Un-Conformist EarthCache (GC1YK8V), which, in particular, includes lots of pictures of rocks (granite, limestone, sandstone) in the gallery.
Great Sandia Un-Conformist, down-low (GC6V0J8).
Sandia Crest Earthcache (GC3KFAF).
Rio Grande Valley transect – soils (GC7MPVN).
Rio Grande Valley transect – terraces (GC7MBQC).
Still other local EC’s highlight rock types – some you will see as part of these EC’s and some you won’t – and one interesting mineral in particular you will see:
Orange Crush (GCA04W2).
Yikes! White Dikes! (GC9PY4C).
Follow that Dike! (GCA6Y5K).
Fun with Fe3O4 (GC9NAXF).
Strata Gem (GC9ZB5Q) and Strata Gem, Part 2 (GCA0DAT).
Welcome to Mazatza-landia! (#1) (GCA9Z1P).
Different kinds of rocks can be found in streambeds that flow north and northeast out of the Sandias and Rincon Ridge on the northern, Placitas side, of the mountains. Your task is to hike to two different streambeds and identify the types of cobble-to-hand-sample-to-boulder-sized rocks that can be found there. The mix of rock types is somewhat different in each of these stream beds. Pictures in the Gallery will help you identify the most common types of rocks you will see. All these pictures were taken from the stream beds you will encounter, hence they are not ideal, “textbook”-like pictures taken from petrology books or online resources.
Note that this EC has you looking mainly for individual rock samples, such as boulders and hand-sized specimens. These particular rocks are not “in-place” outcrops, but rather have been brought to their current resting place after having been eroded out of original in-place rock and transported some distance. An outcrop is an exposed area of solid rock, such as a cliff face or exposed bedrock.
ROCK CLASSIFICATION
Figure 1 illustrates a simplified classification scheme of major rock types. In Petrology (study of rocks), there are three main classes of rocks: IGNEOUS, SEDIMENTARY and METAMORPHIC. Rocks from all 3 of these classes can be observed along the trails and in the streambeds visited for this EC.
IGNEOUS rocks have cooled and crystallized from molten magma. There are 2 main types of IGNEOUS rocks:
Plutonic (or Intrusive) IGNEOUS rocks formed at depth well below the surface of the Earth where cooling rates are slow enough to allow sufficient time for the growth of crystals that are visible to the naked eye. A good local example is the Sandia Granite.
Volcanic (or Extrusive) IGNEOUS rocks formed near (think inside a volcano, such as Cabezon Peak), at (think lava beds at the Albuquerque Volcanos) or above (think ash falls) the surface of the Earth where cooling rates are so fast that developed crystals are often microscopic. Sometimes crystals don’t even form, resulting in a glass (think Apache Tears obsidian); sometimes individual tiny crystals can be seen distributed throughout a background mass of the microscopic crystals (as can be seen in the Andesites at Carrillos Hills State Park).
SEDIMENTARY rocks are formed at or near the Earth’s surface from accumulation of grains or precipitation of dissolved minerals from water. There are 2 main types of SEDIMENTARY rocks:
Clastic SEDIMENTARY rocks are formed from the accumulation, compression and perhaps cementation of clasts, which are grains, or fragments, that have weathered out of pre-existing rocks. Typical clastic sedimentary rocks include, in the order of generally decreasing grain size, conglomerates, sandstones, siltstones and shales.
Chemical SEDIMENTARY rocks are formed by precipitation of minerals dissolved in water. The origin of these dissolved minerals is generally from chemical weathering of pre-existing rocks. Good local examples include, in particular, the Madera Limestone up on the Sandia Crest, but also cherts present in the limestone, as well as gypsum on White Mesa.
METAMORPHIC rocks are formed when pre-existing rocks are subjected to elevated heat and/or pressures that alter the mineralogy and physical texture, or fabric, of the rocks without actually melting them. Rocks can be metamorphosed by widespread Regional effects caused by, for example, mountain building and crustal thickening (and deepening) related to the collision of tectonic plates, micro-plates and island arcs, and by Contact effects, caused by the increase in temperature related to the proximity to intrusion of igneous (magmatic) bodies into pre-existing rocks. Good local examples include the schists and gneisses of the Rincon Hills and Ridge west of FR333.
