Category Archives: sedimentary rocks

It’s Sedimentary, my dear Watson


Sometimes you can’t see the forest for the trees. Take, for example, the sedimentary rocks: a complex family of rocks that reflects the taking-apart, the rotting, the sorting, the dispersion, and the reconstituting of bedrock that operates so vibrantly on the lively surface of our planet. As one of my students wrote, after puzzling sadly over a tray of sedimentary rock samples in his lab, “the sedimentary rocks vary wildly from one another”.

And it’s true:  the most recalcitrant, homogenous, chemically self-satisfied lavas and schists become sands of silica pure enough to make glass, clays fine enough to fire into porcelain, salt you can eat, carbon you can burn, lime you can cast into concrete, and pigments with which you can color your paints. Not to mention some less civilized – we prefer to say ‘immature” – members practically indistinguishable from the rocks that broke down to make them.

Working out a scheme to bring some order to this chemical fiesta is challenging. But in a fundamental way, the sedimentary rocks fall into two camps: those that were transported into place and those that formed in place.

Bedrock breaks down into rubble, sand, clay, and a variety of dissolved minerals here at the Earth’s surface, often mixed with organic material where soils form. This weathered material can be mobilized by slumping and sliding, running water, or wind, and transported elsewhere, exposing fresh bedrock to weathering. The mobilized material is called sediment, and the visible component made up of broken fragments of bedrock, resistant grains of quartz, and particles of clay is called clastic or detrital sediment.

The vast majority of this detrital sediment is transported by streams of running water, where it is  efficiently sorted into various size fractions, and finally dropped, possibly to be remobilized by wind,  waves, or turbidity flows. From this activity comes all the mudstone, shale, sandstone, conglomerate, and breccia that makes up the bulk of the sedimentary record. What all these detrital sediments have in common is the fact that they were transported into place.

Red mudstone and lenses of sandstone in the Abo Formation, NM

Red mudstone and lenses of sandstone in the Abo Formation, near Sena, NM

The invisible component of dissolved minerals mixes freely and anonymously in lakes and lagoons, swamps and seas, where a variety of chemical and biological processes operate to precipitate out salts and silica of remarkable purity. Carbon, carried by invisible carbon dioxide, plays its part, trapped in vegetation, which can be preserved as coal, or bound up in the seas with calcium and magnesium to form limestone and dolomite. Depending upon the role played by inorganic vs organic processes, these sediments are called the chemical or organic sediment. But what they all have in common is the fact that they were formed in place. As geologist Noel James famously quipped, “carbonates are born, not made.” The same could be said about all the chemical and organic sediments.

White, chemically-precipitated gypsum of the Todilto Formation, covering beds of red shale near San Ysidro, NM

White, chemically-precipitated gypsum of the Todilto Formation, covering beds of red shale near San Ysidro, NM

Asking yourself whether the layers of sedimentary rock you’re seeing were transported there, or simply formed where they lie, should trigger a chain of questions that will have you thinking like a geologist in no time!



The Vivid Permian

Earlier this week I made a scouting trip into the Pecos Wilderness, a mountainous wilderness area northeast of Santa Fe. To do this I had to drive around the southern tip of the Sangre de Cristo Mountains, and like practically all travelers before me, I was funneled over the Glorieta Pass, the old route of the Santa Fe Trail around the Rocky Mountains, and the current route of both the Amtrak Southwest Chief, and Interstate 25, which links Denver and Albuquerque.

As soon as Glorieta Mesa begins to rise on the south side of the pass, sedimentary red beds appear in the road cuts and hillsides along the freeway.

Glorieta Pass, looking west

Glorieta Pass, looking west

These colorful rocks are Permian in age. The geologic record of both the Pennsylvanian and the lower Permian systems is well represented in northern New Mexico, and for much the same reason: the tectonic disturbances of the Ancestral Rocky Mountain orogeny. But unlike the rather dull grey tones of the Pennsylvanian sediments, many of our Permian beds are bright with purples, reds, oranges, and warm yellows. Why is that?

