This little black pebble, now sitting on my desk, traveled a lot today. After picking it up beneath a cliff face in southwestern Montana, I carried it in my pocket for a few hours and then drove some 20 miles to this college dorm where I’m staying. It’s the most this pebble has moved for millions of years.
Some time ago–this morning, a week, a year, 10 years, 100 years—the pebble weathered out of a much larger rock and fell to the ground. Its worn, rounded edges tell me that before it became part of that larger rock, it traveled down a stream bed –and its size tells me that its source probably wasn’t too far away. As is typical of stream gravel, its movement was irregular, marked by short bursts of movements during floods separated by longer periods of rest on a gravel bar or in the channel itself. Somewhere along the line, the pebble became buried by more sediment, probably because the land subsided or the river channel switched to another position. Eventually, the pebble and the rest of its surrounding sediment turned into rock.
All this took place some 125 million years ago early in the Cretaceous Period during uplift of an early version of the Rocky Mountains. As the mountains rose, they eroded, shedding material off their flanks and ultimately into streams, where they became worn and rounded, my pebble among them. Today, we can see the folded and faulted remnants of those mountains. They include a sequence of sedimentary rock that records the area’s geologic history dating back more than 500 million years. One of the layers in this sequence, called the Phosphoria Formation, provided the raw material for the pebble. Another layer, called the Kootenai Formation, contains the rock that held the pebble.
Using a handlens, I look closely. The pebble is incredibly fine-grained, with a texture more like a piece of caramel than most typical rocks. It’s chert, made of sub-microscopic silica from single-celled organisms called radiolaria. These creatures float in ocean waters and settle to the bottom when they die. The Phosphoria Formation formed during the Permian Period about 270 million years ago when more than a hundred feet of chert accumulated on the sea floor.
I think that’s what fascinates me the most about this pebble: its very existence speaks to a geologic history from ocean bottom to mountain range. Not only that, but it exists in two time periods: today and the Cretaceous. Throw it on a gravel bar and you couldn’t tell it apart from the modern pebbles.
Perhaps even more extraordinary is how non-extraordinary this pebble really is. Take any handful of loose sand that’s weathered from a nearby sandstone outcrop and look closely: those tiny sand grains are just like the pebble, only smaller. They’re sand grains now, just as they were when they became a part of the nearby rock. And like the pebble, they came from a still older rock –which itself has a history. And it’s all in transit.
To view and/or download more images of sedimentary rocks –or any kind of geology for that matter–please take a look at my website: geologypics.com
Something different here: an essay I wrote about teaching field geology on public lands– published under the same title in the March, 2021 issue of Desert Report with a few extra photos added (please click on the photos to see them larger). I guess I’m hopeful –even optimistic– that I’ll be able to teach field camp again this summer! ******************
Six pairs of eyes stared blankly at me. Cows. Amazing how big those creatures are, especially when you’re sitting on the ground so I was grateful they hadn’t already tried to share my tiny bit of shade. I stood up, shouldered my pack, and walked into the bright sun.
I’m in southwestern Montana, teaching a field geology class. It’s public land, a place I bring my students year after year without fear of locked gates or fences. Besides the great geology, we see awesome views in all directions, innumerable cacti, wildflowers, and sagebrush, occasional wildlife, and of course, cattle. But we come for the rocks: layer after layer, deposited as seafloor sediment, coastal dune, or river gravel over a period spanning some 300 million years. We’re in what’s called the Rocky Mountain Fold-Thrust Belt, where the layers show twists and turns you can’t imagine, and fault zones that caused mountains to rise long before the present landscape even began to form.
And because the area’s managed by the Bureau of Land Management, we can go pretty much anywhere and everywhere over this 3 or 4 square mile area, just like the cows. Each student creates a detailed geologic map. They identify the rocks, draw lines on their maps to show the boundaries between different rock units, and use their maps to interpret the geologic history. It’s difficult work and highly rewarding ‒ and most students complete the course with a new-found sense of confidence and competence.
I spot some students on a nearby ridge and veer off the gravel road to meet them. The sagebrush near the road is high, but as the slope steepens, it gives way to grass and ledges. On cresting the ridge, I call out:
“How’s it going?”
