From Little Treasures Come Big Stories: Travels Through Death Valley National Park

Photo by Mrs. Geotripper

I returned from Death Valley to a load of work back on campus, but over the next few days I'll be posting on some of our adventures. On Saturday morning, we awoke to sunrise on the Mesquite Dunes east of Stovepipe Wells. We grabbed our packs and notebooks and hit the road. We had a lot of ground to cover.

Oddly enough, for having spent two days getting to Death Valley National Park, one of our first stops was outside of Death Valley National Park. This was for the simple reason that we were looking to understand the nature of the rocks that make up the mountain ranges surrounding Death Valley. Because we didn't have enough time to climb most of the mountains, we would need to see what had rolled out of the mountains during the many flash floods and mudflows that had scoured their flanks over the countless centuries. I have honest students, but their conscience would have had a tough time dealing with all the little treasures they were about to find. So we made sure we were outside the park boundaries when we let them out onto the alluvial fans coming down from the Funeral Mountains. For many of my students it was their first experience in finding a fossil.

Fossil crinoid stems. These are rare in oceans today (they are known as sea lilies), but during the Paleozoic era, they covered the sea floor like fields of wheat, and entire rock layers are composed of their fragments.

To most normal people, 300-400 million years of nearly continuous mud deposition is perhaps not the most exciting process to consider. But if that 300-400 million years covers the latest Proterozoic eon and the all of the Paleozoic era, such activity is irresistible to a paleontologist. A rock sequence that covers that time period contains the evidence of the rise of multicelled life on Earth, as well as the first appearance of all of the extant phyla known (plus a few extinct ones). A phylum, as a biologist will tell you, is one of the broader divisions into which all life can be organized. One phyla, the chordates, contains all the familiar animals with a notochord or backbone (fish, amphibians, reptiles, birds, and mammals). There are dozens of others, including the arthropods (bugs and crustaceans) and the molluscs (snails, clams and squids) which make up most of the species known today. A more or less continuous record of deposition makes it possible to detect patterns and trends in the evolution of life on the planet through time.

Grand Canyon National Park has a similar range of rocks exposed in the depths of the gorge, but huge pieces of the story are missing because of episodes of erosion. Where the Grand Canyon has about 4,000 feet of Paleozoic sediments, Death Valley has more like 20,000 feet! How can 20,000 feet of sediment fit into a mountain range that rises no more than 5,000-6,000 feet above Death Valley and other grabens in the region? If you look at the photo of the Funeral Mountains below, the answer is apparent: the sediments in the mountain range have been tilted. To walk through 400 million years of Earth history, we need only to walk a few miles along the base of the mountains.

How is it that sediments could accumulate for such a long time in such stable conditions? Most parts of the crust of the Earth are wracked by extreme tectonic activity like volcanism, folding, and faulting. The Paleozoic rocks of Death Valley accumulated in one of the most geologically "gentle" environments on the planet: a passive continental margin. A billion or so years ago, most of the world's continents were combined in a supercontinent we now call Rodinia. The continent began to break up at the end of the Proterozoic, which is a process that involves severe faulting and rifting, along with vigorous volcanic activity, but as the continents moved further and further apart, the processes became less active and finally stopped. The edges of the continents became a site of more or less continuous shallow marine deposition, and as more sediments were laid down, the crust slowly sank beneath the weight, allowing even more sediments to accumulate.

So, from a bit of wandering across a stony desert surface picking up random fossils, a story is told of massive supercontinents breaking apart and forming huge wedges of sedimentary rock that tell the story of 400 million years of evolution of life on planet Earth. In short, this is why I love teaching geology.

The Abandoned Lands...A Journey Through the Colorado Plateau: The Seven Stages of Grand Canyon Awareness

Stage #1: There's a Really Big Canyon Out There!
This is a response of those seeing the canyon for the first time. Practically everyone has seen pictures of the Grand Canyon, but as we all know, a page or a screen shrinks the canyon to insignificance. There is just that one moment when one walks to the edge of the abyss for the very first time and looks into the depths. It is one moment in a life that cannot be forgotten.