As the Pennsylvanian Period ended, a shallow sea, which had covered much of New Mexico, began to withdraw to the west and to the southeast. Vast regions of Precambrian crystalline basement rock – granite and gneiss – lay exposed in the Ancestral Rockies, deeply weathered by ages of tropical rainfall, and as the climate dried and shifted to seasonal monsoonal conditions, these weathered highlands began to release huge amounts of arkosic sand and red silt across the landscape.

The coarsest material accumulated in alluvial fans near the mountains, trapped in basins still slowly subsiding around the eroding highlands. Sand and finer material spread out in broad alluvial plains, where silt-choked streams wandered across flats alternatively steaming in monsoonal rainfall or baking in the equatorial sun, in an atmosphere charged with excess oxygen. The old Pennsylvanian basins, filled with dull grey marine limestones and black shales, were soon buried under a purplish-red blanket of mud, silt, and sand. In New Mexico these sediments have been given the name Abo Formation, after classic outcroppings near Abo Pass.

A roadcut in the Abo Formation near Sena, New Mexico

A roadcut in the Abo Formation near Sena, New Mexico

As the climate became even more arid, these streams withered away, and fine red sand was mobilized by desert winds over the alluvial plains. Vivid orange dune fields began to cover the muddy plains and sandy channels of earlier times. These dunes were bordered by the remnants of the sea in the southeast part of New Mexico, where the sand stabilized into an arid coastal plain, with blinding salt flats bordering sandy lagoons. These natural evaporating pans spiced the orange sand with dolomite and white gypsum, or yeso, in Spanish, and from this word the bright package of sediments has taken its name: the Yeso Formation.

Orange sandstone of the Yeso Formation lying over Abo

Orange sandstone of the Yeso Formation lying over Abo beds, near Sena, New Mexico

One last assertion of the Permian Sea shaped the durable, mesa-forming rocks that cap these red beds. As the sea spread back toward northern New Mexico, away from the Permian Basin of West Texas, its waves washed the sands of the coastal plain into clean, well-sorted shallow marine bars and beaches. Much of this beautiful yellowish sand blew, on arid winds, away from the beaches and into dune fields of a much paler color than the underlying Yeso sands. Altogether this blanket of clean, and now firmly-cemented sand is called the Glorieta Sandstone, named after the Glorieta Mesa which it crowns.

A closeup of cross-bedding Glorieta Sandstone near the top of Glorieta Mesa, near Rowe, New Mexico

A closeup of cross-bedding Glorieta Sandstone near the top of Glorieta Mesa, near Rowe, New Mexico

Before the sea retreated it laid down a final deposit of hard limestone, thin to absent in northern New Mexico, but noticeably thicker to the west and especially to the south, where its type section in the San Andres Mountains outcrops. The San Andres Limestone is the most widely exposed Paleozoic formation in New Mexico, and, where it plunges into the subsurface, forms both important fresh water aquifers and oil reservoirs. But as far as color goes it echoes the grey marine sediments of the Pennsylvanian days, and marks the end of the vivid Permian.

The Pecos River cutting through the Glorieta Sandstone in Villanueva State Park

The Pecos River cutting through the Glorieta Sandstone in Villanueva State Park







Geologists adore the sedimentary rocks. These are the rocks that contain the Earth’s archives. These rocks outline areas of crustal subsidence and mountain uplifts. They reveal the distribution of ancient environments at the Earth’s surface. They record changes in climate and fluctuations in sea level. They preserve the record of life on our planet.

Of all the sediments, the sandy ones carry the most information about the widest variety of ancient environments: desert, piedmont, river valley, coastal plain, delta, beach, marine shelf and slope, in all their infinite variations.

Muddy sediments are much more abundant in the geological record, but since mud can accumulate anywhere there is slack water, onshore or off, shallow or deep, we depend on interbedded sand to help us understand their story.

