River Rock

River rock is one of those terms used everywhere in the country that everyone knows what means and no one defines. Generally, it’s rock that has a relatively smooth surface. I say relatively, because its not as smooth as a polished tombstone, but it’s not jagged.

If you ask for something more specific, people take the adjective smooth and turn it into a verb. There the trouble begins, because they start to say things like “the erosive action of the moving water from a river smooths and shapes the stones. Other times, they say something slightly romantic like “smooth surfaces created by tumbling around in rivers for years.”

Maybe, elsewhere in the country. Here, even when the Rio Grande is running high, like it was in the above picture take last October under the Griego Bridge in Española, the water only creates rapids. You don’t see rocks being tossed about. More likely you see them laying outside the action where silt can filter through them.

Smoothing and polishing are not the same thing. The second is done by removing material, often by breaking large pieces into smaller ones. In a rock tumbler, the polishing is done by some kind of abrasive grit, something like silicon carbide, with water used as a facilitating lubricant, not as the active agent . The winds here move sand at high enough velocities to blast surfaces.

The above is a piece of granite I found in the area of the far arroyo this weekend. Part of it has striations that left a relatively smooth surface. The darker corner at the top in the picture below was untouched. There the smoothed section has a lighter color because the abrasion removed the tops of the softer, darker mica, but harder flecks of quarts remained.

The smoothing of river rocks is done by filling in rough surfaces with finer sand that drops when the river currents slow. It often is the same general material loosened by wind and deposited in the water. Eventually the sand becomes welded to the surface, like a rind.

I first noticed this with a bit of granite I picked up somewhere along route 554 north of Ojo Caliente last fall. You can see the brown outer skin is very different than the quartz and mica inner stone. If you look carefully, you can see the outer layer isn’t uniform in thickness, but penetrates in low edges to fill the rough places. That’s what creates the coarse textured, smooth feeling exterior surface.

If the river rocks were polished by removal, they’d look more like tombstones or the stone above. Instead, they look like amorphous potatoes. Round, formless, grey lumps.

Someone broke the above rock outside a near neighbor’s drive. Since he works for Cook’s Transit Mix, I assume the rock came from somewhere just north of Española. You can just see the lighter colored outer layer that built up around the granite, especially on the lower curve in the upper left had corner.

Last weekend my immediate neighbor had loads of sand brought into our shared drive by a friend of his who said it came from his yard in Velarde. A broken piece of granite filled with quartz and mica landed near my gate.

Again, you can see the boundary between pock marked grey shell and the uneven texture of the broken face. It may looked like it’s been smoothed by removal, but if you look closely you can see the uneven fill that protects the bright interior with a dull overcoat. In places it has even started to colonize the surface.

A Rock Is a Rock Is a Rock

The young man in the mineral shop who dismissed my gray rock as “just a rock” with none of the special characteristics of a recognized mineral was wrong.

Rocks aren’t just rocks. To begin with, there are only eight elements that go into the composition of most: oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium. They represent great permutations on matter.

Basalt is some combination of oxygen, silicon, aluminum, calcium, sodium and magnesium. The percentages of silicon and sodium are often used to define the type.

Granite is formed from magma and contains oxygen, silicon, aluminum, sodium and potassium.

Quartz is essentially oxygen and silicon.

Rocks don’t limit themselves to just the eight fundamental elements. Basalt, pyroxene and perovskite can contain titanium. Zircons are silicon and oxygen with zirconium.

More important for geologists interested in the history of the planet are their trace minerals. The half-lives of radioactive isotopes of elements like uranium, neodymium and samarium are used to date them. Zircon crystals are the oldest found above ground.

Rocks, as pieces of matter, do have definable characteristics. In the early twentieth century, Norman Bowen crushed them so he could heat the powders until they melted, then heat them more until they boiled. He let them cool in controlled steps.

He ended with iron-magnesium bearing olivine which was formed at a high temperature and pressure. At a specific lower temperature it becomes pyroxene. At another temperature, pyroxene becomes amphibole, and that, in turn, becomes biotite. In contrast, plagioclase gradually transitions from a rock rich in calcium to one rich in sodium without the abrupt phase breaks.

At a set point, they become potassium feldspar, which becomes muscovite, which becomes quartz. These are the three major components of granite, with quartz the least likely to weather away because it was formed in conditions closest to those of the present.

In the past few decades, new technologies like diamond-anvil presses have allowed geologists to return to experiments like those of Bowen, only now they are looking at how rocks are altered at the higher pressures and temperatures deep in the mantle. Instead of changes in elemental chemistry they’re interested in changes in crystalline structure.

When pressure increases, the O^2- ions in olivine are altered into a spinel structure. At even higher pressures, the spinel structure converts to one found in perovskite and another found in periclase.

Near the earth’s surface, enstatite combines silicon, oxygen and magnesium. It too converts to a perovskite-like structure at higher pressures, then near the bottom of the lower mantle to a post-perovskite structure.

All of this makes terminology a bit confusing, since the same thing is identified by its elements, its crystalline structure and its origins. What one thought was just a rock isn’t. There are at least 11 forms of olivine, including basalt which is further broken into five groups. There are 24 types of pyroxene, including enstatite, and 37 varieties of mica, including biotite and muscovite.