Stage #2: How Did This Happen??
There's a big river down there, the Colorado. Obviously, it carved the canyon. But somewhere around stage #5 or #6 one is going to have to ask which Colorado River? The older upstream river or the younger downstream river?

Stage #3: What are all of those colorful layers of rock? I guess we need to know how they happened too.
The walls of the Grand Canyon expose 300 million years of Paleozoic history in 14 different formations representing deposition in shallow seas, tidal flats, river floodplains, and desert dune fields. The layers are about 4,000 feet thick, making up most of the cliffs seen in the canyon.

Stage #4: There are more rocks beneath the flat layers. They're tilted and truncated.
There are 12,000 feet of additional layers hidden in the depths of the Grand Canyon! They're hidden in most places by the Paleozoic layers. They represent an entire Proterozoic history that dwarfs the Paleozoic story in complexity.

Stage #5: There are some really strange rocks in the greatest depths of the Canyon. They aren't layered at all, and in fact seem to be standing on end. What are they?
Those are some very old and contorted rocks in the greatest depths of the Canyon. They are metamorphic gneiss, schist and quartzite bodies that have been intruded by granitic dikes. They are rocks that have been buried miles deep in the crust, and yet were eroded to a flat plain before any of the other rocks of the canyon were ever laid down.

Stage #6: What regional and world-wide events caused the rocks of the Grand Canyon to form in the way that they did?
Grand Canyon is part of a much bigger story. The story starts to emerge as one visits other parks in the region, or one works like a detective in an academic setting to see the links between events like the accumulation and break-up of the Pangaea supercontinent, or the formation of a subduction zone offshore of California and the Pacific Northwest. The story is especially complex in the metamorphic rocks of the inner canyon. The exposures are hard to access, and numerous researchers have spent their entire careers teasing out the story of the oldest rocks.

Stage #7 That darn River. It all seemed so simple at first...
Sometimes the simplest answers are not the correct ones.  In the 1870s John Wesley Powell thought the river existed first, and the land rose while the river cut down several tens of millions of years ago, forming the Grand Canyon. The years have passed, and problems with the model cropped up, to the extent that the simple model had to be discarded. But there was no second model to take its place. The Colorado River carved the Grand Canyon. But not the Colorado River we see today. The Colorado at various times has flowed in directions essentially opposite the flow of the river today. It flowed through canyons in the state of Colorado millions of years before it flowed through the Grand Canyon, and the delta of the Colorado at the Gulf of California has only existed for 5 million years. There has been stream piracy, headward erosion, beheaded streams, collapsed highlands, and tectonic uplift that gave us the gift of the Grand Canyon, but the details have yet to be worked out.

Why do I love the Grand Canyon? It amazes us, and satisfies our curiosity at so many levels. Children can understand the link between a flowing river and the carving of a deep canyon. They can feel the thrill of discovering a fossil for the first time, and understand a little of how it came to be. Casual tourists can be understanding something of the story of the formations that make up the canyon walls, and start to appreciate the immensity of geologic time. Students of geology (like myself so many years ago) can learn the basics of geology and find inspiration for future career choices. Huge mysteries remain to be solved by researchers in the field and in the lab. And the canyon is incredibly scenic. In my next post, we explore one of those beautiful places: the lower reaches at Diamond Creek and Peach Springs Canyon.

Here is the explanation of my "abandonment" theme for this series: http://geotripper.blogspot.com/2012/06/abandoned-landsa-journey-through.html

What's Missing From This Picture? Only a billion years or so...

Geology is good for perspective. The bookends of this particular day: the very very long hour spent trying to restart a dead car in a remote canyon, and a billion years in one hand. I'm on the road researching a field seminar for the AAPG in Arizona and Nevada, and I saw some incredible things.

1.7 billion years ago, fragments of continental crust were colliding to form the core of a new North American continent. Those mountains climbed to the sky, rivaling the Himalaya or the Andes in their grandeur. The time was so ancient that not a bit of life existed on the mountain flanks. The only lifeforms at all were single-celled organisms in the oceans.