Chemical concentrates, like limestone, dolomite, rock salt, gypsum, and coal are fascinating, and economically important, but they each form under very specific conditions. Their presence complements the story told by sand but cannot replace it.

On top of that, the layers of sandstone formed by sandy sediments are often the most eye-catching aspects of the landscape, forming resistant cliffs, rims, and benches when they are interbedded with more easily eroded mudstones and shale. And from an economic standpoint, the more porous sandy sediments are where much of our fresh water, oil, and natural gas is stored.

Santa Fe River carrying sand to the Rio Grande

Santa Fe River carrying sand to the Rio Grande

Continental Environments

Onshore, in continental settings, streams cut and and then fill channels with sand, carrying the finer mud on downstream, or depositing it over the banks during floods. Sediments deposited onshore by running water are called alluvium, and large amounts of sand can accumulate in the alluvial plains built up by streams. Where a stream enter a body of standing water, like the sea or a lake, much of its load of sediment is quickly dropped to form a delta, and sand can be deposited in large river mouth bars. As a delta builds outward, it extends the alluvial plain seaward, or fills the lake.

Coarse sand, mixed with pebbles and cobbles, frequently accumulates as alluvial fans at the margins of basins that flank mountain ranges, filling stacks of ill-defined and overlapping channels.

Fine sand can be remobilized by wind in arid climates, where it often collects in vast dune fields. These dune fields are a kind of large onshore sand bar. Since this sand is windblown, and not carried by running water, it is not considered an alluvial, but rather an aeolian sediment.

Permian red beds, with fossil dunes forming the steep cliff

Permian red beds, with fossil dunes forming the steep, poorly-bedded sandstone cliff

Marine Environments

Offshore, in marine settings, waves and currents rapidly redistribute the sand brought in by streams into beaches, barrier islands, and other kinds of shallow marine bars. Estuaries and lagoons also trap sand in their shallow waters. These environments are collectively called marginal marine or shallow marine environments.

Further offshore, fine sand can  be remobilized by undersea currents flowing in submarine channels cut into the continental shelves, and carried deep on to the ocean floor, where it accumulates as channel fill and thin overbank turbidites in submarine fans.

Shallow marine sand bars

Shallow marine sand bars forming ledges of sandstone

So you can see, sandy sediments truly are the great archivists of the sedimentary record. Learning to read the stories written in shifting sand will greatly enrich your appreciation of the natural world.





You’re not around geologists very long before you hear the word “formation” mentioned. For that matter, almost anybody out enjoying a landscape with rocks in it is apt to use the word themselves, as in “there were the coolest rock formations out by Diablo Canyon!” Geologists cringe just a little when they hear that – not that you’d notice – because to them the word “formation” has a very definite meaning.

This is not a formation. It is, however, "Camel Rock"

This is not a formation. It is, however, “Camel Rock”

A formation is a group of sedimentary strata, volcanic beds, or igneous intrusions with upper and lower boundaries that can be easily traced and mapped across the countryside.

The word ‘mapped’ is critical in this definition. The first step geologists make in their attempt to understand the natural history of a region is to construct a geologic map, showing the relations between the different rocks there. In order to do this, he or she has to make some distinctions among the various kinds of rocks, subdividing the outcrops into “meaningful units” that are large enough to plot on the map and distinctive enough that other geologists can agree on their selection.

Typically the distribution of formations is shown by using different colors or patterns:

A simple geological map of New Mexico

A simple geological map of New Mexico

Because of the way they are chosen, formations have lithologic significance, consisting of a single rock type, or a cluster of closely associated rock types. Formations are the basic Rock Unit of stratigraphy.

This is reflected in the formal names given to formations, based on their definition at a type section, at a specific geographical location, where other geologists can inspect the choice. For example, the Mancos Shale, named after a town in Colorado, or the Redwall Limestone, named after the famous cliff in the Grand Canyon, are cases where the formation is basically one rock type. In cases where the formation is chosen to be a mix of associated rock types (still distinctive enough to trace and map!) the geographic name prefixes the word “Formation”. An example is the Galisteo Formation, named after a village in New Mexico.