“Okay” one of the students replies. We look at each other stupidly. I have no idea who it is.
“Hope you’re having a good day!” I say cheerfully and walk on.
These students aren’t mine. They’re from one of the many other universities that come here to teach field geology skills. With so many students tramping over this landscape every summer, following existing paths to key outcrops or creating their own, breaking pieces off the bedrock so they can inspect fresh surfaces, I sometimes worry that we might love this place to death.
I feel a slight cooling breeze and sit down. In front of me, the rocks form a rounded arch shape ‒ a product of crustal compression that built mountains here some 80 million years ago. The frustration of chasing down the wrong students morphs into gratitude. I think of the many students who’ve told me, years later, how much they learned here and how much it shaped their careers. Many describe how blending physical exertion with their academic backgrounds led to a sense of discovery they’d never before experienced.
I watch a pickup truck approach the cows down on the road. It slows briefly to pass by and then continues up the gentle grade. Multiple use ‒ that’s the BLM mantra. Besides grazing and wandering geology students, this place is open to energy development, timber harvesting, and all sorts of recreation. By comparison, our impact is small ‒ and the geology here will outlive all of us, no matter how many cattle trails we follow, or create, or how many outcrops we hammer. I’m grateful for our freedom to wander, pursuing education anywhere and everywhere here, and to arrive anytime without having to fill out any kind of paperwork.
Ten days later, I’m sitting on a block of limestone, listening to water cascade over some ledges before coalescing into a narrow stream. Above me, soaring cliffs of Helena Dolomite form the headwall to the once mighty Grinnell Glacier. Some of the glacier persists high up in this cirque, but the retreat has been so rapid and recent that the glacial scratches across the bedrock are still fresh. Two of my students sit silently, gazing upwards to the headwall. Another studies a rock with a hand lens. Two others slowly make their way towards my perch.
Glacier National Park hosts a vast wilderness of landforms designated by glacial terminology: peaks called horns, ridges called arêtes, cliffs called headwalls, bowl-shaped valley heads called cirques, rock-bound lakes called tarns, deep U-shaped valleys called U-shaped valleys ‒ all carved by glaciers into this amazing bedrock. I ponder what this place must have looked like 15,000 years ago when glaciers were at their maximum. The peaks and some ridges would be recognizable but the valleys between them would be filled with ice that extended in long strands to the edge of the Great Plains.
One of my students practically shrieks with excitement. I’ve been waiting for this: they found the stromatolites, fossilized algae ‒ Earth’s oldest easily visible life form. Resembling an onion in cross-section, these thin concentric layers of rock formed when dome-shaped bodies of algae in shallow clear water trapped sediment as they grew. The ones here, which reach a meter or more in diameter, are the largest and most dramatic I’ve seen.
I watch the other students hurry over as the contemplative mood lights into a spontaneous revival. One student starts laughing. Another exclaims, “Look at this one!” another says, “Oh God!” another: “And the glacial striations go right across them!” Another turns to me quizzically: “How old are these rocks?”
I smile. “I just read that they were all deposited between 1.4 and 1.47 billion years old ‒ so somewhere in there.”
Another question: “And these glacial striations, maybe 10 or 15,000?”
I keep smiling. This spot is one of my all-time favorite places. We hiked the five miles up here for sunrise and now, mid-morning, still have the place to ourselves. The landscape is so fresh and raw ‒ so untouched ‒ that we feel a primeval kinship with this rock and ice.
I finally answer. “Or younger. This place was under ice just thirty years ago. There’s still some glacier left, just across the lake. So much change, huh?”
Many of my students have visited national parks before, but this is the first time they’ve come as geologists. They now see things through the lens of geologic time and process ‒ and for the first time, they are applying their knowledge to a landscape that not only surrounds them, but is beautiful and pristine. They learned important skills earlier in the course on BLM land and here they combine those skills with their human spirits.