The forces pushing the mountains upwards ended, and the ice and rain tore away the flanks. The gigantic peaks eroded to hills, and in hundreds of millions of years, the hills eroded to nearly flat plains. They were gone, but the rocks that formed the roots of those mountains remained. Around 530 million years ago, the continent had begun to break up, and the edges subsided slowly while a shallow sea advanced. The beach sands swirled back and forth across the roots of the ancient mountains.

An unconformity is a buried erosional surface. Sometimes the time missing might be a few hundred thousand or a few million years. The unconformity in the Grand Canyon is profound, representing a gap of a one thousand million years. Get to the right place, and you can place your hand across 1,000,000,000 revolutions around the sun: nearly a quarter of the age of the Earth.

The problem with the Great Unconformity, as the early geologists called it, is that it is mainly exposed in the bottom of the Grand Canyon. There are only a few ways to see it and touch it: you can float down the Colorado River in a raft, or you can walk down the long difficult trails from the rim of the canyon. Or, as it turns out, you can drive.

I left Las Vegas this morning and traveled up Lake Mead Boulevard. The growth of the town has slowed with the depression, but over the last twenty years I've watched the urbanization of the valley wash up the slopes of Frenchman Mountain on the eastern edge of town like a tsunami. Fortunately, the wave stopped at the boundary of the city limits and the federal lands around Lake Mead. It turns out that the rocks making up the Great Unconformity are exposed no more than a mile from the residential neighborhoods of Sin City. The sand layers of the Tapeats Sandstone cover the weathered granite and schist of the 1.7 billion year old basement complex.

The geologists at the University of Nevada, Las Vegas knew of the treasure in their backyard. Years ago they set up a trail and some very nice interpretive exhibits. I regret to say that vandals and garbage dumpers destroyed the exhibits in a short time, but you can still see the rocks.

If the exposure lacks anything, it is grandeur. The oldest rocks of the Grand Canyon are at the bottom of one of the deepest and longest gorges in the world, and there is much to be said for seeing the rocks in their full perspective, underneath a mile of subsequent sedimentary layers. When time or health is short, hiking or rafting is not feasible. But I was on a mission today, to see the only place in all of the Grand Canyon where one can drive to the Great Unconformity. It is well-known to rafters as it is the take-out point for the river voyagers. It's called Diamond Creek, and Peach Springs Canyon. The road starts at the administrative headquarters of the Hualapai Nation in Peach Springs, and winds 19 miles and 3,500 feet down to the Colorado.

Anyone who has seen the Grand Canyon, either in pictures or in person, has to wonder how any road could intrude into such rugged terrain. There are so many cliff-forming layers that a road seems next to impossible (which it is, given that only one exists). But Grand Canyon is also crossed by faults and folds, and the disruption of the cliff-forming layers is what makes roads and trails possible. The road starts innocently enough, dipping into rolling terrain composed of Mio-Pliocene sediments that have huge significance in understanding how the Grand Canyon was carved. We actually follow canyons that originated at the end of the age of the dinosaurs (the Cretaceous Period) and were subsequently filled, and then exhumed in the last hiccup of geological time.

Heading deeper into the canyon, we see some evidence of deformation; in the picture below, we can see a gentle monocline crossing the picture where the layers step down in elevation without splitting. A major fault zone, the Hurricane fault, crosses the canyon in front of the fold. The road turns and follows the fault zone.

The gorge becomes deeper and deeper, and we pass into older and older layers. The Mississippian Redwall limestone forms the highest cliffs, and the lower cliffs are dominated by the Devonian Temple Butte limestone and Cambrian Muav limestone. The gentler slopes cover exposures of the Cambrian Bright Angel shale. The rocks on the left side of the canyon sit around a thousand feet lower than the rocks on the right due to displacement along the Hurricane fault.

The cliffs grow higher and higher, and we finally reach the base of the Paleozoic rocks with exposures of the Tapeats sandstone, the same layer seen at Frenchman Mountain in Las Vegas. The Tapeats can be seen in the first picture of this post, forming the prominent layers at the lower left. The orange-brown rocks at the base of the cliff are the 1.7 billion year old granite and schist.

