A Great Geologist Passes: John Shelton

John Shelton passed away on July 24th at his home in La Jolla, California.

If you have been following my blog from early on (the olden days of January 2008), you will know that I am thrilled by plane rides, and the unique perspective that aerial photographs add to one's understanding of geology. I haven't talked much about why I feel this so strongly. I didn't get to fly much as a child and teen, but I lived in southern California, and I was surrounded by high mountains: the San Gabriel and San Bernardino Mountains were only a short drive away, the Sierra Nevada and the Mojave Desert just a little farther. My favorite weekends were those spent climbing mountains that gave me a view. If I was stuck at home, I climbed to the very top of a deodar tree in my backyard that gave me a look at the surrounding neighborhood (I thought as a child that it must have been 50 feet high, it was more like 20...). When I took my first geology class in 1975, I was introduced to the perfect book that would feed my mania for high places: Geology Illustrated by John Shelton, published in 1966.


Shelton was a great teacher, a pilot, and a photographer. If the teachers among you have ever shown a slide with an aerial view of a geologic feature, there is a good chance that he took the picture. His set of 400 aerial photographs is still sold in geoeducational catalogs. His book, despite being out of print for decades, is considered one of the 100 most influential books in science for the last century by American Scientist. Get it if you can at the used bookstores or online, it is priceless. My dog-eared copy is one of my single most treasured books.

John was a graduate of Pomona College (in 1935), and received his PhD from Yale in 1947. He taught at Pomona College from 1945 to 1960, and then he served as a scientific advisor for the American Geological Institute from 1962 to 1974, developing a series of scientific films with Encyclopedia Britannica. If you remember seeing "Beach: River of Sand", "Why do we still have Mountains?", "How Solid is Rock?" and lots of others, you were also influenced by Shelton's contributions to the geological sciences. In retirement, he assisted in the development of the "Earth Revealed" telecourse series.

As a young student at Pomona College, I had a single opportunity to meet Dr. Shelton, and we corresponded a bit when I began searching for a geological career. I appreciated his assistance, but I can barely imagine any books that were more influential in defining the direction of my own career, and as a teacher, his pictures are one of my most valuable classroom resources. I offer my condolences to his family, and I want them to know how much I appreciate his contributions to the geological sciences, as well as the inspiration he provided in the direction of my life.

Picture from the San Diego Natural History Museum

More information about John Shelton can be found here, or here , and info on the exhibit of his photography at the San Diego Natural History Museum can be seen here (through November 2).

Time Beyond Imagining - A Brief History of the Colorado Plateau: Old Salt

So, what do the Arches at Arches National Park, the Potash Mine just outside Canyonlands National Park, the tragic 2008 earthquake in the Sichuan Province of China, and the Southern Appalachians have in common?

Well, ok, not much, but bear with me. In the last post on the geology of the Colorado Plateau, we were exploring the formation of a rather extensive mountain range in southwest Colorado, where the Rocky Mountains are today (but these mountains, the Ancestral Rockies, were eroded away in the late Paleozoic era). We were left wondering why mountains could be rising in the interior of the continent, when most mountain ranges seem to develop along continental margins.

The breakup of the supercontinent Rodinia in late Proterozoic and Early Paleozoic time had a huge impact on the patterns of sedimentation in the Colorado Plateau, with a series of transgressions and regressions that gave rise to the formations of most of the recent posts. The continents remained as separate entities through much of Paleozoic time, but landmasses began to converge upon one another, and a series of collisions produced Himalayan-sized mountain ranges (which is coincidently the reason the Himalayas happened: Asia and India are colliding). A collision between a portion of Europe and the eastern part of North America raised the northern Appalachian Mountains. Asia and Europe connected to form the Ural Mountains, and in Pennsylvanian time, Africa crashed into the southeastern United States to raise the southern Appalachian Mountains. The supercontinent of Pangea was being constructed, bit by bit.

