An outcropping of Mancos Shale in the Cerrillos Hills

Of all the rocks in the American West, perhaps the least loved – or at least, the most overlooked – is the Mancos Shale. Rather than forming the spectacular buttes and spires that draw people to the Southwest, this dull grey rock underlies the long hot drives between the National Parks and Monuments you are longing to visit. It forms sullen badlands, here and there, that seem to suck up the famous light rather than casting it back to you in glowing cliffs and purple-shadowed mountains. Even the toughest desert grasses struggle to grow on it. And yet, as so many other unpromising encounters in life do, these rocks conceal a fascinating story behind their reclusive and crumbling facade.

First, a word about a name. In order to reconstruct the history of the Earth from the rocks – or for that matter, to plan a mining venture or prepare a suitable foundation for a big construction project – different kinds of rocks are mapped and cross-sectioned on a variety of scales. To this end, packages of similar rocks are assigned to what are called formations. To a geologist, a formation is not an interestingly shaped rock, like, for instance “Camel Rock” just north of Santa Fe, or the “Sword of Damocles” in Carlsbad Caverns, but rather, a package of rock that is easy to distinguish from surrounding rocks,  and sufficiently large enough to be plotted and traced out on a map. Formations are given formal names, with capital letters to prove it, based on a geographical feature near the place where the unit is particularly well-displayed, or first described. It so happens that the thick grey beds of the Mancos Shale crop out beautifully near the town of Mancos, Colorado, close to Mesa Verde National Park. Hence the name.

Shale is the most abundant of the sedimentary rocks because it is made of that most ubiquitous of substances: mud. It doesn’t have any of the seductiveness of sandstone, hinting of ancient beaches and desert dunes, nor the allure of limestone, redolent of tropical seas and atolls. . . but there sure is a lot of mudstone in the geologic record. Shale is mudstone that exhibits, well, shaliness: 

Shale

It’s fissile – the shale splits easily into thin plates and flakes, in a crude reflection of the billions of microscopic platy clay minerals, aligned by settling out of water – picture throwing a deck of cards up in the air and noting how they end up flat all over the floor – along with the effects of tiny partings of coarser silt among the mud. This property is particularly well developed when the mud has settled in quiet water – for instance, deep on the floor of the ocean, below the stirring effects of waves. Shale comes in many dull earthy colors, reflecting its muddy heritage, but the Mancos Shale is generally a dark grey to nearly black, thanks to its rich content of organic carbon. That’s another clue, by the way, that this mud collected in somewhat stagnant waters, where  decay was inhibited by low oxygen.

Evidence of life isn’t entirely missing from these ancient muds, however, and that’s the reason for this post. I know there are a fair number of fossil lovers out there, and if you have a walk in our new Cerrillos Hills State Park, south of Santa Fe, or brave the ruts of the old Waldo Road from Cerrillos to I-25, you can spend some pleasant time looking for marine fossils from the seas of the Cretaceous Period. Two important caveats: First – no collecting is allowed in the State Park. You can look, photograph, or sketch, but the fossils stay. And second: if you decide to try the Waldo Road, be sure you have a moderately high clearance vehicle, and don’t even think of taking the road if rain threatens. The Mancos Shale is full of “swelling” clays and the dusty roadbed will liquify in minutes into a dangerously slick and tenaciously sticky mess, quite capable of fossilizing your car. No one will come out to help you, either, until the mud dries.

The "Devil's Throne" along the Waldo Road

 In places the bedding planes of the shale are covered in what are called trace fossils. Trace fossils are records of animal activity preserved in sediment, rather than remains of the animal itself. A footprint is a good example. But check these patterns out:

Feeding traces in the Mancos Shale

Some sorts of creatures spent their time browsing around on the muddy floor of the sea, either searching for tidbits to eat, or, more likely, eating the mud itself to strain out the organic matter in it, for nutrition. Feeding traces like these, as well as filled-in burrows, and the occasional tiny rows of tracks (looking like someone was trying to row a boat in mud) are perfect examples of trace fossils, and are very characteristic of the Mancos Shale. At times the sea floor was literally crawling with life, in spite of the murk and low oxygen.