It would take only a moment's walk to lay one's hand on the unconformity, but we also wanted to see the Colorado River. We were nearly 16 miles into the canyon, with only three to go. We got into the car, turned the key....and nothing. Not a click, not a warning light, nothing. 16 miles into the depths of the Earth, and we had a dead car.

So we had the longest hour I've ever felt (aside from the births of my children), trying anything and everything to get the car going again. A construction crew stopped by, five guys in one truck, and we did the engine worship thing where all six of us stood around the open hood and jiggled connections. A river runner came up the road with his boat trailer. He had room, so Mrs. Geotripper jumped in to go arrange for a tow truck. River runners like this guy drive this road all the time, so the Missus had a regular Mr. Toad's Wild Ride up the bumpy road. Meanwhile, I stayed with the car, pretty much preparing to camp out for the night. The ranger for the Hualapai tribe showed up, and we did some more jiggling of engine connections, and he helped me get the transmission unstuck, but we still weren't getting anything when I turned the key. In final desperation, we tried the obvious, and pulled out the jumper cables. They worked.

I zoomed up the road, picked up Mrs. Geotripper, and sped on to Williams without turning the car off once. Thus, our unexpected night in a motel, and a delay, looking for a saturday mechanic before we head on to Grand Canyon tomorrow. And internet access! We'll see what surprises tomorrow holds for us.

By the way, the ocotillos were blooming in the lower canyon...enjoy!

NOTE: Tourism is almost the sole source of income for the Hualapai tribe. You need to pay an exploration fee of $25/person at the Hualapai Motel in Peach Springs. Looking at the town of Peach Springs, I was glad to give them some support.

Vagabonding across the 39th Parallel: A Canyon Where Cameras Stand Sideways

You walk to the edge of the precipice, and you lose all sense of proportion. The rocks at your feet drop off in vertical cliffs into the darkness nearly 2,000 feet below. The opposite rim seems close enough that you are tempted to try and throw a rock across (but it is more than a thousand feet away).  You have never seen a canyon quite like Black Canyon of the Gunnison.

It didn't take some of you very long to figure out where we were in the last vagabonding post; I was describing a place that could have been the much more familiar Grand Canyon National Park, a place with a less visited North Rim, and a heavily trafficked South Rim, but we were in Colorado instead. Black Canyon of the Gunnison National Park preserves one of the most unique canyons in the world, a canyon so deep that cameras start standing sideways in the hands of photographers leaning over the rim. The canyon just doesn't fit in the picture any other way.

The canyon seems impossibly steep. The walls are made of extremely tough metamorphic gneiss and schist, intruded by dikes and sills of equally tough granite pegmatite (an extremely coarse-grained variety of granite; many of the crystals of quartz, feldspar and mica are more than an inch across). The rocks are some of the oldest to be found anywhere in the western United States; they date to Proterozoic time, around 1.7 billion years ago.

 A canyon that in many places is deeper than it is wide. It defies belief.

The Painted Wall is the highest unbroken cliff in Colorado with a drop of more than 2,000 feet. The gneiss and schist exposures are dark, while the pegmatite dikes are lighter in color. The rocks record a complex story of continent-terrane collisions, intrusions and mountain-building episodes. The rocks on the rim preserve a relatively flat surface representing a profound amount of erosion...many vertical miles of rock are missing, having been converted into an unimaginable mix of pebbles, sand and mud distributed by rivers that long ago ceased to exist.

The very existence of the canyon is a handy mystery. It carves through what is essentially the top of a mountain ridge, and looking at maps, it looks like the river should be flowing someplace else. One has to drive a long way uphill to reach the south rim. The canyon is an example of a superimposed drainage, one whose pathway was established by external factors (lava flows from adjacent mountains) until it was trapped into flowing across a place that seems to make no sense.