(As a side note, I should point out that when geologists speak of colliding and crashing continents, we are living in a totally different time reality. The colliding continents are moving no faster than 5 or 6 inches a year. Yes, India is crashing into Asia, but it has been happening now for 40 million years)

The collision of Africa and the SE United States explains the Appalachians, but what about the tragic earthquake in the Sichuan Province? The Ancestral Rockies formed at about the same time as the Appalachians, but the two ranges are separated by a thousand miles or so. In the same way, the mountains of central China are far from the Himalayas, but their origin is related to the buckling of the crust related to this continental collision. The process of buckling produces earthquakes, and in highly populated regions this produces tragedy. Peripheral buckling of the crust along ancient fault lines related to the collision with Africa is at least one explanation for the origin of the Ancestral Rockies.

The Potash Mine? Just south of the town of Moab, and on the edge of Canyonlands National Park, there is an exceedingly odd sight: the evaporation ponds of a potash mine. The turquoise blue, seen here from Dead Horse Point, clashes with the red beds of Permian formations. At the mine, holes are drilled into salt layers, and water is injected to dissolve the salts and bring them to the surface. The brine is evaporated, and the salts are harvested and transported away.

The salt is also related to the formation of the Ancestral Rockies. When the crust is buckled and mountains rise, in other places the crust is pushed downwards. A feature called the Paradox Basin developed southwest of the Ancestral Rockies. It flooded with seawater, but for some reason the connection to the ocean was cut off, and the water evaporated, leaving behind thick deposits of salt and other evaporite minerals. Water flooded in again, and the cycle was repeated, at least two dozen times. Eventually the salt layers were thousands of feet thick.

What is the connection to Arches National Park? Well, this post has become long, and I will discuss that issue next time!

Time Beyond Imagining - A Brief History of the Colorado Plateau, Part 7: Mountains Rise

Where did all the dirt come from??? During the previous post, we discovered that something had caused thousands of square miles of land across Arizona, Utah, and Colorado to be inundated with mud and sand more than 1,000 feet deep. These rocks formed above sea level, as deltas, floodplains, river channels, and sand dunes. In the Grand Canyon, these layers of red sediments are called the Supai Group and Hermit Shale. In southern Colorado and Utah, related rocks are referred to as the Hermosa Group, the Halgaito Formation, the Cedar Mesa Sandstone, and the Organ Rock Formation (the Organ Rock Formation makes up the base of the spires of Monument Valley).

On the other hand, today's photograph shows a canyon near the eastern edge of Colorado National Monument, one of those oft-overlooked gems that are scattered about the Colorado Plateau. The grayish scrub-covered rocks in the bottom of the canyon are composed of the same 1.7 billion year old metamorphic rocks that were seen in the Grand Canyon and Black Canyon of the Gunnison. Unlike the Grand Canyon, there is no Grand Canyon Supergroup, no Tonto Group, no Redwall Limestone, no Supai Group or Hermit Formation. None of the rocks of the Grand Canyon are found here at all. The rocks forming the canyon walls are Mesozoic in age (less than 245 million years old), with the lower slopes composed of Triassic Chinle Formation, and cliffs of Wingate Sandstone (more on these later). So we have a second mystery: why are the Paleozoic rocks missing in southwest Colorado when they are thousands of feet thick in adjacent regions?

Pioneering geologists in the region soon recognized that the source of the sediments found in Arizona were mountains that had risen in southwest Colorado during Pennsylvanian and Permian time. The mountains were pushed so high that all the overlying Paleozoic sediments and a considerable amount of the Proterozoic metamorphic rocks were washed onto the coastal plains, leaving a denuded surface of metamorphic rocks that were not covered by sediments until Mesozoic time. These mountains are referred to today as the Ancestral Rocky Mountains or the Uncompahgre Uplift. As we look at the unconformity between the Proterozoic and Mesozoic rocks, we are looking at the eroded roots of what must have been an extraordinary mountain range rising from the interior of the North American continent.

Which leads to the next vexing question: what were mountains doing within the continental interior? Mountains most often form along a continental margin (think of the Andes, Cascades, or Coast Ranges). For a long time, no particularly good explanation could be offered to explain the development of these mountains.

More in the next post! And a salty surprise!

Time Beyond Imagining - A Brief History of the Colorado Plateau, Part 6: Seeing Red (Really!)

