One of my absolutely favorite outcroppings in the mountains above Santa Fe is this little stream-polished window into the depths of the Earth’s crust, along Little Tesuque Creek, not far above the Bishop’s Lodge Resort:

Hornblende schist - a kind of amphibolite - along Little Tesuque Creek, Santa Fe, New Mexico

Hornblende schist – a kind of amphibolite – along Little Tesuque Creek, above Santa Fe, New Mexico

The strain displayed by this dark schist is hard to miss. Outcroppings of metamorphic rocks are abundant in the ancient crystalline core of the Santa Fe Range, but few of them exhibit such dramatic stretching as this example. Here’s a close up of a structure known as boudinage:

Boudins of more quartz-rich layers in the schist

Boudins of more quartz-rich layers in the schist

These stony strings of sausages are the result of differing competencies among the mineral components of the schist. The smooth borders attest to the overall ductility of the rock as it was being slowly sheared, deep in the plutonic realm of the middle crust.

These schists are sturdy, and rounded cobbles of this rock are common throughout the foothills and stream beds west of Santa Fe. Most of them glitter in a dark way, as tiny prismatic crystals of hornblende, aligned by shearing strain, send back a little light. Harder to see is their matrix of white plagioclase feldspar, which probably makes up over half the rock’s mineral content.


One of the joys of teaching is, oddly, learning. Teaching a basic class in physical geology to a wide range of students at the local community college brings me weekly opportunities to hone my communication skills – to try to find concise ways to summarize complex topics in an accurate, yet simple, manner. The struggle to do this teaches me far more, I suspect, than I teach my students.

Volcanism is a case in point. It’s a big topic. Is there a way to capture its essence in a blog-sized post? Here’s a try:

As I’ve mentioned before, New Mexico is the Volcano State. The sheer variety of volcanic features here is unrivaled by any other state in the country, including Alaska and Hawaii. And yet all of this volcanic activity can be encompassed between two poles: effusive eruptions and explosive eruptions.

Effusive eruptions

Effusive behavior refers to relatively quiet outpourings of molten lava from a volcanic vent. Effusive eruptions of fluid basaltic lavas are exceedingly common, forming stacks of thin flows that pile gently into broad shield volcanoes, or spread out across the countryside in lava plateaus.

Valley of Fires, a vast flow of basaltic lava, in southern New Mexico

Valley of Fires, a vast flow of basaltic lava, near Carrizozo, New Mexico. Click to enlarge.

Effusive extrusions of viscous silicic lavas, like dacite or rhyolite, are much less common. These eruptions typically form short, thick, glassy flows, piling into rubble-covered lava domes that usually plug their own vent.

An arc of rhyolitic lava domes in the Valles Caldera

An arc of rhyolitic lava domes in the Valles Caldera, west of Los Alamos, New Mexico. Click to enlarge.

Explosive Eruptions

Explosive eruptions occur when violently expanding gases fragment molten lava into clouds of pumice, scoria and volcanic ash. The first phase of many otherwise effusive eruptions of basalt is a gassy discharge that builds a one-shot, steep-sided cone of scoria, commonly called a cinder cone.

A cinder cone you can walk into, near Grants, New Mexico

A cinder cone you can walk into, near Grants, New Mexico. Click to enlarge.

Eruptions of somewhat more silicic lavas, like andesite, dacite, and trachyte often alternate between explosive and effusive phases, over tens of thousands of years, building “composite” cones of lava and ash that can grow into large – but unstable – mountains.

Mt. Taylor, near Grants, New Mexico, is a classic composite volcano

Mt. Taylor, near Grants, New Mexico, is a classic composite volcano. Click to enlarge.

Although thankfully uncommon, very large eruptions of silicic rhyolite lava can be catastrophically explosive, burying thousands of square miles of countryside under  incandescent blankets of ash flow tuffs up to a thousand feet thick. Events of this magnitude probably alter the global climate. These eruptions leave circular zones of collapsed crust, miles across, called calderas, so large that they are difficult to recognize from the ground.

The Valles Caldera from space

The Valles Caldera from space. Click to enlarge.

What lies below

Magmas trapped and crystallized as shallow intrusives in the feeder systems beneath volcanoes – dikes, sills, and shallow stocks – are often grouped together with the other volcanic rocks.  Where erosion exposes these formerly buried structures, fascinating landforms result, complementing the flows, cones, craters, and calderas of modern New Mexico.