In sedimentary formations, the strata within a formation tend to be more or less parallel, or conformable, with one another.

The Galisteo Formation, showing a mix of strata

The Galisteo Formation, showing a distinctive mix of conformable strata

Because of the way they are chosen, formations also have a genetic significance. Each one records a time of fairly uniform environmental or depositional conditions, different from adjoining formations. Understanding this is vital in our attempt to work out how conditions changed with time.

Finally, formations have a built-in time significance. By virtue of the principle of superposition – younger layers must rest on older layers – formations can be put into a relative time order, from oldest to youngest. In the case of igneous intrusions, the principle of cross-cutting relationships serves the same purpose. If an intrusion cuts through another body of rock, it must be younger in age than the rock it intrudes.

That’s a lot of significance packed into one common word! “Formation” truly is a useful concept in geology.

And yet, as geologists – especially petroleum geologists – soon discover, the formation is not the perfect stratigraphic bookmark we tend to think it is.

Among sediments, depositional environments can exist simultaneously, side by side, in a given area. Think of shrimp boats dragging their nets through the mud while swimmers frolic on clear sandy beaches at the shore. These environments – one accumulating mud, the other sand – migrate with time. A barrier beach may slowly build out over a muddy marine shelf, which, millennia later, will show up in the geologic record as sandstone over shale. We’d very likely define two formations in our mapping – say, the Point Lookout Sandstone over the Mancos Shale – and consider that the sandstone is everywhere older than the shale. But we would be wrong. The two formations actually interfinger and there are places where the sandstone here is the same age as the shale there.

Such formations are called diachronous – “passing through time”. Most sedimentary formations are diachronous to some extent. Now this may sound like the sort of hair splitting only a stratigrapher could enjoy – but understanding these sorts of relationships can be critical in defining potential petroleum traps, sources, and seals in an oil-bearing basin. An entire branch of stratigraphy called sequence stratigraphy has developed among oil companies (rooted in the insight of perceptive geologists long before, I must emphasize) in order to establish accurate time lines within and across formations, repackaging the strata into a different kind of “meaningful unit” called a sequence.

But these are subtleties we can let rest for now. You have to start somewhere, in every science, and the notion of a formation – properly used –  is one of the first stepping stones in geology. So bite your tongue next time you hike in Zion National Park or wander through Carlsbad Caverns. A geologist might be listening.


The Pennsylvanian System in New Mexico

In the last post I discussed an episode of crustal disturbance that created a system of uplifts, basins, and mountains centered in Colorado, but connecting with related disturbances in Oklahoma and North Texas, during the late Paleozoic Era. In Oklahoma the structural features related to this tectonic activity are called the Wichita System. In Colorado they are called the Colorado System, or, much more commonly, the Ancestral Rocky Mountains, because of the remarkable coincidence of the main uplifts with the modern uplifts and mountain ranges of the Southern Rocky Mountains, which we admire today.

This tectonic activity started during the Mississippian Period and died away in the Permian Period, moving vaguely from east to west over time. In Northern New Mexico, the largest impact was during the Pennsylvanian Period and the early part of the following Permian. Because so little geologic activity had occurred in the area prior to the Ancestral Rocky Mountain orogeny, the effects of the disturbance are striking in the sedimentary record here.

Nearly 60 percent of New Mexico was covered with sediments deposited in the Pennsylvanian System as shallow seas and sediment-shedding uplifts rippled up and down an area where, formerly, nothing but ancient granite, schist and a few thin outcrops of Mississippian limestone baked in the Paleozoic sun. Basins sagged on the continental crust and made space for mud, sand, and gravel weathered and eroded from the Ancestral Rocky Mountains, which were now practically rotting in the heavy rainfall and tropical weather of the time. Exotic plants and insects thrived and coal swamps darkened the margins of the basins. In times when sea level stood high, the warm seas clarified and marine organisms multiplied, secreting lime and leaving hard parts.  Limestone is particularly abundant in the shallow marine shelves that surrounded the basins in New Mexico.