Parts of the Grand Canyon are really red. Extremely red. So red, that a hike through the red layers leaves stains on socks that will never, never come out. So red that it spills over and turns the surface of the underlying rocks red too (see the last post about this). Red enough that the river that carries this sediment to the sea (or at least used to), the Colorado, means Red River.

There are several red formations in the Grand Canyon, but the subject of today's post is a group of layers that are collectively known as the Supai Group and Hermit Formation. A group is a collection of formations that share some characteristic, such as similar rocks, age or depositional environment. There are four formations that make up the Supai Group, and they are a bunch of tongue-twisters: the Watahomigi, the Manakacha, the Wescogame, and the Esplanade. They are red. Did I mention that?

All five formations are visible in the first photo. The lower prominent cliff is the Redwall Limestone, which takes the color by staining from the overlying Supai Group (it is actually gray). The Supai Group formations and the Hermit Formation are a series of alternating ledges and cliffs that are bright red, leading to another prominent cliff, the white Coconino Sandstone near the rim of the canyon. The individual formations are not easily distinguished. The easily eroded Hermit Formation is best seen in the second picture, where it is exposed just below the white cliff of Nankoweap Butte (as seen from Pt. Imperial on the North Rim). Collectively, these layers are the thickest of those seen from the rim of the canyon, totaling something like 1,000 feet. The first three formations date from the Pennsylvanian Period (just over 300 million years ago), and the Esplanade Sandstone and the Hermit Formation are early Permian (just less than 300 million years ago).

What does the red mean? It means iron oxides, which form in the presence...of oxygen. Very few of the layers discussed so far in this series formed above sea level. The red beds formed in floodplains, river channels, delta deposits, and sand dunes, all above sea level. They are terrestrial deposits. So the Supai Group and the Hermit Formation cover thousands of square miles in northern Arizona to a depth of more than a thousand feet, which leaves us with a huge question...where did all this sand and mud come from? And why are Pennsylvanian and Permian deposits on other parts of the Colorado Plateau very different from those in the Grand Canyon?

More in the next post!

Do you have a Photographic Memory?

I do. But not in the normal sense of the word. I forget most things in my past, but those old memories are reawakened by looking at photographs taken at the time. The most vivid example came to me early in life, in my mid-twenties. As a reward for actually graduating from high school, a friend and I walked the John Muir Trail along with some side trips during the summer of 1975 (and unfortunately a few short months before taking my first class in geology). Five or six years later, I decided that the trip, which I consider one of the pivotal events of my life, should be preserved as a written memory.

As I started to describe the trip, I realized that huge gaps existed in my memories, and since I had only five rolls of Instamatic film (remember those cameras, older people?), I was only taking two or three pictures a day. But those pictures were the key to my unlocking the memories. I only wish I could have taken more. So much has been lost.

As such, I came across an older post on saving older pre-digital age photographs by Dr. Jerque at Geologic Froth that deserves your attention. I have thousands of old slides that I never have the time to peruse, and the good doctor makes that point that it is a lot easier to look at digital images than it is to pull up slides and run them through a projector or screen tray.

Today's picture? The gawky teenager on his first major expedition into the Sierra wilderness, just few months before his entry into the world of geology. The moments we wish we could get back....

Time Beyond Imagining - A Brief History of the Colorado Plateau, Part 5: Seeing Red

Well, not exactly seeing red; it is more like seeing gray (I'll explain in a moment). The Redwall Limestone is one of the most prominent layers in the Grand Canyon, forming 400-500 foot cliffs almost anywhere that it is exposed (and in the Grand Canyon, that would be everywhere). It exerts a huge control on human access to the inner gorge of the park; literally all trails are located on the basis of how they can surmount or circumvent the Redwall cliffs (usually where a fault or joint system cuts through the layer).

The Redwall Limestone formed in Mississippian time, around 330-340 million years ago. The lime formed in a shallow equatorial environment. Rocks like it are found all across North America, the result of the spread of one of the greatest of the Paleozoic epicontinental seas. Fossils are extremely common, including brachiopods, bryozoans, corals, sponges, nautiloids, and fish. The rock is generally gray in color; the characteristic red color comes from seepage of iron oxides from the overlying Supai Group.

