I’m not hallucinating from the effects of hypothermia, nor am I suffering from a deadly virus. This essay does not address my vulnerability to risky stocks or ultraviolet light. I often take photographs with different exposure settings, but that’s also about photons. No trench coats either. Exposure to new ideas is my intent, to reveal the truth about rocks. It’s not a deep dark secret that our planet is an enormous rock, approximately 22,000 miles in circumference, with an outer shell comprised of many persuasions. Much of our rocky crust is hidden by oceans, forests, cities, and soil, but exposures still await the potential admirer. And it’s not just about the rocks. Secrets from the Earth’s distant past are lurking between the grains and layers waiting to be discovered.
My earliest rock discoveries were not exposures at all but seemingly random occurrences of rounded pebbles and cobbles in the black dirt of northern Illinois farm country, relics from retreating glaciers during what geologists call the Pleistocene Epoch. When I was in fifth grade in the 1950s, my father moved the family to Geneva, into a 19th century fixer-upper. My parents got the property cheap because it needed fixing, lots of it. The renovation included some landscaping: ground leveling, tree removal, several sloping rock gardens, a new patio, and an inclined flagstone walkway along the north side of the house. The limy fossiliferous flagstone fascinated me. I would admire the fossils, wondering how old they were and how they formed. That was my first exposure—sort of. I later learned the slabs were thin beds of 450 million-year-old Ordovician dolomite, most likely extracted from real exposures at a quarry along the Fox River south of town. The sedimentary layers were exposed by torrents of water from melting glaciers 10,000 years ago.
During high school, I ventured west with friends, into the northern Illinois countryside. I saw glacial features I had only read about. On one occasion, I visited the Kaneville esker, a sinuous ridge of water-borne sand fashioned in melt water tunnels under a waning continental glacier. Glacial cavities were chocked with sandy, pebbly debris. Later, the esker survived as a meandering shelf, preserving a dramatic final scene in the last act of a continental performance. The esker was so young it was just soft sediment waiting for humans to destroy it. I drove by the esker several times through the years as the ridge grew smaller. A local sand and gravel company ultimately obliterated most of it for use in construction and cement manufacture. A remnant is preserved today in a nearby forest preserve.
I discovered my first in-situ rocky exposures near Oregon, Illinois when my parents visited friends and relatives there. Bluffs along the Rock River and road cuts along Illinois Highway 64 had a buff, off-white appearance due to weathered exposures of the St Peter Sandstone, another Ordovician-aged rock. It was deposited in a shallow sea around the time my favorite flagstone was a mere limy mud, but the St Peter is a clastic rock. Quartz sand originally derived from Precambrian granite was winnowed and abraded by the continuous motion of marine currents giving it that distinct weathered whitish color. After burial, grains were cemented together by more silica. It is a remarkable rock unit, first described in the 19th century for exposures along the Minnesota (then called the St Peter) River in Wisconsin. It’s exposed throughout the central United States both east and west of the Mississippi River. The St Peter was valuable to early settlers who used it to make high quality glass products and abrasives.
My exposure collection expanded significantly during my undergraduate college years in western South Dakota. I time travelled, beyond the Ordovician by hundreds of millions of years. I viewed exposures representing two billion years of geologic time by leaving Badlands National Park and driving 75 miles west to Rapid City on I-90, and then 25 miles farther west into the heart of the Black Hills. The gray, muddy, bone-bearing beds of the Chadron and Brule Formations of the Badlands were part of the 50-million year old Tertiary Period. Those swampy lowlands hosted now extinct creatures, such as oreodonts, entelodonts, hypertragulids, and brontotheres. Exposures along the hiking trails in the park were bone-bed spectacles, femurs, clavicles, vertebrae, radii, carpals, mostly tiny fragments carried first by scavengers and then by rivers to their current resting places.
