Time Beyond Imagining - A Brief History of the Colorado Plateau, Part 3: LIFE!

Fossils! Do you remember that first moment that you found a fossil as a child? Or did you never have that experience? I vividly remember exploring a meadow on the Kaibab Plateau north of the Grand Canyon as a 10-year-old. I didn't know what they were at the time, but later learned that they were disks from crinoid stems and sponges from the Permian seas 300 million years ago. I was impressed; I was not even disappointed that they weren't dinosaurs. I can look back and see this moment as one of the seminal events that led to my choice of geology as a field of study.


All kids go through a fossil phase. I also know that most geologists and paleontologists simply never outgrew it. And so it is that today's post is about fossils. Particularly the earliest complex life that evolved on our planet.

600 million years ago (give or take, what's a few million years between friends?), the continents of the world were in the process of splitting and breaking up. Many laypeople have heard of Alfred Wegner's Pangea, but we are not talking about that particular supercontinent (although we will in a later post). The continent that existed around a billion years ago has been named Rodinia. Such continents seem to be unstable, perhaps due to the buildup of heat from underneath. The land is pushed upwards in vulnerable places, the continent splits, and new seas are formed. As the continents move further apart, they subside and the shorelines around the margins of the continent invade the land. All over the world, the seas were transgressing across continental surfaces, forming shallow epicontinental seas (I am guessing that Hudson Bay comes closest in today's world as an example).


The split in North America cut across the southeastern part of California, and through central Nevada. During the Cambrian Period, the transgressing seas covered most of the southwest, with the exception of a portion of Utah called the Transcontinental Arch. The region was located close to the equator, so the ingredients were there for life: nutrients, shallow water, warmth. Life was about to get a whole lot more complex than the single-celled organisms that had existed for billions of years. The details are for other experts to describe, but multi-celled life appeared, and thrived.


Look at the picture of the Grand Canyon. Down in the depths there is a prominent terrace called the Tonto Platform. The platform exists because the Bright Angel Shale cannot form cliffs for very long, being easily eroded. Just below lies a cliff of Tapeats Sandstone, and above are a few slopes and ledges of the Muav Limestone. These three formations, sand, shale and limestone, record the invasion of the shallow sea across the Grand Canyon region.


Pick up enough fragments of the Bright Angel Shale, and you may be lucky enough to find the signs of the explosion of life that swept across the planet around 540 to 500 million years ago. Worm trails and burrows are plentiful, but the prizes will be the discarded exoskeletons of trilobites.


Trilobites are arthropods in the same phylum as the insects, crustaceans, arachnids and other jointed-legged creatures. Because they were one of the first animals to possess a hard shell, their fossils tend to dominate the fossil record, even though a large number of soft-bodied animals were doubtlessly around (see one of my first posts). They walked and burrowed in the sediments of the ocean floor; some grazed on algae, some were mud processors, some may have been predators. They ranged in length from a quarter inch to one monster that exceeded three feet in length. Their closest living relatives are probably the horseshoe crabs (see this NOVA geoblog article). Some were capable of rolling up like pill-bugs to protect themselves. They grew by molting, so not every discovery is a dead creature, simply a cast-off shell.


Other early creatures included brachiopods, sponges, and strange cone-shaped archaeocyathids, which may have been distantly related to corals.

Earth: The Biography on National Geographic Channel

It looks like the National Geographic Channel has put together a great documentary on Planet Earth, called Earth: The Biography. It is always good to see the planet getting the glossy treatment in a way that instructs and inspires at the same time. I enjoyed the BBC's Earth series a few months back even though it was mostly about animals and plants and stuff! Check it out!

Time Beyond Imagining - A Brief History of the Colorado Plateau, Part 2

Look at a picture of one of the most famous geologic localities in the world, the Grand Canyon in Arizona: in this case, from the South Rim, in the vicinity of Desert View on the eastern edge of the park (we were on the North Rim during our recent trip, but this view works better for today's discussion). Sedimentary layers stacked thousands of feet deep, recording hundreds of millions of years of deposition in shallow seas, river floodplains, and desert dune fields. But take a closer look: there are two utterly separate stories revealed here. One story is told by the horizontal layers making up the upper walls of the canyon. Their story will be told on a different day. Concentrate instead on the tilted layers visible in the bottom of the canyon.