As I have noted in previous blogs, I became a geology major largely as a result of a 1976 field trip into the canyon. The Redwall Limestone loomed large on that journey. Really steep and TALL! Water sources were widely separated, and we loaded up more than a gallon apiece at a spring at the base of the Redwall, since we would have none for the remainder of our journey out of the canyon. The climb up the cliff was tough, but along the way we explored a copper mine (my first time seeing azurite and malachite), and once we made it to the top of Horseshoe Mesa, we explored a pair of limestone caverns (complete with ropes and helmets that we had dragged during the entire journey).

The caverns were another geological revelation to me. Who knew there are hundreds and hundreds of caves in the Grand Canyon (besides a number of geologists and spelunkers)? But there are! Even in Mississippian time, sinkholes and caverns formed on the surface of the Redwall. The sinkholes filled with sediments from the overlying formation, and indeed an entire formation went unnoticed in these sinkholes right up until the 1980's (the wonderfully named Surprise Canyon Formation).

The caves that are open to exploration today formed much more recently, when the groundwater table was higher, around the time that the canyon itself was being carved. Rather few of the caves are actually accessible to people on foot. Most often, the caves open out as alcoves at the top of the Redwall cliffs, and are easily visible from most points on the rim. Some of the caves were accessible to Pleistocene mammals, and at least one of the caves was full of ground sloth droppings (a precious scientific resource that got burned up a few years ago).

A final note, about caves. Publicity is the death knell of a cavern. I made it back to that cave on Horseshoe Mesa about 25 years later, and it was a mess, not much more than a featureless hole in the ground. It may be that I didn't get into the back reaches that I remembered, but every visit to a cave does damage, and this one has sustained a lot. Take care when in caves; they can't be fixed in our lifetimes.

Time Beyond Imagining - A Brief History of the Colorado Plateau, Part 4: Something is Missing!

The Grand Canyon is assumed by many to be an encyclopedia of earth history, that the rocks exposed there reveal a complete sequence of sedimentary layers showing the march of world events. It is not quite like that...

Imagine that you are interviewing two candidates for a job opening. One of them has a perfect resume, with a complete job history going back for years, with no time periods for which a job was not being performed. The other candidate has a rather sloppy resume, and there are years at a time where no job is mentioned. It might be easy to choose whom you would hire, but on the other hand, who do you find the most interesting, the most mysterious? Who do you want to know more about, even if you would never hire him or her?

Geology on the Colorado Plateau is much the same as a choppy resume. Each layer is a mere glimpse at a short period of earth history, but huge gaps exist between many of the layers. These gaps, mentioned in previous posts, are called uncomformities. They result when exposure produces erosion that removes previously existing rocks. There are at least 14 of these gaps in the Grand Canyon section.

In the previous post, we talked about the Tonto Group, the three Cambrian-aged layers that were laid down sequentially as the shallow epicontinental sea covered much of the North American continent. These layers formed around 545-515 million years ago. If there were a complete unbroken record, the next layers would be Ordovician (490-445 million years ago, roughly) and the layers above those would be Silurian (around 445 to 415 million years ago). But they aren't there. Something like 100 million years has gone missing!

The next layer in the sequence at Grand Canyon is the Devonian-aged Temple Butte Limestone. It is a fresh-water limestone that was deposited across a rough eroded surface. In many parts of the Grand Canyon, it doesn't even exist as a continuous layer, because it was eroded itself before the next formation was laid down on top of it. I have not actually seen it myself in my travels inside the canyon (and certainly no digital images, thus a mysterious looking canyon sunset for a picture today!).

The layer is of great interest because although vertebrates are known from middle to late Cambrian time, and they evolved widely in Ordovician and Silurian time, the Temple Butte Limestone is the first Grand Canyon formation that has fish fossils. They are the bony plates of fish who scuttled back and forth in the channels near the Devonian shoreline.

Geology is interesting to me, not because the layers of the earth provide all the answers to all the geological mysteries, but because so much of the record is missing. A geologist has to be a very intuitive detective, working with limited information and tantalizing clues. But what a great journey!