For every mile I drove west beyond the Badlands in one of my clunkers toward the oldest rocks in the Hills, I passed (on average), 17 million years of time. I dashed through the entire Mesozoic and Paleozoic Eras before a fifth grader could say Euomphaloceras septemseriatum (my favorite fossil name). By the time I reached Rapid City, I was driving through the Cretaceous Period. The fabricated dinosaurs on Skyline Drive stand as smiling sentinels for arriving tourists. Forty minutes later I reached a remarkable exposure. At the east end of Custer State Park, I walked to the crest of a nearby hill, and stared at an irregular surface below me. There, red-weathering sandstones, shales, and conglomerates of the 500 million-year-old Deadwood Formation rest on two billion-year-old metamorphic rocks, originally deposited as sands and shales, later squeezed, baked and deformed into schists and gneisses. The amount of missing time at that rock boundary astonished me. The surface represents 1.5 billion years of missing time. What happened during that immense interval? What seas advanced and retreated? What mountain ranges rose and weathered away. It’s a humbling sensation, one of deep-time wonderment. The entirety of human existence including extinct Hominids, encompasses at most a few million years. Mt Rushmore National Monument is minutes away. There, I gazed at The Faces, four presidents carved into the Harney Peak Granite, an intrusive igneous rock that crystallized deep in the crust 1.7 billion years ago. The sculpted exposure is now honoring the tenure of four American presidents in the historical context of a measly 230 years.
Rocks like the St Peter, Deadwood, and Harney Peak are called formations because each has unique characteristics that are geographically and stratigraphically defined. Like everything else, rocks change in time and space, an integral part of the rock cycle. When formations were first described in the 18th and 19th centuries, they were given type locations and descriptions. Geologists published this information so others could view the details and corroborate rock characteristics. A formation is simply a mappable rock unit, meaning that it can be traced and identified at different locations and recognized on a geologic map. Formations are often subdivided into members. Several similar formations can be included in a group. If a unique rock unit is identified over a wide region, beyond its type location in discontinuous exposures, its name is usually retained. Names are often changed. Occasionally, confusion reigns. Geologists are often characterized as lumpers or splitters based on how they define and use rock names. I’m a lumper.
One of the grandest exposures of all is the Grand Staircase, a stair-step montage of colorful cliffs and bluffs, descending through geologic time from Powell Point in southwestern Utah to the north rim of the Grand Canyon, and then farther yet, down into the canyon and inner gorge to the Colorado River. Powell Point sits atop the Aquarius plateau at 10,000 feet, home to high stands of Englemann spruce and ancient Bristlecone pine. It’s a windy, desolate place with a beautiful view. The tortuous gravel road to the top is rough in places, ending where a trail begins. A short walk ends at The Point on an old lava flow, a black rubble-strewn landscape in a state of natural disarray. Basalt forms a resistant layer capping the plateau, holding older rocks in place, the eruptive remnant of Oligocene volcanism some 25 million years ago. I’m at the top of the Staircase now and look around. I can see Tropic, Utah in the distance and the pink cliffs of the Claron Formation at Bryce Canyon National Park 10 miles southwest.
I continue my journey through older and older rocks, first through the White Cliffs of the Navajo Sandstone at Zion National Park, then the Kayenta Formation at Vermillion Cliffs National Monument, and the Chocolate Cliffs of the Kaibab Limestone at the north rim of the Grand Canyon. I look down into the immensity of the rift, at multiple formations, red, gray, and brown Paleozoic rocks, and then into the darker inner gorge and the Vishnu Schist and Zoroaster Granite at river level. They are appropriately called basement rocks, now at the very bottom step of the staircase, rocks of Proterozoic age, 1.7 billion years old.
The most incredible exposures are often those separating crustal plates, continental-sized slabs of Earth’s exterior, crashing into each other or waiting to meet their fate in the distant geologic future. Geologically active plate margins are guaranteed to offer volcanic eruptions, earthquakes, continental rifts, and majestic mountains. Exposures of mangled rock north of Cantwell, Alaska immediately come to mind. There, the Denali Fault crosses the Richardson highway creating a jumbled collisional mishmash so intense that a highway sign warns drivers not to stop along the route. The Cantwell Formation is so fractured and broken that blocks are constantly falling from steep cliffs onto the roadway. The fault trends east-west and then south, a right lateral suture separating the Pacific plate from North America. If you look north from the south side of the fault at Cantwell toward Denali, North America is moving to your right about one centimeter (on average) each year. That adds up to one meter in 100 years, 10 meters in 1,000 thousand years, and one kilometer in 100,000 years. For those who only understand miles, that’s one mile in 160,000 years! In 2002, a 7.9 magnitude temblor rocked the region reminding local inhabitants that the fault is alive and well.