It doesn’t help that only some of these can be seen. Anything with a post-perovskite structure immediate changes to something else when the pressure relents which it must if it’s to make a journey from the center of the earth where it may end up as simple quartz in my arroyo.

Mica (top) from near Albuquerque; granite with bands of gray quartz and shining specks of mica (middle) and a thin piece of quartz (bottom) from the far arroyo.

Quartz

My bookshelf is beginning to resemble something hidden in the backroom of a museum. Sedimentary rocks kept from crumbling on my floor lie in baggies. None are particularly attractive. They simply will be informative if I ever put them side by side to learn the differences in the different sediments in this area.

My camera doesn’t like any of them. I fantasize about the silicon chip rebelling at such ugly dates and give it nicer silicon oxide to look at.

White colored quartz is common in the arroyos, usually as pebbles, but sometimes in larger hunks. But more aesthetically pleasing are the small, broken ones with exposed facets that capture light.

White Sand

There are things you always say you’re going to do, and something always intervenes.

Every time I drive north towards Taos from Velarde I tell myself I really must pull over on the way back and look. Fallen lava boulders litter the right shoulder as you rise. By necessity, the turn offs are all on the other side. The road has too many blind spots to simply cross over.

But then, when you’re coming home, you’re, well, coming home. It’s a different mental state. There’s never time to pull over.

Well, I finally did it. When I was coming back from the Dixon area a few weeks ago, I pulled over in some of the places between Embudo and Velarde where the rift is narrow, the Rio Grande close to the road.

The look up towards Taos isn’t quite as dramatic as it was when I was driving north - but then it’s like the drive down La Bajada Hill - there are no turn offs when the rocks are the most menacing. The turn offs are only where there’s room, which, by definition, is not where it’s most exciting. Perspective is different from fifty feet across the road.

The river turns out to look like a river there, like any river anywhere in the country where water actually flows and doesn’t simply ripple around sand islands as it does in Española. Nice, unusual actually for New Mexico, but conventional still the same.

Then there’s the unexpected, the thing you didn’t know was there because you never stopped.

In this case, there was a patch of white sand near the river with Russian thistles and purple asters. You think, wait a minute, white sand? New Mexico?

The White Sands in the Tularosa Valley are gypsum dunes. This glittered in the sun. It had to be ground quartz, which of course is as much silica as sand. In fact ground pure quartz provided the material for Venetian glass makers.

Quartz has the greatest weather resistence of any of the rocks in the area. It’s often the last remaining eroded rock from the Sangre de Cristo. This “dune” looks suspiciously like how that sedimentary grey rock I saw earlier in the day beyond Dixon would look if everything soft disappeared and left only the quartz and shining mica. And it photographs the same way, too brown and out of focus, or all glare.

I Am Not a Camera

The sun’s rising later. It’s dark when I wake at six. Instead of getting up, I check the heated mattress pad is still connected and snuggle back in. It’s the best time for unstructured thinking.

Some people have everything organized. Before they go on a trip, they’ve read the guidebooks, have an itinerary, know what to expect. Perhaps they’re anxious they’ll never get another chance, that this is their one opportunity and they can’t afford to blow it.

I haven’t classed them yet as a separate species, but it’s a possibility.

Or maybe I should be thankful my mother was too intimidated by Spock and Freud to do anything serious about toilet training.

I learned long ago there’s no place in this country I can’t return to. That doesn’t mean things won’t change. Some evenings clouds are so special I wish could remember them. But there’s always the possibility for something wonderful some other night, some other summer.

The freedom of retirement is that time constraints are gone. If I didn’t get quite the picture I wanted Saturday I can drive back today. Or will be able to soon. Please let that woman who interviewed for the job yesterday come to terms with my boss.

A camera matters if you’re not blessed with a painter’s imagination and skills. I may not be able to recall those clouds, but it should remind me.

And so, this morning as I lay in bed I returned to the problem of photographing rocks that apparently send no signals digital equipment can recognize. I thought maybe I should simply take a picture of the same section of that rock with each light setting on the new camera to see the differences.

Maybe tonight.

This morning there was more light coming through the kitchen window and camera settings made do difference. The rock was determined to be brown and out of focus.

I took out the other camera, the close up one, which is turning out to be much more difficult to learn than I expected. It rarely gives me what I want when I first try, but always teases me with possibilities. It makes clear, it’s always my fault it doesn’t do what’s expected.

It folds in half, so it can be set it on its base in a V or triangle to take pictures. However, the lens is at an angle to the flat rock and gets blinded by reflecting light. I tried standing it on its end in a great U so the lens was perpendicular to the rock and things got better.

Then I decided to take a wet paper towel and wash the rock in a few places. As a child I learned it was worth while washing quartz, but useless if not dangerous with sandstone. The idea of washing a sedimentary rock is quite alien.

However, I discovered the wet rock photographed better than the dry one. Perhaps the water plays with the lens so the light reflects differently. It was finally possible to see the bits of mica and the flakes of quartz, though the focus was still fuzzy, the glints bright blurs, and the light glaring.

I have no choice but to master these cameras - a new one won’t be better, just different. But I curse them a lot, because throwing them across the room would, contrary to most rules of punishment, actually hurt them more than me.

Please, I beg them, if you can’t see what I see, can’t you please show me something better?

Tell me, is that pebbly surface of dark carbon and white silica something the land looked like before it hardened into rock? Can you show me the past?