With a little more luck you’ll find some of these:

Inoceramus fossil

This is the impression of a marine bivalve, called Inoceramus, which was common worldwide in the Cretaceous seas that covered much of the planet.

With even more luck – as in more luck than I’ve had so far – you might find an ammonite fossil, or a shark’s tooth that drifted down long ago to bury itself in the dark mud at the bottom of the Interior Cretaceous Seaway.

It’s hard to imagine, as you stroll through these arid and dusty hills, that all this country, from horizon to horizon, and from Gulf of Mexico to Arctic pole, was covered by a shallow ocean, less than 100 million years ago. And if you are inclined to stimulate your imagination even further, find a copy of the amazing new book “Ancient Landscapes of the Colorado Plateau“, by Ron Blakey and Wayne Ranney, published by the Grand Canyon Association. It’s the closest thing you’ll find, currently, to sneaking a peak at God’s old atlas of the Earth.

The Rio Grande and the Cerros del Rio from the White Rock Overlook

When you are making that drive to Bandelier National Monument, or taking a day trip to hike in the Valle Grande National Preserve, don’t forget to make a short side trip through the little town of White Rock to enjoy the view from the White Rock Overlook. It’s just a few minutes jaunt off of Highway 4 – turn at the “white rock” and follow the signs through a neighborhood and a pleasant community park to the end of the street. (White Rock is a bedroom community to Los Alamos and the National Laboratory, and I never drive through it without hearing a Rod Sterling voiceover in my head. This place is suspended in 1950′s paranoia. Just keep driving. . . ) There’s parking and a little viewing platform:

The path to the viewpoint

and a spectacular view into the canyon of the Rio Grande – the Caja del Rio – waiting for you:

Looking northeast toward the Rockies from the Overlook

In the warmer months you’ll probably see a kayaker far below, or maybe a string of rafts. 

I like to take students here. The river has nearly dissected the small volcano that forms La Mesita and you can see some interesting details from the viewpoint:

La Mesita

And if you look at the rocks you’re standing on, you can see some the characteristic features of a basalt lava:

Vesicular basalt underfoot at the White Rock Overlook

There are hundreds of “Black Mesas” scattered over the American West, and more often than not, they are held up by a resistant flow of black basalt forming their flat tops. This flow, which is actually related to the cluster of low volcanoes across the river from you, has a good display of gas bubbles frozen into the rock – vesicles – and also shows the very characteristic crusty white weathering of basalt in an arid climate like ours. Basalt is full of calcium-rich feldspars, and in dry climates the element leaches out to form these strikingly white caliche crusts. This is so diagnostic that you can impress your friends with your rock-identifying skills, with this simple observation.

So keep your camera ready and follow the signs to the White Rock Overlook the next time you head west out of Santa Fe. This is definitely one of those “off the beaten path” stops that you’ll want to make when you come to visit us.

New snow along the East Fork Jemez River Trail

The Pacific storms that have been soaking Southern California have been doing us a bit of good, here in Northern New Mexico, and this past weekend was the perfect opportunity to strike out for the high country and see what Nature has put in the storehouses. Our choice this time was one of my favorite walks up in the Jemez Mountains, a trail along a little steep-walled canyon so beautiful that a friend of mine calls it “Beaver Valley”, after some half-remembered Disney fantasia from childhood. It’s an idyllic hike in the summer, with a cheerful creek winding along the flat floor of a narrow canyon crowded with spruce and dotted with wild rose and iris. I’d never seen it in the depths of winter, and now was the time.

The real name of the trail is the East Fork Jemez River, and our point of departure was the Las Conchas Trailhead, just off Highway 4 not far after you leave the Valle Grande in the Valles Caldera National Preserve. It’s about 57 miles from Santa Fe.

Las Conchas Trailhead

The drive up was beautiful. The last snow squalls from the departing storm were still blowing through the mountains and the forest, flocked with fresh white, was almost hypnotic. Of course we pulled over at the Valle Grande overlook to have a look at the snow:

Snow squall over the Valle Grande in the Jemez Mountains

It is impossible to capture the scale of this mountain park, but you can get a measure of the expanse by noting the height of those full-grown trees at the foot of the mountains. (At other times of the year, you can pull up with the other visitors and listen to people arguing if those little specks way out there really are a herd of elk)

The Valle Grande is just a small part of the great volcanic caldera that blocks out the center of the Jemez Mountains. It has held a number of crater lakes in the recent geologic past, which accounts for its forest-free floor. The fires below are banked for the time being, however, and now, in winter, the Valle becomes a dazzling bowl of snow. You really must see it.