Fifty years of rock hounding and banging have produced a multitude of memorable exposures and fond memories for me. The Falls of the Ohio River near Cincinnati encompasses exposures of Early Paleozoic fossil-rich limestone with beautifully-preserved brachiopods, trilobites, and bryozoans. Those exposures are only a few miles from the Creation Museum in Petersburg, Kentucky where visitors can view a replica of Noah’s Ark, dinosaurs mingling with humans, and a biblical history of the Great Flood.
The Paros Island quarries of Greece have yielded some of the finest marble. From tiny Cycladic figurines to the Parthenon, marble has been ubiquitous to Greece. Exposures there reveal the complex geologic history of the Mediterranean region and metamorphic processes that transformed the marls, but also the fascinating archaeologic history of the Greek and Roman cultures.
While a grad student at Syracuse University in the 1970s, I visited a marble quarry in the Adirondack Mountains in upstate New York. That marble fascinated me because it was coarse-grained and had flecks of graphite. Graphite is pure carbon, forged by metamorphism, an artifact of living creatures inhabiting a limy sea during the Proterozoic, more than a billion years ago, during a mountain-building episode associated with the creation of the supercontinent Rodinia.
Everyone’s Bucket List must include a spiritual pilgrimage to southwestern Minnesota to see the oldest rocks in the United States. The Morton Gneiss, exposed along the Minnesota River near the town of Morton is 3.5 billion years old and represents part of the earliest accretion of the continent. The oldest dated rock unit on the planet still sits (unfulfilled) on my bucket list, the Acasta Gneiss of the Northwest Territories, dated at nearly four billion years!
Sooner or later, fledgling geologists stumble onto a specialty, maybe paleontology, seismology, sedimentology, environmental geology, or other discipline. Geologists are drawn to their favorite exposures for many reasons. They may love looking at tiny sand grains under a microscope, deciphering the evolutionary history of one-celled organisms, digging trenches along faults, or looking for oil or gold. For me, it was all about trying to understand the geologic history of a region during an exciting time and place. I needed to study exposures, lots of them. I wandered the southwestern Montana landscape looking for outcroppings of Cretaceous rocks. Often, exposures required four wheeling on back country roads. Some exposures were very difficult to reach.
The drive to the Paint Pots was arduous. It was my third or fourth attempt to get there. I was trying to reach the exposure. I was collaborating with a fellow graduate student on the relationships between adjacent rock units in the area. It was a clear, sunny morning in the summer of 1982 after a night of continuous rain. The route from the end of the paved road to our destination was only about eight miles long, initially a gravel route, then a rough clay track, often wet, deeply rutted, and steep in places. I was especially worried about one area along the clay track where it descended from an upland bench along the south side of Lima Peaks into the Deep Creek drainage, a tributary of Sawmill Creek.
Lima Peaks is a majestic range just north of the Idaho state line in Beaverhead County, Montana. Getting across Deep Creek required a steep descent along the deeply-rutted track, bounded on the right by a precipitous rocky slope and on the left by a 75-foot drop to the valley bottom. The track was wet and ponded in places. When I stopped the Bronco at the edge of the bench and looked down into the Deep Creek drainage, I saw a half-dozen cowboys milling around their horses, a temporary break from moving cows to nearby Deep Creek Spring. Their horses were grazing on lush green grass. We waved of course, the polite thing to do. They looked surprised to see a vehicle but waved back in that shy cowboy way and stared for a moment anticipating our intent.
I scanned the surroundings and conferred with my colleagues. We studied the rutty, wet bentonitic track on both sides of Deep Creek, about 150 feet of slippery rutted trackway ahead of us. Down and then up. It was warm with a lot of residual humidity from last night’s rain. The clay was slowly drying but still very wet. The cowboys continued to stare. I couldn’t turn back now. Cattlemen don’t like cowards, and besides, everyone in southern Beaverhead County knew about the geology students working in the area.
Our only chance to save face was to guide the Bronco onto the slippery track in 4-wheel low and descend as slowly as possible to the creek, then increase speed and shift up on the rutted uphill side. Easier said than done. I yelled to the cowboys that we were coming down and nervously got into the vehicle. I eased into the track, hoping that low gear would keep us from gaining speed. The wet clay was like ice, and we started accelerating. I tried to keep my foot off the brake, hoping the track would hold us. I glimpsed the cowboys scrambling in several directions to avoid a potential tragedy. Luckily for us, the slick roadway was straight down to the creek level, and the Bronco stayed in the track. But we continued to accelerate and then plunged into the shallow rocky stream bed. I turned the steering wheel slowly to the left, shifted into higher gear, and pressed the accelerator. We left the cowboys behind as the Bronco started to climb the back side of the valley, fast at first, then slower and slower as we approached the bench with spinning wheels. I could feel the slipping, but we carried just enough momentum to move forward. The clay was dryer at this end of the drainage, saving us from the embarrassment of sliding back down. I felt relief at first, then anxiety, realizing that at the end of the day I had to repeat performance in the opposite direction.