Cabezon Peak, a basaltic volcanic neck, west of San Ysidro, New Mexico

Cabezon Peak, a basaltic volcanic neck, west of San Ysidro, New Mexico. Click to enlarge.


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






Born of Fire

To the surprise of many, New Mexico is sometimes called the Volcano State. It’s not that we have any erupting volcanoes – at present. But the sheer variety of volcanic features here is unrivaled by any other state in the country, including Alaska and Hawaii. We are definitely an igneous state.

Back in fifth grade you probably heard about the three great groups of rocks on Earth: igneous, sedimentary, and metamorphic. Igneous rocks are rocks that crystallize from melts called magma. Magma is a mix of liquid silica-rich melt, suspended crystals, and dissolved gas like water vapor and carbon dioxide. It’s hot: 1300 to 2400 degrees Fahrenheit, hot enough to glow like fire. When it cools, it freezes in complex ways into igneous rocks – “born of fire”.

Most magma remains trapped in the Earth’s crust. But when it gets out, it forms volcanoes, named after Vulcan, the Roman god of fire, with his smoky forge Vulcano in the Mediterranean Sea. Magma extruded at the Earth’s surface is given an older Italian name, lava, which refers to both the flowing melt and the rock into which it cools. Lava flung into the atmosphere by the explosive expansion of dissolved gas forms a variety of fragmented and glassy materials called pyroclasts - “fire fragments”Lavas, loose pyroclasts, or tephra, and pyroclastic material consolidated into rock, called tuff, collectively form the volcanic rocks.

Since volcanic rocks are quenched at the Earth’s surface, they typically have fine-grained, glassy, or fragmental textures. It is usually easy to recognize a volcanic rock in the field, but assigning them to their specific family – basaltic, andesitic, trachytic, dacitic, or rhyolitic – can be frustratingly difficult. Much depends on finding and identifying small suspended mineral crystals to help out, which is something few of us do on a regular basis. Like algebra. Learning to simply recognize a lava, and to name variations based on texture, like pumice or obsidian, is an easy and rewarding undertaking for natural history buffs.


Magmas trapped deep in the Earth’s crust belong to the plutonic realm, named after a darker god, Pluto, the Roman ruler of the underworld. These magmas are intimately associated with the underworld rocks which they intrude, rocks which have been changed by heat, confining pressure, and shearing stresses into metamorphic rocks.

Plutonic rocks, having cooled slowly deep in the crust, with all their juices sealed in, typically have coarse-grained, visibly crystalline textures – granitic textures. This makes it a little easier to assign them names in the field – granite, granodiorite, tonalite, diorite, gabbro, monzonite, syenite – but since magmatic rocks are mixes, not species, there is always some blurring and overlap. Learning to mentally gauge whether the rock is rich or poor in dark minerals, and rich or poor in visible quartz, helps out here. Light-colored, quartz-rich members of the granite and granodiorite family are much more common than the others.

Simply finding a plutonic or metamorphic rock in the field means something has transported that messenger from the underworld up to the surface. What could it be?


There are any number of rock-identification guides and classification schemes both online and off, but one sweet site you might want to visit has been created by our Australian cousins: Igneous rock types. Go have a look!





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.




What on Earth are you doing with Howard Bannister’s rocks?


The classification of the igneous rocks is a morass from which you would be well advised to steer clear. Even Judy Maxwell said, “I can take your igneous rocks or leave them. I relate primarily to micas, quartz, feldspar. You can keep your pyroxenes, magnetites, and coarse-grained plutonics as far as I’m concerned.”

Personally, I love the igneous rocks. Nevertheless, there is one coarse-grained plutonic up in the mountains above Santa Fe which has given me fits in trying to classify. And it points perfectly to the sort of look-alike confusion which plagues the field identification of these rocks.

Here’s the rock:

The speckled rock along Tesuque Creek

The speckled rock along Tesuque Creek

Ideal countertop material, you might say. I asked a hiking companion what he thought it was and was told “it looks just like the granite back home up in the Sierra” – the Sierra being the Sierra Nevada Mountains in California. And it does look just like those granites, except for the caveat going though my head – the curse of an education – that, as the geologist P.B. King relates, in the Sierra Nevada, “true granites in the technical sense are rather minor, most of them being the somewhat more mafic quartz monzonites, granodiorites, and quartz diorites”.