I've been to Black Canyon of the Gunnison many times over the years, but always via the paved roads on the South Rim out of Montrose, Colorado. The park could never be mistaken for a place like Grand Canyon, as it lacks stores, gift shops, hotels, and hordes of tourists. But the North Rim was always mysterious to me.

I could see a single gravel road, and on rare occasions, a car traveling slowly along the edge. The rim was so close and yet incredibly remote, impossible to reach without setting aside a good portion of a day. I wondered what it was like over there, with no pavement, no tourists, just the wind and silence.

My chance to visit the North Rim of Black Canyon happened in July while we slowly made our way home while vagabonding across Colorado and Utah, roughly following the 39th parallel. We had left the Rocky Mountains behind, and when we crossed McClure Pass, we entered into the Colorado Plateau, the high uplifted region extending across the entire Four Corners region. When we paused in Paonia, an old coal mining town, I glanced at the map and realized we were only a few miles from the remote part of the park. The road was gravel, but not at all difficult to negotiate, and after an hour or so we arrived at the only developed spot, a ranger station. The ranger seemed gratified to have a couple of visitors.

We passed a small primitive campground (mostly empty), and walked out the edge of the abyss. We were alone on the trail. It was a calm day, and the canyon swallowed up any noises. We stared at the cliff faces and marveled at the vertical walls. The patterns of the rock were complex and almost hypnotic.

 I loved this place.

We finished our walk and jumped in the Subaru to traverse the rim drive for a few miles. We saw one other car. We couldn't help but stop every few feet to snap another picture. One thing I found amazing was the persistence of life. Mature trees were clinging to life on vertical ridge lines that could barely have enough surfaces to develop soils.

The shadows were getting longer, and we still had quite a few miles to go. We headed out the access road to the north, and found our way back on to the highway to Ouray. We had a nice view north to the Elk Mountains and an interesting looking volcanic neck. It was a nice visit.

The next day we would be heading into Utah...

The Other California: A Canyon as Deep as the Grand, and a Road for no Reason

OK, not quite as deep as the Grand, but who's to quibble over a few hundred feet? That's what I love about California: it has a lot of famous places that everyone has heard about, places like Yosemite, Death Valley, and Big Sur, but it also has many corners that are known locally perhaps, but are not all that prominent in the public consciousness. They are parts of state parks or national forests, sometimes private lands, and they offer geologic treasures that would be national landmarks in any other setting. That is the theme of my sporadic posts on the Other California, a description of these unheralded places.

The beautiful skyline in the photo above comes from the high point on the Glendora Ridge Road, a paved highway with no picnic areas, no campgrounds, no signed overlooks, and no particular destination or purpose that is obvious. The road connects Baldy Village at the east end of the San Gabriel Mountains with roads rising out of the San Gabriel River and the urban sprawl of the Los Angeles Basin around Glendora and San Dimas.
For such an unpublicized road, the Glendora Ridge Road is spectacular. It winds across the top of a high ridge, a fault block caught between the now inactive San Gabriel fault and the highly active Sierra Madre-Cucamonga fault system along the steep mountain front. The rocks include beautiful exposures of Proterozoic gneiss and schist, some of the oldest rocks in California, along with Mesozoic or early Cenozoic granitic rocks (some gneiss is shown in the photo at the end of this post). To the north, the road provides a bird's-eye view of one of Southern California's more dramatic wild areas, the Sheep Mountain Wilderness. It always seemed inconceivable to me that such a place could exist just 10 miles as the crow flies from the urban sprawl of the Los Angeles basin. The wilderness encompasses nearly 8,000 feet of vertical relief from the bottom of the East Fork of the San Gabriel River to the summit of Mt. San Antonio (Mt. Baldy). The canyons are immense, with depths exceeding 4,000 feet in a few places. The rivers (another SoCal rarity) are so rugged that few trails reach them, and at least one of them is a natural trout fishery (Upper Fish Fork). The wilderness is an ideal habitat for the rare Nelson's Bighorn Sheep. I was lucky one year while hiking on the boundary of the wilderness; I chanced upon a herd of a dozen or more. Being who I am, I fumbled in my pack for a camera (one of the old-fashioned kind with this stuff inside called 'film'). They heard the noise and skittered away. I got a great picture of a dust cloud...