This was the Age of Coal. In Europe and Asia the Mississippian and Pennsylvanian Systems are lumped together as the Carboniferous System. Although the ‘carbo’ refers to coal, it might as well refer to carbon, because the creation of enormous amounts of limestone requires the extraction of enormous amounts of carbon dioxide from the environment, and that carbon was buried just as thoroughly as the carbon sequestered in the huge coal measures (and petroleum deposits, for that matter) of the time. Oxygen levels in the atmosphere must have been freakishly high by the end of the Pennsylvanian Period.

In that careless way the Earth has of disregarding all her previous efforts, the deepest Pennsylvanian basin, and the thickest strata in Northern New Mexico and Colorado, were deposited in a trough that subsequent mountain-building activity casually pushed up to form a range of mountains we now call the Sangre de Cristo Mountains. In Colorado these strata were distorted almost out of recognition, but in New Mexico, the rocks rode up in a fairly intact manner. Here is a view looking south over Pecos Baldy, in the heart of the Pecos Wilderness, showing stratified, smoothly-weathered Pennsylvanian rocks in curved fault contact with the Proterozoic quartzite holding up the peak:

Pennsylvania strata on left, rugged quartzite on right
Pennsylvanian strata on left, rugged quartzite on right. Click on the image to enlarge.

A little further south, you can see a beautiful exposure of Pennsylvanian strata at Daltons Bluff, just upstream from the little village of La Posada. There’s even a bit of Mississippian limestone thrown in, visible as the lighter grey rocks at the bottom of the pile, near the river:

Looking down the Pecos River at Dalton's Bluff. Click on the image to enlarge.
Looking down the Pecos River at Daltons Bluff. Click on the image to enlarge.

Geologists have climbed up and down this outcropping like ants.

On the Santa Fe side of the mountains, the Pennsylvanian rocks are not nearly this thick. And what little there is of them is preserved haphazardly in patches and fault blocks low on the western side. It is likely that the block of crystalline basement rock that forms the Santa Fe Range today formed a shallow platform over which only thin layers of sediment were deposited. Subsequent uplift and erosion of the range stripped off most of what did get deposited. The rest went down with the ship, so to speak, when the Espanola Basin floundered into the Rio Grande Rift.

Nevertheless, the beds that are left on the west side of the mountains are easy to access and display such a variability of rock types, sedimentary structures, fossils, and stratigraphy that you could easily illustrate half a textbook on “Sedimentation and Stratigraphy” with them. Most of the rocks are tilted so that you can walk up and down the section by following dry washes:

Dry wash


The Pennsylvanian Period was a time in the Earth’s history when sea level fluctuated frequently and with large amplitude. Although New Mexico was near the equator, much of Gondwana, the southern complex of continents slowly assembling into Pangea, was over the South Pole and enduring cycles of continental glaciation. Geologists suspect that, similarly to the Quaternary Period in which we live, sea levels rose and fell with the waxing and waning of continental ice sheets. Near Santa Fe you can walk out at least one cycle of sea level change, starting with beds of limestone deposited in a clear, well aerated, subtidal environment:

Shallow marine limestone
Shallow marine limestone

Higher up the section the limestone struggled with influxes of mud as sea level fell and the sea became murky with the outbuilding of a shallow delta.