I thought it might be diorite. Diorite is an interesting construction, a French name built from the Greek root dior izein, ‘to distinguish’. Diorite is a granular igneous rock made up of bright white feldspar and dull black hornblende, with a classic “salt and pepper” appearance that every first year geology student learns to identify on sight.

Unfortunately, diorite is very difficult to distinguish from gabbro, another dark speckled igneous rock, which is what another hiking companion (understandably) always thought it was.

There’s a reason field geologists carry around that little ten-power hand lens, and when you look at this rock up close, you discover that most of the dark minerals are the black mica called biotite, and that there is an awful lot of quartz mixed in with the white feldspar. This throws the ball back into granite’s court, petrologically speaking, and there is a surprising name for the common hybrid between granite and diorite: granodiorite. So that’s where I finally decided to pigeonhole the rock. A very dark granodiorite.

Except that I found out its real name is tonalite.

The point is, the point is… oh god, I’ve forgotten my point.



What to take away

My last post on the variety of natural features in Northern New Mexico led me to consider just what geological insights a traveler to our beautiful country could take away with them. It’s easy to be overwhelmed with loads of scenery, lots of facts and examples, and a few really unfamiliar words.

Looking out into the Espanola Basin from the Sangre de Cristo Mountains

Looking out into the Espanola Basin from the Sangre de Cristo Mountains

There are, however, some basic concepts that a geologist would love to feel you’ve carried home:

  • Northern New Mexico has a spectacular landscape which conceals a rich and eventful geologic history.
  • This history can be pieced together from the rock record, some of which you have just encountered.
  • Geologists attempt to understand the rock record in terms of processes which we can see operating today. This works well for rocks that form at or near the Earth’s surface, less well for rocks that form deep in the crust.
Santa Fe River carrying sand to the Rio Grande

Santa Fe River carrying sand to the Rio Grande

  • Sedimentary and volcanic rocks form at or near the Earth’s surface. On the continents, these rocks form an extensive cover that rests upon an older crystalline basement. The crystalline basement is made up of metamorphic and plutonic rocks that originally formed far below the surface.
Mississippian dolomite resting nonconformably on Precambrian crystalline basement

Paleozoic sedimentary strata resting unconformably on Precambrian crystalline basement

  • Because sedimentary strata and many volcanic lavas and tuffs were originally laid down in horizontal layers, we can infer subsequent crustal movements by their displacements. This includes deformation by flexure, offset by faulting, and elevation above or below sea level.
  • Sedimentary rocks contain the Earth’s archives. They outline areas of uplift and subsidence, record the distribution of ancient environments, and track changes in sea level and climate over the ages. They also preserve the record of life on Earth.
  • Volcanic rocks contain the Earth’s clocks and compasses. They freeze in radioactive elements that act like an hourglass, which we can use to measure the age of the rock. They freeze in magnetic minerals aligned with ancient magnetic fields, which we can use to measure continental drift and plate movements.
  • The metamorphic and plutonic rocks, many of which formed from deeply buried volcanic and sedimentary rocks, contain a blurred, but fascinating, record of conditions and movements deep in the Earth’s crust.
  • The leveling process – weathering and erosion at the Earth’s surface – works swiftly and ceaselessly to wear down the high places and fill in the low ones. Landscapes of great relief and drama like New Mexico’s point to ongoing tectonics and volcanism.
Slot canyon in rhyolite tuff

Slot canyon in rhyolite tuff at Kasha Katuwe Tent Rocks


Santa Fe and Northern New Mexico

Rowe Mesa above the Glorieta Pass into Santa Fe

Rowe Mesa above the Glorieta Pass into Santa Fe

I’ve been leading a few guided hikes lately, in the countryside around Santa Fe, and while I’ve lived and hiked here for many years, I’m still amazed at the diversity of natural features we enjoy in this corner of the Southwest. Finding a suitable walk for guests with geological interests is never a problem. And when you add in the rich overlay of human cultures in New Mexico, almost any walk becomes a dream-like journey though times past, from symbols with which we can resonate, to artifacts of an almost alien world.