The Las Conchas Trailhead opens off Highway 4 at a place where the East Fork of the Jemez River enters a box canyon it has cut through the tortured rocks of the South Mountain Rhyolite. This rhyolite is a thick flow of silica-rich lava erupted around 550,000 years ago, during the waning stages of volcanic activity in the Jemez. The flow blocked drainage inside the caldera for a while, but the lava was overtopped by water and a narrow canyon was soon carved through the resistant rock. A subsequent episode of backfilling gave the canyon a flat floor, which accounts for its unique attractiveness, and makes a summertime walk delightful.

Usually when you pull up to the trailhead you have a suspicion that you have stumbled into an REI commercial. Cattle Call Wall, which you can see in the picture below, is usually thick with rock climbers, and there are always many more just inside the canyon, shouting happily to each other and jingling their carabiners.

Cattle Call Wall at the Las Conchas Trailhead

There were no climbers on Saturday. The summertime crowds of hikers were missing, and the gurgling creek was muted by ice and buried under about two and a half feet of new snow. A few hardy snowshoers had broken a path – bless them – and my friend and I wound our way into the hushed winter paradise within.

Snow-covered bridge over the Jemez River

Let me just mention that crossing these very narrow bridges, on an unstable icing of over two feet of new snow, is somewhat . . . challenging. There’s not a lot of margin for error, and it’s really really hard to put one foot in front of the other when you are wearing snowshoes. Always be sure to bring someone along to help pull you out of the creek! But don’t let such minor obstacles stop you from enjoying the glorious snows of our New Mexico winter.

The Rio Grande and beyond, from the White Rock Overlook, on the way to Bandelier

One of those must-do day trips on your visit to Santa Fe is a drive to Bandelier National Monument, just south of Los Alamos, a place where you can walk along the shaded Rito de Frijoles under the pines and cottonwoods and have a close look at the Anasazi cliff dwellings that perch along the canyon walls. It’s an old park whose value was recognized very early on, and you are sure to enjoy a visit. But don’t forget to pay attention to some of the other remarkable sights you’ll see on your drive there.

Bandelier National Monument sits on the eastern flank of the Jemez Mountains west of Santa Fe, a rounded range of blue mountains that forms the stage for our spectacular sunsets here. (And you know the quip: Santa Feans take half their salary in sunsets) Steep-walled canyons are cut in the Pajarito Plateau that flank the Jemez, one after the other, dividing the land into narrow pinyon-covered fingers over which Highway 4 winds and dips on its way from the Rio Grande into the great Valles Caldera National Preserve in the heart of the mountains. This plateau was formed in two great acts of creation (disregarding some minor canyon-cutting that came afterwards) when the Jemez supervolcano disgorged its fiery guts not once, but twice, approximately 1.6 and 1.25 million years ago. Incandescent floods of ash formed a vast apron of tuff all around the older volcanic highlands – part of which makes up the Pajarito Plateau – and the bulk of the highlands themselves collapsed into a circular depression, called a caldera, 12 miles across. I could write about this stuff for days (and no doubt will in future) but for now I want to highlight a key outcropping you can’t help but notice as you cruise up from the Rio Grande on Highway 502 from Pojoaque to Los Alamos, just before you make your turn on Highway 4:

The Bandelier Tuff exposed along Highway 4

Your first thought, upon seeing this cut, is that the Highway Department has sprayed that disgusting, so-called “stabilizing” concrete over the rocks to hold the loose stuff in place. (Can you tell how geologists feel about those tricks?) But you would be wrong. This cliff exposes almost the entire record of those two great explosive eruptions. The lower, greyish-white part of this roadcut is a great fall of pumice that overwhelmed the landscape during the first stage of the cataclysmic eruption 1.6 million years ago. Above the pumice are thick layers of rhyolitic tuff (tuff is consolidated volcanic ash) laid down by massive pyroclastic flows of magnitudes never witnessed by man. The top of the orange tuff, which erupted 1.25 million years ago, marks the top of the Pajarito Plateau.