Interbedded limestone and mudstone
Interbedded limestone and mudstone

Above these little coarsening-upward cycles of silt and sand appear:

Sand introduced into the basin as the shoreline approaches
Sand introduced into the basin as the shoreline approaches

Many of these beds show beautiful ripple marks on their bedding planes. You can practically feel the shifting tides, reflected in the stone:

Ripple marks on a bed of sandstone
Ripple marks on a bed of sandstone

Higher yet a bed of very well-cemented sandstone containing coarse grains of quartz and feldspar, and showing the lenticular bedding of alluvial channel fill, announces the arrival of the shore. The basin has filled to sea level:

Arkosic channel-fill sandstone forming the floor of the wash
Arkosic channel-fill sandstone forming the floor of the wash

The siltstones that lie on top of this bed contain fragments of plant fossils, like these giant horsetail ferns:

Calamites fossils
Calamites fossils

These deltaic sediments are soon overwhelmed, however, by the return of the sea, and are buried in mud, interbedded with thin beds of limy silt full of marine fossils:

Marine fossil hash
Marine fossil hash

Finally even these beds disappear into thick, organic, featureless shale:

Thick marine shale
Thick marine shale

The sea has returned.

Cycles like these are the story of the Pennsylvanian System all over the planet. The reason we have a record of this kind in Northern New Mexico is the disturbance of the Ancestral Rocky Mountain orogeny, making space for sediment to accumulate, and making highlands to supply the sediment. The mountains that once graced the sounds and bays of tropical New Mexico have long since vanished. The only evidence of their existence the detritus they left behind.





Red beds

My recent scouting adventures here in Northern New Mexico have reminded me just how important “red beds” are to our colorful landscape. Red beds are sedimentary rocks, usually sandstones, siltstones, or shales, that are stained various shades of red and orange. As you make the drive up to Ghost Ranch from Santa Fe, on your way to visit the Georgia O’Keeffe country, you abruptly pass from a pastoral river valley lined with small farms and green cottonwoods to this lurid scene:

On the road to Ghost Ranch

On the road to Ghost Ranch

Or you take a drive up from Albuquerque to Jemez Springs and watch the sandy, juniper-studded badlands give way to flaming cliffs:

Entering San Diego Canyon on the way to Jemez Spings

Entering San Diego Canyon on the way to Jemez Springs

Rocks like these are so ubiquitous here that the first great summary of the State’s geology, published by N. H. Darton in 1928, was called “Red Beds” and Associated Formations in New Mexico.

The reddish colors in red beds are due to ferric oxides – oxidized iron (rust, basically) – that coat the tiny mineral grains that make up sandstone, siltstone, and shale. It doesn’t take much: as my artist friends tell me, a little red goes a long way.

Except for the obvious fact that these stained rocks must have contained a few iron minerals and have been exposed to free oxygen sometime during their genesis, understanding the conditions under which red beds form has been surprisingly elusive. For a long time it was assumed that they formed in ancient deserts and always recorded the existence of a hot arid climate. This was based on analogy with modern red deserts like those found in the American Southwest and Australia.

Many modern deserts are grey, however, and most of the soils and sand of the red deserts are reworked from the red rocks that already outcrop there. Permian rocks, like those around Jemez Springs, and Triassic sediments, like those you see at Ghost Ranch, are famous for their red beds and these rocks crop out all over the American Southwest, contributing a vast share of modern sediment.

More recently red beds have been taken to give evidence of seasonally dry conditions – monsoon climates – in the past. Modern areas with monsoons are generally considered semi-arid, hot and dry much of the time, then soaked in rain. Such alternative drying, then wetting with oxygenated water, seems to agree with a chemistry that would stain sediment with iron. During the Permian and Triassic Periods the Earth’s continents were assembled into a vast supercontinent named Pangea, whose climate must have varied dramatically from the modern dispersed continents. Parts of Pangea may have experienced mega-monsoon conditions (to go with its “supercontinent” status, I guess) and this has been used to explain the prominence of red bed from that time.

But lately doubts have arisen. The safest thing to say is that the red color indicates former good drainage in the sediments. Terrestrial conditions. Other clues hidden in the red sediments must be sought and added to understand the ancient climate in which they formed. Here is another set of New Mexico red beds seen along the “Turquoise Trail” that links Santa Fe with Albuquerque:

The Galisteo Formation near Cerrillos

The Galisteo Formation near Cerrillos, New Mexico

These rocks are Eocene in age and record the weathering and erosion of the first ranges of the Rocky Mountains as they were born. This was a very lush and wet time here, almost tropical compared to the modern climate. Perhaps red beds are forming in the Amazon basin, today.