We cross the stream and start picking our way up the ledges on the other side, finding more stromatolites as we go. I hear my students talking about landscape and time. Like me, they can relate to today’s landscape, and can imagine the scene during the glacial maximum some 15,000 years ago, but the shallow inland sea in which these rocks formed seems beyond comprehension. Its age, about 1.4 billion years, makes it all the more inconceivable. And what does the word “about” mean in this context anyway? If the rocks formed 1.41 instead of 1.40 billion years ago, they would be a full ten million years older. As geologists, we bandy these ages around with comfort, but when we really think about it, we can’t comprehend 10 million years, let alone a billion.
I stop and consider a series of stromatolites in front of me. They’re exposed on both the front and upper sides of the ledge, giving three-dimensional views of their mounded shapes. Weathering and erosion rounded off many of the broken edges, accentuating the stromatolite’s appearance. For a second, I think I can see them breathe.
Our little group falls silent as we watch a rain squall farther down the valley. What a vast open-air cathedral we’re in! Public lands, be they national parks or forests or open BLM land, house these wonderful places and welcome everyone who will make the journey.
To see more geological photos from from SW Montana or Glacier NP, please see my geology photo website, where you can use keywords to search among >4000 images and download them for personal or instructional uses for free.
Published in 1974 by the US Geological Survey, the geologic map of the United States beautifully lays out our country’s geology. If you’re stuck at home these days –as most of us are—you can gaze at this masterpiece and go anywhere!
Southern Willamette Valley. Eugene’s in the center of the map
At their simplest, geologic maps read like road maps: they tell you what rock unit or recent deposit forms the ground at a given place. So right here where I am in Eugene, Oregon, I can see that I live on “Q” –which stretches north up the Willamette Valley. “Q” stands for Quaternary-age material (2.5 million years to present), which is typically alluvial material –or sediment deposited by rivers and streams. Just to the south of me lie a variety of older volcanic and sedimentary rocks. You can look them up in the map’s legend using their symbols. Here’s Rule #0: white areas, being mostly alluvium, mark low areas; colorful areas indicate bedrock, so are typically higher in elevation.
The beauty of this map is that you can see the whole country at once, and using just a few rules, can immediately glean the underlying structure of a region.For any of the maps, photos, and diagrams here, you can see a larger size if you click on the image.
Rule 1: Rock bodies without internal contacts appear as color swatches (or “blobs”) as seen on the block diagram below. These rocks include intrusive igneous, undifferentiated metamorphic, and flat-lying sedimentary rock. (If you’re not sure what geologic contacts are, please see my recent post on the nature of geologic contacts.) Read more…
Geologic contacts are the surfaces where two different rocks touch each other –where they make contact. And there are only three types: depositional, intrusive, or fault. Contacts are one of the basic concerns in field geology and in creating geologic maps –and geologic maps are critical to comprehending the geology of a given area. For those of you out there who already know this stuff, I’ll do my best to spice it up with some nice photos. For those of you who don’t? This post is for you!
Depositional contacts are those where a sedimentary or volcanic rock was deposited on an older rock (of any type). Intrusive contacts are those where igneous rocks intrude older rock (of any type). Fault contacts are… faults! –surfaces where two rocks of any type have moved into their current positions next to each other along a fault.
In a cross-sectional sketch they may look like this:
And here are some photos. Click on the image to see it at full size.
So how do you tell them apart in the field? If the actual contact surface isn’t exposed –which is usually the case– you have to use some indirect observations. Here are some general rules that can help. Of course, each “rule” has exceptions, described later. Read more…
I always try for window seats when flying and I always try to shoot photos out the window –with varying results! So often, the window’s badly scratched, there are clouds, it’s hazy, the sun angle’s wrong –there are myriad factors that can make good photography almost impossible from a commercial jet. Last year though, I had a few amazing flights with clear skies and a great window seat –and I’ve now loaded nearly 100 images onto my website for free download. Here are 10 of my favorites, in no particular order. You can click on them to see them at a larger size. They’re even bigger on my website.
Mt. Shasta at sunset. Volumetrically, the biggest of the Cascade Volcanoes, Mt. Shasta last erupted between 2-300 years ago –and it’s spawned over 70 mudflows in the past 1000 years. From the photo, you can see how the volcano’s actually a combination of at least 3 volcanoes, including Shastina, which erupted about 11,000 years ago.