A few years later, Shell Oil drilled a 15,000-foot exploratory well a few miles east at Junction Creek and obliged the U.S. Forest Service by building a narrow but solid gravel road with culverts and ditches all the way to the Paint Pots and beyond. Hundreds of loads of gravel were shoveled into trucks from the base of nearby Shine Hill, and a continuous road bed of quartzite and chert pebbles made life a lot simpler for geologists!
The Paint Pots was a jumble of overturned sandstones, mudstones, and shales; reds, whites, grays and pinks, turned upside down to the south against the base of Lima Peaks. It was a deeply eroded, pastel-colored bentonitic mess of an exposure. The area was severely faulted, uplifted, and folded, the result of compressive stresses and mountain building events during the Late Cretaceous. My task was to measure and describe this exposure, and to interpret how it formed and related to other exposures in the region. I visited the Paint Pots more than 30 times over the years, first to do a detailed stratigraphic analysis, and later to refresh my memory and further ponder its origins. I found river channels, lake sediments, mud flats, volcanic ash beds, and even a few fossils, including some bone fragments, clams, and snails. The color differences were due to post-depositional chemical changes caused by volcanic eruptions, and by groundwater. These rocks were formed when a seasonal wet-dry climate dominated the region. Ancient soil horizons, called paleosols, were abundant. I had to travel miles in every direction to find time-equivalent rocks. The Paint Pots was my gold standard.
For several years in the 1990s, I went back and forth on the new road, gathering additional information about my favorite exposure. I had black organic-rich shales analyzed for fossil pollen, I collected volcanic ash beds containing tiny feldspar grains for radiometric age dating, and I developed a theory on how the Paint Pots fitted into the regional geologic framework. Why were they there? What caused them? Then, I was finished. Much later, I retired and occasionally returned to fish for Brook trout in Sawmill Creek or to climb the Lima Peaks. I always stopped to admire the poetic beauty of the area. Geologists enter the field for a variety of reasons. Some work in economic geology, looking for oil or minerals, others study geologic hazards or environmental problems, and some conduct research or teach to help answer questions for future generations. My primary interest has always been poetic geology, to better understand nature, its beauty and meaning, a spiritual experience.
In the early 2000s I stumbled on a paper by Varricchio, Martin, and Katsura in the Proceedings of the Royal Society entitled First Trace and Body Fossil Evidence of a Burrowing and Denning Dinosaur. Very cool, I thought, the fossils were found in Montana. Then I read on, suddenly realizing where they found the dinos—the Paint Pots. I was flabbergasted. My favorite exposure! They excavated one adult and two juvenile partial skeletons in a sediment-filled burrow. The creature was a burrow maker, the first such discovery of its kind. The skeletal makeup supported digging habits. I contacted David Varricchio and visited him in the field. He graciously showed me dinosaur tracks and some bone fragments. Later, I got a call from one of his new doctoral students, LJ Krumenacker, who continued David’s work on burrowing dinosaurs in a larger region of Montana and Idaho. LJ asked me to be on his dissertation committee. I enthusiastically agreed. LJ discovered burrowers of the genus Oryctodromeus in the Wayan Formation of southeastern Idaho, in beds that are time-equivalent to The Paint Pots. LJ and I spent several days together on multiple occasions looking at the Wayan and discussing how it was similar and different from The Paint Pots. LJ later graduated and became Dr Krumenacker.
I’m currently collaborating with Giulio Penasci, another student of David’s. Giulio has discovered dinosaur tracks, bones, and egg shells in younger exposures stratigraphically above the Paint Pots. He’s also attempting to more precisely define the environment in which these ancient creatures lived. Years ago, I included these beds in an existing rock unit, the Frontier Formation, an often muddy bentonitic floodplain deposit formed along the western margin of an interior seaway that sliced North America in half 90 million years ago. These are the same slippery beds that nearly embarrassed me in front of several cowboys 35 years ago.
THADDEUS DYMAN 