Mount San Antonio (10,064 feet) is the highest point in the wilderness. The summit is hidden by clouds in the pictures below, but the mountain is one of the most familiar sights on the Southern California skyline. Thousands of people hike to the summit every year along a trail that starts at the Mt. Baldy ski area. There are two other approaches to the summit, but the number of hikers every year on those routes probably numbers in the dozens. A long and rugged trail approaches from the north part of the wilderness over Dawson Peak and Pine Mountain. Perhaps one of these days my brother will guest-blog about the time we, uh, "misplaced" him on this trail during an Explorer Scout backpack trip...


The other trail to the summit starts at Baldy Village and climbs almost 6,000 feet to the summit. To call this a difficult trail is nearly an understatement, but climbing it was one of the great adventures of my youthful days. Hot and dusty in the lower reaches, and spectacular views the whole way up. It's a good place to catch sight of the bighorn sheep.

There is no official Sheep Peak, by the way. It is the informal name of the prominent peak in the center of the wilderness area, 8,007 foot Iron Mountain (the prominent peak on the right in the picture below). I've never had the privilege, but the climb to the summit is said to be one of the toughest hikes in the south state, with great views and a chance to explore some old gold mines along the way.
Glendora Ridge is paved and well-maintained. That it exists is kind of a mystery to me, and I welcome any explanations as to why it was built. According to Russ Leadabrand (a travel author and Sunset Magazine contributor in the 1960s and 70s), the road was constructed in the early 1930s. I have thought of one possible reason. After my comments a few weeks back about the hazards of living in Southern California, especially adjacent to the high mountain ridges, it occurs to me that Baldy Village would have but a single access road if Glendora Ridge Road wasn't there. A single way out in the event of a major earthquake, a major flood, a major fire, or big landslide. The same problem applies to the developments in San Gabriel Canyon off to the west, a highway that also connects with Glendora Ridge. It looks to me like Glendora Ridge Road is the evacuation route... Whether it is or is not the escape route, it is a great road to drive. Unfortunately the local kids with their newly minted driver licenses know about it too, and a Google search on the subject of the road is likely to turn up stories of tragic car crashes as much as anything else (one of my high school buddies survived a 400 foot plunge off the highway). But for us last month it was uncrowded, and with the clear December skies, a beautiful excursion looking towards the high mountains, and the crowded valleys below.

There are some great rocks exposed along the highway...

Time Beyond Imagining - The Oldest Rocks on the Colorado Plateau

Looking at the oldest rocks of any region requires that we free ourselves of the present-day geography. In the case of the Colorado Plateau, the landscape that existed 1.7 billion years ago was very far removed from the relatively stable arrangement of horizontal sedimentary rocks and gently faulted and folded monoclines and upwarps that exist today. It was not at the same latitude and longitude, possibly not even in the same hemisphere. The land we now know as the Plateau was connected to a large landmass that today is Australia, Antarctica or Siberia. How could everything be so completely different? Amazingly enough, movements of 2-3 inches a year, added up over millions and billions of years, are more than enough to account for the huge changes we see in the rocks. Entire continents move laterally, collide, split up, and reconfigure the world's geography on a constant basis.

The oldest rocks of the Colorado Plateau are much changed from their time of origin. The original sandstone, shale and volcanic rocks have been contorted by heat and pressure brought on by burial deep in the crust, as much as five miles or more. They may date from hundreds of millions of years before the time they became metamorphic, perhaps 2.0-2.2 billion years before the present time. Very little of the original textures can be detected in these metamorphic rocks. Still, we can make some reasonably accurate conjectures about how these rocks formed.