These events were set up by persistent volcanic activity beneath the Jemez volcanic field, beginning perhaps 16 milion years ago. Eruption after eruption softened the crust beneath the mountains, and a kind of cosmic indigestion developed in the bowels of the field, nurtured by the relentless activity. And then, around 2 to 3 million years ago, renewed pulses of exceedingly hot basaltic magma from the Earth’s mantle began to push queasiness toward catastrophe. 

The rocks beneath the mountains began to melt wholesale, forming a vast chamber of viscous and gas-charged magma. The crust that supports continents like North America is lighter than the mantle below it, is richer in silica (most rocks are made of oxygen and silicon – silica – bound together with light metals like aluminum), and is usually richer in ‘wet’ minerals that release water when they melt. Magmas that are oversaturated in silica are ‘sticky’. They are so viscous that it is difficult for them to reach the Earth’s surface. The lavas they produce are lighter in color than basalt (which bleeds up darkly as a partial melting of the mantle beneath the crust), and form rocks called dacite or rhyolite. These magmas are capable of trapping dangerous amounts of superheated steam and other gases. If they do make it to the surface, the violent expansion of those trapped gases has terrible results.

Some of the basaltic magma leaked out around the flanks of the Jemez Mountains before the big events to come. You can see one of these flows at the base of the outcropping:

Flow of black basalt capped by an ancient orange soil

There’s plenty of space to park and get out of your car at this roadcut, and if you do, you can put your nose on these dense black rocks and see tiny greenish flecks of the Earth’s mantle suspended within. These flecks are unmelted fragments of the mineral olivine, a simple, heavy silicate rich in iron and magnesium. (Larger gem quality crystals of olivine are called peridot, the gemstone for August. Sorry; none of those here. I’ve looked.) What little gas these magmas contain bubbles out peacefully. You can see some of the last of the bubbles trapped in the basalt if you look.

This lava bed sat at the Earth’s surface long enough for a soil to develop on its top. That’s the reddish-orange clay you see above the dark basalt. No doubt it supported a thriving forest on the fatal day when, somewhere to the west, the dangerously swollen magma chamber beneath the mountains could contain itself no longer and blew out.

The day of death

A massive jet of silica-rich magma fountained skyward with such force that it propelled a boiling cloud of ash into the stratosphere, miles above, within minutes. Clots of stiff rhyolitic magma, bursting with gas and incandescent with heat, swelled into a glassy foam – pumice –  light enough to float on water, but not light enough to stay suspended in the blackening atmosphere. The bulk of the magma simply ruptured into a choking ash. The pumice began to fall back to earth in a hellish hail of such magnitude that, here at least, it buried the shaking ground as much as 20 feet deep. (Winds blew ash and smaller bits of pumice over the Santa Fe area and well into Texas) As pictured above, you can put your hand on the very place where the first particles reached the ground.

Loose fragments of pumice. There's a quarter for scale.

Worse was yet to come. You can get a hint of this if you examine the top of the pumice layer, where it becomes more stratified. Parts of the eruption column began to collapse and send hurricane-force gusts of pumice and ash across the blanketed landscape in pyroclastic flows hot enough to sterilize everything they touched. This is what happened (on a smaller scale) to Pompeii and its unfortunate citizens. If you want to relive their experience, treat yourself to the climactic scene in Robert Harris’ “Pompeii“. Remarkable.

Fortunately for the history of the Western World, what happened at Pompeii stopped at this point. Things didn’t stop in the Jemez Mountains. The magma chamber was too large, too over-pressured, and too unstable to be ventilated in one central eruption. Multiple vents began to open up in a roughly circular pattern miles across over the crown of the chamber, each one screaming with expanding gas. The crust inside this ring began to collapse into the chamber, like a huge piston, carrying down an entire range of mountains (which were immediately swamped in ash so hot it fused back into liquid rock) So much magma was ejected into the atmosphere that the eruption columns, in a continual state of collapse, spread pyroclastic flows over hundred of square miles, filling valleys, burying mountains up to their necks, and and smoothing the landscape into a sterile plateau of lifeless ash. 