Aerial view of Mt. Shasta, a Cascades stratovolcano in northern California.
If you want to see more aerials of Mt. Shasta (shot during the day) –and from a small plane, go to the search page on my website and type in “Shasta”.
Meteor Crater, Arizona. Wow –I’ve ALWAYS wanted to get a photo of Meteor Crater from the air –and suddenly, on a flight from Phoenix to Denver, there it was!
Aerial view of Meteor Crater, Arizona
Meteor Crater, also called Barringer Crater, formed by the impact of a meteorite some 50,000 years ago. It measures 3900 feet in diameter and about 560 feet deep. The meteorite, called the Canyon Diablo Meteorite, was about 50 meters across.
Dakota Hogback and Colorado Front Range, near Morrison, Colorado. Same flight as Meteor Crater –and another photo I’d longed to take. It really isn’t the prettiest photo, BUT, it shows the Cretaceous Dakota Hogback angling from the bottom left of the photo northwards along the range and Red Rocks Amphitheater in the center –then everything behind Red Rocks, including the peaks of Rocky Mountain National Park in the background, consist of Proterozoic basement rock.
Aerial view of hogback of Cretaceous Dakota Formation and Colorado Front Range.
Distributary channels on delta, Texas Gulf Coast. I just thought this one was really pretty. Geologically, it shows how rivers divide into many distributary channels when they encounter the super low gradients of deltas. And whoever thought that flying into Houston could be so exciting!
Distributary channels on delta, Texas Gulf Coast
Meander bends on the Mississippi River. My mother lives in Florida, so I always fly over the Mississippi River when I go visit –but I was never able to take a decent photo until my return trip last October, when the air was clear, and our flight path passed just north of New Orleans. Those sweeping arms of each meander are about 5 miles long!
Meander bends on the Mississippi River floodplain, Louisiana
Salt Evaporators, San Francisco Bay. Flying into San Francisco is always great because you get to see the incredible evaporation ponds near the south end of the bay. I always love the colors, caused by differing concentrations of algae –which respond to differences in salinity. And for some reason, salt deposits always spark my imagination. Salt covers the floor of Death Valley, a place where I do most of my research, and Permian salt deposits play a big role in the geology of much of southeastern Utah, another place I know and love.
Salt evaporators, San Francisco Bay, California
Bonneville Salt Flats and Newfoundland Mountains, Utah. And then there are the Bonneville Salt Flats! They’re so vast –how I’d love the time to explore them. They formed by evaporation of Pleistocene Lake Bonneville, the ancestor of today’s Great Salt Lake. When the climate was wetter during the Ice Age, Lake Bonneville was practically an inland sea –and this photo shows just a small part of it.
Aerial view of Bonneville Salt Flats and Newfoundland Mountains
Stranded meander loop on the Colorado River. I like this photo because it speaks to the evolution of this stretch of the Colorado River. Just left of center, you can see an old meander loop –and it’s at a much higher elevation than today’s channel. At one time, the Colorado River flowed around that loop, but after breaching the divide and stranding it as an oxbow, it proceeded to cut its channel deeper and left the oxbow at a higher elevation.
Stranded meander loop (oxbow) on the Colorado River, eastern Utah
San Andreas fault zone and San Francisco. See those skinny lakes running diagonally through the center of the photo? They’re the Upper and Lower Crystal Springs Reservoirs –and they’re right on the San Andreas Fault. And you can see just how close San Francisco is to the fault. As the boundary between the Pacific and North American Plates, its total displacement is about 200 miles. See this previous post for more photos of the San Andreas fault.
San Andreas fault zone and San Francisco
And my favorite: Aerial view of the Green River flowing through the Split Mountain Anticline –at Dinosaur National Monument, Utah-Colorado. Another photo I’ve so longed to shoot –but didn’t have the opportunity until last year.
The Green River cuts right across the anticline rather than flowing around it. It’s either an antecedent river, which cut down across the fold as it grew –or a superposed one, having established its channel in younger, more homogeneous rock before cutting down into the harder, folded rock. You can also see how the anticline plunges westward (left) because that’s the direction of its “nose” –or the direction the fold limbs come together. The quarry, for Dinosaur National Monument, which you can visit and see dinosaur bones in the original Jurassic bedrock, is in the hills at the far lower left corner of the photo.