NEED FOR SIMPLIFICATION
The geology of the bedrock – from which rocks in streambeds originated – in the northern Sandias area around Placitas is quite complex. For purposes of this EC, however, we are going to reduce the complexity and worry only about a subset of rock classes.
Note that the types of bedrock metamorphic rocks in the area from which metamorphic rocks in streambeds were sourced are quite varied. For purposes of this EC they are all being lumped together into either Gneiss or Schist. In practice, academics examine the detailed mineralogy of metamorphic rocks to identify the grade of metamorphism to which the rocks have been subjected. So, for example, different kinds of bedrock schists that have been mapped include “amphibole-biotite schist”, “andalusite-biotite schist” and simply “mica schist”. Other types of related metamorphic rocks in the area include phyllites and amphibolites. One type of metamorphic rock that appears to be absent, or least not common, from the northwestern Sandia Mountains area is quartzite, which, however, is extremely common down in the Manzanos. Sometimes it’s hard to decide whether a rock might be a schist or a gneiss, and “schistose gneiss” is actually a classification “catch all” that has been used in the literature. Mention of all this detail here is not meant to add confusion, but to be straightforward and call attention to the need to “gloss over” a lot of the details for the EC.
NOTES
Note that many of the metamorphic rocks in this area appear to be blackened to some extent with a coating of desert varnish (usually manganese oxide). This means that identifying some rocks may be more challenging, hence justifying an increase in the Difficulty.
Note also that some rocks are coated with a white or light-colored layer of re-precipitated calcite (calcium carbonate).
ROCK SAMPLE CATALOG
Figures 2-5 show examples of the more common rocks to be encountered when doing this EC. These pictures may be used to help identify rock samples in the dry streambeds.
Figure 2A shows examples of local Igneous Granite rocks. Often the Sandia Granite is found with coatings or veins of a green mineral, which I believe is epidote, as that has been reported to be commonly present.
Figure 2B shows grains of the mineral magnetite (Fe3O4): magnetite is not a rock, of course, but the abundance of magnetite that can be found here in streambeds is worth noting. Also shown are examples of chunks of quartz.
Figure 3A shows Metamorphic Schist rocks. The schists can sometimes be identified by how much they sparkle in sunlight, but this sparkling is difficult to capture in a photograph. Schists are often characterized by a general alignment of platy minerals, such as micas, that have formed during metamorphism, resulting in a texture called “schistosity”. Sometimes desert varnish will blacken the schists, suppressing the sparling effect and texture.
Figure 3B shows Metamorphic Gneiss rocks. There is probably a gradation between these particular schists and gneisses, but what I usually use to classify a gneiss is the presence of pervasive banding of the different minerals. The banding can be convoluted or more linear in pattern. Again, desert varnish helps to blacken many samples, hiding some of the banding.
Figure 4A shows Sedimentary Limestone rocks. Many of the limestones here are various colors of gray, and have very small grain size. Many of the limestones are fossiliferous, with the fossils being large enough to see well, although many fossils are tiny and others are broken up into fragments. Sometimes fossils that were originally made of calcium carbonate have been replaced by silica that can be shades of orange (see “Orange Crush”, GCA04W2).
Figure 4B shows Sedimentary Limestone rocks colored yellow/ochre/red by iron oxides. This is a common phenomenon that can be observed on outcrops on many trails in the Sandias, especially the northern Sandias from what I have seen.
Figure 5A shows Sedimentary Clastic rocks, in particular sandstones, siltstones and shales. Many of these rocks are various shades of red, but can be buff, yellow and tan in color. For purposes of these EC’s it is not necessary to be accurate when trying to differentiate between a sandstone and a siltstone, or between a siltstone and a shale, based on particle grain size.
Figure 5B shows Sedimentary Conglomerate. These rocks can be found in the various streambeds, and can contain quite large pieces of various rock fragments. These rocks may be found as individual hand specimens, but also as actual outcrops along cut banks, for example, or in the stream beds, so they are an exception to the “rule” stated earlier that all the rock samples of interest encountered during these EC’s are not actual outcrops.
Figure 6 takes a big step back, showing a view looking west from a hilltop to the east looking out over some of the area where you have been exploring streambeds. Note the general topographic slope going down to the right; note also the pattern of sub-parallel incised streambeds, which are highlighted in white in the bottom picture.