Look at the photograph again:

The Bandelier Tuff

These pyroclastic flows left a deposit which, in this picture, extends from the place where the ash beds turn a brighter white all the way to the base of the orange cliff. It is called the Lower Bandelier Tuff. The reason it is called the Lower Tuff is that, unbelievably, this entire cycle repeated itself 400,000 years later, with even greater force. That eruption left the next massive layer, the Upper Bandelier Tuff, which forms the steep orange cliffs at the top of the pile. This upper tuff is the layer in which Frijoles Canyon, in Bandelier National Monument, is carved, and it is a record of the eruption that formed the Valles Caldera just beyond the monument.

There are two recent publications available in Santa Fe that you must look for if you find these sights as fascinating as I do. The first is Kirt Kempter and Dick Huelster’s “Valles Caldera”, which is a fold out map with associated text and photographs, published by High Desert Field Guides. The second is Fraser Goff’s “Valles Caldera: A Geologic History“. Both of these publications are intended for the lay audience and both of them are well worth having. You can find copies for sale here in Santa Fe at The Travel Bug and Collected Works Bookstore. If you manage to make your day trip without them, you’re sure to find copies at the respective bookstores at the headquarters of Bandelier National Monument and the Valles Caldera National Preserve.

P.S. The matter of Pompeii shouldn’t be taken lightly. There is a partially submerged caldera just west of Naples, called the Campi Flegrei, that is only a little smaller than the Valles Caldera – and it is only the latest scar of some truly catastrophic eruptions on the Italian peninsula.

"Grand Central" in the Cerrillos Hills

Last week I mentioned that I would be giving a little overview of the geology of the Cerrillos Hills, and here it is. Before I begin, however, I would like to direct the attention of those more serious inquirers and rockhounds to an article by a true expert on this area, Stephen Maynard: “The Geology of the Cerrillos Hills“. My intention is to highlight some of the bold features you might see on a walk in the new State Park on a sunny afternoon, with a few photographs to guide us along. If this piques your interest, don’t hesitate to build some context by having a look at Stephen’s excellent and easy to read summary.

Everyone has seen pictures of volcanoes, but did you ever wonder what it might look like under the smoking mountain? A walk in the Cerrillos Hills will give you that opportunity, with a little guidance from the geologists. All that pent-up magma, liquid and mobile, seeking a new equilibrium in the Earth’s crust above those places where it has been born, exerts a tremendous amount of pressure as it wedges its way up through the rocks. In places it pauses and pools horizontally, splitting the crust and lifting it up, forming a sort of gigantically hot flat pancake in the crust. If the magma freezes there, the body of rock it leaves is called a sill. (Think flat, like a window sill)

If the pancake of magma continues to grow, eventually its roof will rupture in vertical cracks, and the hot batter will squirt up into the splits with great violence, propping them open. If the magma freezes here, the body of rock it leaves is called a dike. Exposed by erosion at the earth’s surface, these features often stand up like walls or dams, hence the name. Here’s a small dike you might walk by in the park:

A dike exposed by erosion in the Cerrillos Hills

While these splits and ruptures can cease at any time, if they do continue upward and breach the surface, the magma gets out. We call the “getting out” a volcanic eruption.

In our part of the American West, it so happens that a very thick body of shale – mud that has accumulated on the floor of an ocean or a lake, and subsequently compacted into rock – stood in the way of the ascending magma and led to some unusual effects. Here’s a diagram, taken from an interpretive display in the park (a picture being worth a thousand words):

Magmatic blisters

Laccoliths are sills with bulging roofs, bowing up the rocks above, like a blister. In the Cerrillos area, these blisters actually stacked themselves one above the other, forming – in the fevered imagination of a geologist – something like a stony Christmas tree. The relative weakness of the thick shale encouraged this phenomenon. To put you out of your suspense, the magma did eventually reach the surface, forming a volcano, but persistent erosion dispersed the volcano and etched its way down into the stack of sills and dikes among which we can walk, today.