Split Mountain anticline and Green River, Utah-Colorado
So these are my ten favorites from 2019. Thanks for looking! There are 88 more on my website, at slightly higher resolutions and for free download. They include aerials of the Sierra Nevada and Owens Valley, the Colorado Rockies, including the San Juan Volcanic Field, incised rivers on the Colorado Plateau, and even the Book Cliffs in eastern Colorado. Just go to my geology photo website, and in the search function type “aerial, 2019” –and 98 photos will pop up. Boom!
Smith Rock, the Crooked River, and modern Cascade volcanoes from Misery Ridge.
The view from outside the small visitor center at Smith Rock State Park offers a landscape of contrasts. The parking lot, and nearby camping and picnic areas, are flat, underlain by the edge of a basaltic lava flow that drops off in a series of steps to a narrow canyon, some 120 feet (37 m) below. The Crooked River, which rises about 100 miles (162 km) away in the High Lava Plains, fills much of the canyon bottom. Across the canyon, tan cliffs and spires of tuff, another volcanic rock, soar overhead. Smith Rock itself forms a peninsula of this rock, enclosed by a hairpin bend of the Crooked River. The tuff erupted 29.5 million years ago in the largest volcanic eruption to occur entirely within Oregon. Read more…
We all know the importance of water—our bodies are mostly water, we need it to survive, it’s the second most important ingredient in coffee… Geologically, it facilitates almost everything we know, from erosion to magma formation to rock fracture. I’m often struck by how so many of water’s unusual properties are determined by its chemistry and molecular structure –and in a very understandable way.
waterfalls in Fjordland, South Island, New Zealand.
Water molecules are polar
Many of water’s properties stem directly from its polar nature –and its polar nature comes right from its molecular structure. Here’s how. Read more…
Where can you see some rocks? It’s winter and everything’s covered in snow –or you’re visiting family in some place where there’s virtually no bedrock exposed anywhere –or you’re simply stranded far from any good rocks in the center of a big city.
Take yourself on a field trip to a granite countertop store! You might not see very much real granite, but you will see some other types: folded gneiss, pegmatite, amphibolite, quartzite, maybe even some granite… and a lot of amazing metamorphic and igneous features and faults –and they’re all polished and none are covered by vegetation.
I needed a rock fix the other day while visiting my mother in SW Florida –so I drove to a granite countertop store. And wow— I saw all sorts of great stuff, a lot of which related to faulting and fracturing, and a lot of it could go right into a geology textbook. In Florida!
Red garnet along with quartz and feldspar in gneiss -a metamorphic rock.
Most of us love landscapes –and many of us find ourselves wondering how they came to look the way they do. In most cases, landscapes take their shape through the combined processes of weathering and erosion. While weathering and erosion constitute entire fields of study unto themselves, this primer outlines some of the basics—which pretty much underlie all the further details of how natural processes shape landscapes.
Aerial view of incised meanders of Green River, Utah.
Two definitions: weathering describes the in-place breakdown of rock material whereas erosion is the removal of that material. Basically, weathering turns solid rock into crud while erosion allows that crud to move away.
Weathering Weathering processes fall into two categories: physical and chemical. Physical weathering consists of the actual breakage of rock; any process that promotes breakage, be it enlargement of cracks, splitting, spalling, or fracturing, is a type of physical weathering. Common examples include enlargement of cracks through freezing and thawing, enlargement of cracks during root growth, and splitting or spalling of rock from thermal expansion during fires.
Spalling of volcanic rock–likely from thermal expansion during a fire.
While Iceland hosts an amazing variety of awesome landscapes, what stands out to me most are its incredible exposures of the Mid-Atlantic ridge. To the north and south, the ridge lies beneath some 2500m of water, forming a rift that separates the North American plate from the Eurasian plate. The rift spreads apart at a rate of some 2.5 cm/year, forming new oceanic lithosphere in the process. But in Iceland, you can actually walk around in it!
Please click on any of the images below to see them enlarged.
Geologic map of Iceland as compiled from references listed below.