No matter what ultimately created them, red beds make a big contribution to the scenic beauty of New Mexico and indeed, all of the American Southwest. And their elusive origin reminds us that geologic investigation, like all the sciences, is never static.


The Colorful Folds of the Sierra Nacimiento Mountains

Sedimentary rocks hold many charms for geologists. They contain the Earth’s archives, recording the distribution of ancient environments, outlining areas of subsidence and uplift, and tracking changes in climate and fluctuations in sea level over the ages. They preserve the history of life on Earth in the fossil record. They contain mineral fuels like coal and petroleum.

But there is an additional, almost incidental aspect of their record-keeping abilities which never fails to fascinate even casual observers. It’s tucked into the formidable phrase “original horizontality”.

Because sediments at the Earth’s surface are originally laid down in approximately horizontal layers, dispersed by moving water or blowing wind and settling under gravity, we can infer subsequent deformation of the outer crust by their displacements from the horizontal. Deformation by flexure – or folding – is one of the most striking manifestations, and some truly spectacular examples can be found here in New Mexico along the southern flanks of the Sierra Nacimiento Mountains, only a short drive west of Albuquerque.

Here is a Google Earth Image of the San Ysidro anticline, an up-arched buckle in the Earth’s crust beautifully outlined by a layer of white gypsum encircling a core of red shale:

The San Ysidro Anticline

The San Ysidro Anticline

Erosion has scraped out the soft center of this fold, exaggerating its appearance. Other sedimentary beds tilt away from the elongated center of the fold – its axis – in all directions. The axis of this anticline dips below the surface – or plunges – toward the lower left.

This part of New Mexico is just on the edge of the Colorado Plateau, which stretches from here far to the north and west. The Colorado Plateau is famous for its colorful strata, laid out in buttes and mesas or exposed in deep, sheer-walled canyons, and these wild color contrasts make this area of folding more spectacular than most.

Here is a view of the fold taken from the opposite direction which includes its companion fold, a plunging syncline:

Complementary plunging syncline and anticline

Complementary plunging syncline and anticline

Synclines are down-buckles in the Earth’s crust, U-shaped in cross-section. Rumple your napkin up by pushing it across the table and you’ll get the idea of why these folds come in pairs.

Sedimentary rocks make a cover – a sort of thin blanket – at the top of the Earth’s crust, resting on older and unrelated crystalline rocks like granite or schist. Geologists often refer to this ancient foundation as the crystalline basement. Just to the north of these folds, an upthrust of the crystalline basement rocks in the Sierra Nacimiento has flexed these same strata sharply upward, almost to the vertical:

Along the western front of the Sierra Nacimiento

Along the western front of the Sierra Nacimiento

Scenes like this are common throughout the Southern Rocky Mountains and the Colorado Plateau. Some famous examples include the Flatirons near Boulder, Colorado and the Garden of the Gods, near Colorado Springs.

One pleasant byproduct of folding sedimentary strata, at least for geologists, is the fact that you can go up and down the geologic record by walking among the tilted strata, rather than scaling a cliff or drilling a well. Here is the transition from the colorful shales and sandstones of the Morrison Formation, to the right, into the duller grey and yellow shales of the Dakota Formation, off toward the left:

Tilted Mesozoic strata in the east limb of the San Ysidro Anticline

Tilted Mesozoic strata in the east limb of the San Ysidro Anticline

A scramble along one of the dry washes that cuts the flank of the anticline will let you examine a huge thickness of rock.

Of course, you may decide that today is not the day for scrambling down a gulch of slippery shale, and are content to simply enjoy the view:

Enjoying New Mexico

Enjoying New Mexico

It’s all good.