TOOLS OF THE TRADE
- Slips of paper, index cards, “stickies” or similar.
- Bold marker (Sharpie, perhaps).
- Magnet (even a refrigerator magnet will do).
REQUIREMENTS
Navigate first to GZ on the Boot Leg Trail and then to Stage 2 on the Sidewinder Trail. As you hike to these locations, start observing the various rocks you encounter along the way. It is a little more interesting to do GZ first and then Stage 2, but this is not strictly necessary.
At GZ (Boot Leg) stroll up and down the dry streambed, being sure to go upstream about 140 feet and downstream about 100 feet.
At Stage 2 (Sidewinder) again stroll up and down the dry streambed, being sure to go upstream about 140 feet to where you come to a “Y” bifurcation of the streambed.
At GZ and Stage 2, obtain a general impression of the streambed just from visual inspection at a distance. For example, determine what the prevailing color is, observe any cut banks on the sides of the streambed.
At GZ and Stage 2 collect at least 3 different rock types that you can identify. See of you can find a rock from each of the main classes (IGNEOUS, SEDIMENTARY, METAMORPHIC). If you are not confident you can do that, you could try to find 2 different sedimentary rocks (limestone and sandstone perhaps) plus an igneous rock. Or if you are really determined you could try to find 2 different metamorphic rocks (gneiss and schist) plus an igneous or sedimentary.
Take photographs of your samples, including “name tags”, which can be small sheets of paper, index cards, “stickies” or the like. Figures 2-5 show examples of how I did this. If your examples are all hand-sized specimens, you could perhaps assemble all 3 into the same photograph. In your pictures be sure to include some unique item of yours that can be used as a scale.
While you’re there, tap the surface of the streambed gravels with your magnet and see what happens.
Send your pictures and answers to the following questions to the CO. Do not include your rock identification pictures in your log, although other more general pictures can be loaded if you want. If your rock identification pictures are labelled correctly, and are additional good examples compared to what is already in the figures, some of them can be loaded to the gallery by the CO later.
QUESTIONS
Were you able to find an example of IGNEOUS, SEDIMENTARY and METAMORPHIC rocks at each Part streambed location?
What was the striking difference in overall color between GZ (Bootleg) and Stage 2 (Sidewinder) streambeds?
What rock types appeared to be most abundant at each streambed? Why is there an apparent difference in the population of rock types observed at each streambed location?
At Stage 2 streambed, upstream at the “Y” bifurcation, what unusual rock did you find and where did you find it? Hint: this rock is not necessarily a hand-size specimen, but bedrock in cut banks and forming streambed bedrock. What do you think is the origin of this rock?
Running water in the form of streams and rivers carries sediments from where they were eroded out of rocks upstream to where they were deposited downstream, in streambeds for example. The size of the sediments able to be carried will depend on the force of the running water. For example, high-energy Rocky Mountain streams and Glacier Valley streams with strong running water can contain boulders and large rocks, while low-energy river deltas (think Mississippi River) contain very fine-grained silt and shale sediments. Remember you were asked to observe any cut banks in the streambeds.
Where did the hand-to-boulder-sized rocks in the streambeds come from? Figure 6 provides a hint.
Permission
Permission for placement of this EarthCache was obtained from the Cibola National Forest and National Grasslands, Sandia Ranger District, Tijeras, NM.
References
Berkley, J. L. and J. F. Callender, 1979, Precambrian Metamorphism in the Placitas-Juan Tabo Area, Northwestern Sandia Mountains, New Mexico, New Mexico Geol. Soc. Guidebook, 30th Field Conf., Santa Fe Country.
Bonewitz, R. L., 2012, Rocks and Minerals, Smithsonian Nature Guide, Dorling Kindserly.
Brandes, N., 2021, New Mexico Rocks!, Mountain Press Publishing Company.
Connell, S. D., 2008, Geologic Map of the Albuquerque – Rio Rancho metropolitan area and vicinity, Benalillo and Sandoval Counties, New Mexico, Geologic Map 78, New Mexico Bureau of Geology and Mineral Resources.
Julyan, R. and M. Stuever, editors, Field Guide to the Sandia Mountains, University of New Mexico Press.
Schumann, W., 1993, Handbook of Rocks Minerals & Gemstones, Houghton Mifflin Company.