The upper layers of the Grand Canyon, the Paleozoic rocks, are about 4,000 feet thick. The tilted layers below, the Grand Canyon Supergroup, are closer to 12,000 feet thick, some three times thicker. How in the world did more than two miles of sedimentary layers get tucked into the bottom of the Grand Canyon, between the very ancient metamorphic rocks and the flat-lying Paleozoic rocks?

The basic story is as follows. Following the formation of the Mazatzal/Yavapai mountains in early Proterozoic time, erosion reduced the mountains to a nearly flat, featureless plain (see my previous post on the Oldest Rocks). At various times the land subsided or sea level rose, leading to the deposition of thousands of feet of sedimentary rocks: sandstone, shale, limestone, and some occasional volcanic flows. As in the older rocks, few fossils are found; life on the planet consisted mostly of single-celled organisms. Exposed land was barren of life.

The southwest part of the continent was stretched about 700-800 million years ago by forces related to the break-up of the supercontinent Rodinia. The crust broke into a series of fault blocks that tilted the sedimentary rocks, and raised a series of mountain ridges that would have resembled the faulted mountains of central Nevada and eastern California (the Basin and Range Province). These mountains may have been thousands of feet high, but as in previous events, erosion brought the mountains down, and the layers of the Grand Canyon Supergroup disappeared across the region, except in a few places where downdropped blocks protected the sediments from erosion. Only a few ridges, a few hundred feet high, projected above the plains. This surface was later inundated by rising Paleozoic seas.

So, much of the story of the Grand Canyon involves what is missing more than what is present. Some 500 million years elapsed between the Mazatzal mountains and the formation of the layers of the Grand Canyon Supergroup. Another 250 million years is missing before the deposition of the Paleozoic sequence. These gaps in the rock record, indicated by buried erosional surfaces and abrupt changes in the rock types (metamorphic topped by horizontal sediments; tilted sedimentary layers topped by horizontal sediments) are called unconformities. The two unconformities in the depths of the Grand Canyon are among the most famous in the world.

I love these particular rocks...they are difficult to access, only by long steep trails, and I have only stood on them once. But it was my first extended geology field trip; a five day backpack down the New Hance Trail and up the Grandview Trail in 1976. I came out of the canyon convinced that I wanted to be a geologist. I was able to place my hand on the unconformities, I understood for the first time the idea of the earth's crust as a history book, I touched for the first time some of the oldest fossils on the continent. For icing on the cake, I even saw the Northern Lights for the first time in my life while we were camped on Horseshoe Mesa!

What is next? Life finds a way....the Paleozoic story of Grand Canyon and the Colorado Plateau!

An Annotated Look at our Colorado Plateau Journey

If you want a sneak peak at some of the sights from our recent Colorado Plateau journey, taken by an excellent photographer, check out http://www.mrhollister.com/getout/geology_trips/2008_Colorado_Plateau/index.html. Ryan has a great site for his high school students, including huge numbers of great photographs from all round the American west. And he performed wonderfully as a volunteer on our recent trip. Thanks for your able assistance!

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!

Time Beyond Imagining

Still decompressing after a "long" trip of two weeks through geologic time on the Colorado Plateau. It is always difficult to comprehend the contrast between the hours and days of one's human existence with the millions and billions of years that characterize the geologic history of our planet and Universe. I expect to follow this theme in my blogs for a few days as I sift through the pictures of our journey.

The unusual view above comes courtesy of astronomical lunar cycles and Landscape Arch in Arches National Park. The arch has an opening nearly as long as a football field, and is only 6 feet or so across at the thinnest point. It is rather a wonder that the span still stands, and the future prognosis of the arch is probably measureable in human, not geologic, terms. A large chunk fell from the span around 1991.

The arches in the park are formed in the Slickrock Member of the Jurassic Entrada Sandstone. The unit has been pushed upwards by bodies of mobile salt from the late Paleozoic Paradox Basin. The upward pressure in the salt anticline has fractured and jointed the sandstone into a series of "fins", long narrow walls of rock. The base of the fins lie in the Dewey Bridge member of the Entrada (or Carmel), which is easily eroded, allowing a small window to form at the base of the fin. Chunks of rock fall from the window, until an arch develops. There are hundreds in the park.