Here’s an outcropping along the railroad tracks that might help put things into perspective:

You'll see this driving into the village of Cerrillos

The pale grey stuff on the left, with the skirts of loose talus, is the shale. (It has a name, the Mancos Shale, about which more in another piece) The craggy orangy-grey cliffs, forming the little peak on the right, is a partially exposed sill of frozen magma. Keep in mind, from this perspective, the magma didn’t so much push up through the shale as out toward you. And it froze in place far underground. The little layer of orange stuff capping the grey shale on the left is a modern blanket of loose rock eroded from the sills and dikes and spread out as a thick rocky soil.

The magma frozen into the sills and dikes in the Cerrillos Hills has a very distinctive texture. Here’s an example:

Andesite porphyry

You can see a thick speckling of white crystals and clots of black crystals suspended in a greenish-gray mass of stone. An igneous rock with this sort of texture – visible crystals floating in a fine grained groundmass – is called a porphyry. To a geologist’s eye, this texture indicates at least two episodes of cooling. And the stony appearance of the groundmass is a clue that the final episode of cooling was fairly rapid and occurred under low confining pressures, a characteristic of volcanic activity.

By the way, that dark mineral you see is rich in iron, so as these rocks weather at the surface, they acquire a patina of rust. That’s why the rocky outcroppings in the hills are more orange than grey.

As if all this blistering wasn’t enough, in a second episode of igneous activity, a big slug of magma of somewhat different composition forced its way through the pile of laccoliths to feed another generation of volcanism. Some of this magma froze into a large, roughly cylindrical plug – called a stock – right in the middle of our stack of sills, and when erosion hacked its way into this mass, it left the stock standing in relief. It’s big. We call it Grand Central, now, and you can see a picture of it at the beginning of this entry.

This second episode of intrusion was sufficiently forceful to dome up and distort the entire package of shale, sills, and dikes. And this mass of melted crust had an additional cargo of elements humans find either useful or attractive – like gold, silver, copper, lead, and zinc. Now we’re talking!

An intrusion of this magnitude takes a long time to crystallize and cool off, and as it does so, all kinds of secondary effects can occur. The heat of the magma sets groundwater into motion. Much of this water is superheated and aggressively corrosive. The crystallizing magma itself rejects volatile elements like hydrogen (i.e. more water) chlorine, and sulfur. It also rejects elements that don’t fit into the atomic framework of the bulk of the minerals that are crystallizing: the heavier metals that we love to use in wedding rings and bullets and car batteries and telephones. This hot brew of chemicals seeks its way toward the surface as best it can, staining everything it touches and leaving behind crusty residues of exotic minerals rich in those valuable metals.

In the Cerrillos Hills a system of fractures oriented in a northeasterly direction guided these potent juices to create bleached and iron stained zones of rock the old miners called veins or lodes. Erosion gradually unearthed some of them (with weathering effects adding lovely new complexities, one of which is called turquoise) and miners both ancient and modern began poking around for the riches:

Prospect pit in the Cerrillos Hills

The ancient ones (and not a few modern rockhounds) sought turquoise, which was used for adornment and was traded far and wide. They also used some of the lead minerals to make pottery glazes. The pragmatic Spanish had no use for turquoise, but lead was always useful for bullets, copper for utensils, and silver for money. The turn-of-the-last-century Anglo miners loved that silver too, but also had industrial markets for lead, zinc, and copper. (There’s not much gold in the Cerrillos Hills, but in the rugged mountains you can see just to the south, the Ortiz Mountains, there was – and is – plenty of gold) Turquoise experienced a new vogue in jewelry and small mines were developed to find it. (You always wondered why the box from Tiffany’s had that particular color, didn’t you?)

As usual, the richer and easier to find deposits were exploited to the point of exhaustion. There’s still a faint halo of copper minerals in the rocks, exploitable by modern mining techniques – but it would require the removal of the Cerrillos Hills themselves to get it (not to mention an ocean of unavailable water) and this is unlikely to happen anytime soon. But for now, we can admire the efforts of the early miners, preserved in the park, and we can enjoy a unique natural museum of subterranean activity – under the volcano – laid out for anyone who takes the time to look.