Inscriptions on El Morro

Inscriptions on El Morro, a cliff made up of the ancient dunes of a Jurassic desert

Four great provinces of the American West come together near Santa Fe, to account for this diversity. We sit at the foot of the southernmost range of the Southern Rockies, a group of mountains bordered on the east by the Great Plains, and buttressed on the west by the Colorado Plateau. A rift valley bisects these regions from north to south, bringing a prong of the fourth province, the Basin and Range, into our mountain setting.

All of these regions stand far above sea level, basking in the sharp light and dry air of their high altitude settings. The Colorado Plateau averages 2 km above sea level, and a few peaks in the Sangre de Cristo Mountains reach 4 km. Even Albuquerque, in its basin along the Rio Grande, is 1.6 km above the sea. Rocks are well exposed in this high and dry country, and getting out to see them is always a pleasure.

And the variety! All four provinces host young Cenozoic volcanic features: lava flows, ash-flow tuffs, volcanic cones and domes, as well as good exposures of sub-volcanic structures such as dikes, laccoliths, necks, and stocks.

A Pliocene basalt flow on La Bajada Mesa

A Pliocene basalt flow on La Bajada Mesa

Ancient Precambrian metamorphic and plutonic rocks are extensively exposed in the cores of our mountain uplifts.

A boulder of migmatite high in the Sangre de Cristo Mountains

A boulder of migmatite high in the Sangre de Cristo

In each province sedimentary rocks form a colorful blanket, carrying a record of environmental change that ultimately spans the late Paleozoic, Mesozoic, and Cenozoic Eras.

Permian red beds

Permian red beds

A visit to Santa Fe and Northern New Mexico is an invitation to explore a vast and varied natural history with only a little time and effort. Immerse yourself in Deep Time and you will find your travels here enriched in ways you never expected.









I was on a mountain excursion earlier this week with some 14 year-old hikers, high in the ancient rocks that outcrop east of Santa Fe, when their attention was drawn to this luminous rubble along the trail:

Milky quartz float along the trail

Milky quartz float along the trail

I picked up a piece and asked them what they thought it was. “Crystal?” asked one. “Quartz!” asserted the other.

They were both right. The coarse-grained granite that splits the gneiss in the foothills of the Sangre de Cristo Mountains above Santa Fe spills its crystalline contents all along the mountain trails, mixing with the pines, penstemons, and desert plants there. It covers the ground with pink feldspar and adds icy accents of quartz and mica. Everyone notices the quartz.

The components of granite: feldspar, quartz, and mica

The components of granite: feldspar, quartz, and mica

Each of these minerals is composed of chemical elements common in the Earth’s crust. Among these elements, oxygen is by far the most abundant, making up 47 percent of the rocks by weight, and a whopping 96 percent by volume! Oxygen is a big atom. When you look at a granite mountain like Pikes Peak you are basically looking at a big pile of oxygen with some impurities in it. This fact never fails to impress me.

In second place is the element silicon, making up around 27 percent of the crust by weight. Since oxygen and silicon can link together chemically, it follow that their compounds utterly dominate the composition of the Earth’s crust. There is so much oxygen and silicon around that their simplest combination, two atoms of oxygen sharing one atom of silicon, or silica, is exceptionally common. We know it as the mineral quartz.

Under ideal conditions, such as in the cavities of mineral veins, quartz can be found as transparent, six-sided prisms, each terminated by a pyramid with six shining faces. These noble light-forms are perennially fascinating – visit any mineral shop or New Age bookstore – and they seems to captivate everyone who sees them. The ancient Greeks gave the name krystallos (clear ice) to these forms, considering them a kind of ice that had been eternally frozen. Transparent quartz is still called rock crystal, and from that simple beginning, any mineral or chemical substance that develops symmetrical forms bound by planar faces is now known as a crystal.

A crystal of quartz

A crystal of quartz

With the development of the atomic theory of matter, we now realize that the beautiful symmetry of a crystal is simply the reflection of its internal structure, a lattice-work of linked atoms repeating themselves in endlessly in three dimensions. Even when a mineral is confined and unable to develop its ideal outer expression of crystal faces, its internal order may still reveal itself when it is broken. Cleavage planes in feldspar scattered along the forest trail flash back at you like mirrors when the light is right, literally reflecting the mineral’s crystalline structure:

A cleavage plane in feldspar shining along the trail

A cleavage plane in feldspar shining along the trail

So both of my young hikers were right. In two words they accidentally captured an ancient linkage that gave a permanent name to the crystalline nature of minerals.








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.