As a rule, the conditions that form schist or gneiss require a major mountain-building event, such as the collision of major continents (like India and Asia today), or by the collision of a terrane with the edge of a continent (such events have happened in Alaska and Indonesia). The best evidence, illustrated very nicely in the Blakey reconstructions linked below, shows a regional continental arrangement somewhat similar to the situation that exists today in southeast Asia. North America was a smaller continent, and only a small corner of California existed as dry land. Two island masses, the Yavapai and Mazatzal islands were moving north towards the continent. They made contact, one after the other, about 1,700 million years ago, causing an extensive mountain range to rise. The range extended through New Mexico, Arizona, California, and an unknown distance west. We can't say for sure how tall the mountains were, perhaps 10-20 thousand feet, but we can say that they were absolutely barren of life: only rock, gravel and maybe snow. Even in the seas, nothing above the level of single-celled life existed.

The mountains rose to the sky, the mountains eroded. And eroded. Over several hundred million years, rock was removed until nothing remained but a nearly flat plain near sea level. Whatever else happened in this interval is lost to us forever, for erosion, by definition, removes information. But the roots of the mountains still exist for us to study, in the Inner Gorge of the Grand Canyon, at Frenchman's Mountain outside Las Vegas, in the Black Mountains of Death Valley, on the Uncompahgre Uplift in Colorado (and Unaweep Canyon), at Colorado National Monument and Black Canyon of the Gunnison National Park, and at other sites scattered in the deepest canyons on the Plateau and around the Basin and Range, especially in southern Arizona.

The rocks, in my humble opinion, are some of the most beautiful one can find. The biotite and muscovite mica shines like glitter, the black and white layers form incredible folds and crinkles (crinkles, there's a geological term for you), and the occasionally there are the bright greens of epidote and the red-browns of garnet. It is humbling to hold rocks from a different time and place far changed from the world we know today.

Paleogeographic maps from Ronald Blakey at Northern Arizona University

Time Beyond Imagining - An Intro to the Colorado Plateau

I would like to use a series of posts to illustrate the incredible story revealed in the rocks of the Colorado Plateau where I held class for the last two weeks. The Colorado River and her tributaries have laid bare a wide swath of the earth's crust, exposing more than two billion years of earth history. Few places offer so much information, exposed so vividly, within an area that can be toured over a matter of days.

The principle of superposition tells us that in a sedimentary sequence, the oldest layers will be exposed deeper within the surface, presuming that no overturning has taken place. It is when we find the base rocks upon which the deepest sedimentary layers are laid that we find the truly ancient rocks. On the Colorado Plateau, this happens in a couple of places: the Inner Gorge of the Grand Canyon, at Colorado National Monument, and especially at Black Canyon of the Gunnison National Park. If you wish to pick up a few pieces of the ancient crust, you might check some of the quarries in Unaweep Canyon south of Grand Junction, Colorado and other locales on the Uncompahgre Highlands; collecting is generally allowed on Bureau of Land Management and National Forest lands.

Black Canyon of the Gunnison is extraordinary. Over time, the deep crust in this region has been lifted and pushed upwards into mountain ranges thousands of feet high, in Proterozoic time, in late Paleozoic time, and now in Cenozoic time. Mountains rose, and mountains were eroded away. Eroded so completely that shallow seas washed over the remains of the mountains and covered them with hundreds and thousands of feet of additional sediment.

Today, the Gunnison River is misplaced, in a sense. Standing on the rim, one is struck by the way the land slopes away from the canyon walls; the deep valley more or less crosses a mountain ridge. The formation of the present-day canyon, which at 2,000 feet is often deeper than it is wide, is an entire post in itself, to be saved for another time. I want to concentrate on the ancient rocks that form the canyon walls.

The Proterozoic is what we call the time period from 2.5 billion years ago to about 545 million years. It is a vast expanse of time; all the story of complex multicelled life on planet Earth took place in the most recent 545 million years. The Proterozoic was four times as long!

The rocks exposed in the canyon walls at Black Canyon are not sedimentary; they have been changed by heat and pressure into metamorphic rock: dark biotite mica schist, gneiss, and hornfels. On the Painted Wall of Black Canyon, seen in the photo above, one can also see light colored dikes of granitic rock that melted and intruded into the older rocks about 1.4 billion years ago.

The story of these most ancient rocks on the Plateau begins with tomorrow's post!