A Ghost Rock Speaks

Photo by Greg Willis.

“Geologists have been somewhat chary about acknowledging that this soft, white, calcareous material is in reality chalk.” (Calvin 1895)

In 1868, biologist Thomas Henry Huxley presented a lecture to the working men of Norwich titled On a Piece of Chalk—the chalk that every carpenter carried in his breeches-pocket (1). But everyday utility was mentioned only in passing; Huxley’s objective was much more ambitious. “I have been unable to think of any topic which would so well enable me to lead you to see how solid is the foundation upon which some of the most startling conclusions of physical science [geology] rest” (all quotes by Huxley unless noted). He then proceeded to recount the story told by chalk.

Graham Young recently wrote of rocks as ghosts. Unlike human ghosts—disembodied souls, fleeting suggestive figures—a rock ghost has a physical presence. Yet it too is disembodied—an altered piece of a former landscape, or as Young noted: “a concrete relic of the lost place and time in which it was formed.” Such ghostly remains provide a glimpse into the past, but it’s often dim. Sometimes though, we’re lucky enough to meet rock ghosts that speak in great detail. Chalk has been especially generous in this regard.

Chalk is a form of limestone, and as such, is composed of calcium carbonate (CaCO₃). The source and structure of the calcium carbonate are what make chalk distinctive—remarkably white and soft enough to write with (2), “almost too soft to be called rock.” In his lecture, Huxley explained that a microscope reveals chalk to be a mass of tiny discs called coccoliths, which were also discovered in abundance on the seafloor during the 1857 survey of the route for the first transatlantic telegraph cable. This probably surprised most in the audience, but there was no arguing. Chalk clearly had formed in the sea!

Coccolithophore covered in coccoliths. Note scale bar—how many coccoliths must fall to the seafloor to produce 1000 vertical feet of chalk?! (source).

Coccoliths are the calcareous scales of coccolithophores—marine phytoplankton (3). Being plants, they photosynthesize, making food from sunlight, water and carbon dioxide. They also extract calcium from seawater, and combine it with carbon dioxide to manufacture calcareous scales. When they reproduce or die, their coccoliths separate, and slowly drift down and settle on the seafloor. Insignificant debris you might think, but coccolithophores are so incredibly abundant that their scales form thick layers of limy muck. With the pressure of overlying sediments, the muck becomes rock. But because the seemingly delicate coccoliths are tough and hard to compress, the rock is soft—chalk.

Huxley moved next to an even more surprising but unavoidable conclusion revealed by chalk—the nearly-incomprehensible amount of time involved in its formation. “… it must have taken some time for the skeletons of animalcules of a hundredth of an inch in diameter to heap up such a mass as that.” He explained his rough calculation of the minimum time required to build the thousand feet of chalk underlying Norwich, based on rate of coccolith accumulation—12,000 years!

At that time, the idea that one small part of nature’s work required 12,000 years was mind-boggling to most people. Now we see it as a gross underestimate. Probably coccoliths accumulated for something like ten million years to build England’s chalk.

White cliffs of Dover, England, seen from the deck of the ferry to France; Makiko Itoh.

Huxley continued, with still more startling news: the earth has changed radically many times. Just look—the disembodied soul of a seafloor that we know as chalk now stands above water and sometimes far from any sea. Therefore it must have been “upheaved & converted to dry land,” forcing us to conclude that “the earth, from the time of the chalk to the present day, has been the theatre of a series of changes as vast in their amount, as they were slow in their progress.”

But here Huxley hit a dead end. It was clear from the rock record that the earth’s surface was continually subjected to elevation and depression. But when asked “Why these movements?” … the rocks were silent.

“I am not certain that any one can give you a satisfactory answer to that question. Assuredly I cannot. All that can be said, for certain, is, that such movements are part of the ordinary course of nature, inasmuch as they are going on at the present time.”

Huxley with sketch of gorilla skull, c. 1870 (source).

Huxley had to end chalk’s story there, but fortunately our accumulated knowledge has grown significantly in the 150 years that have passed. The theory of plate tectonics now takes care of Huxley’s unanswered question. The earth’s crust consists of giant plates that move, stretch, split, collide, dive under, and override—producing uplift and downwarping in the process. It was during a time of major crustal change, when sea levels were at a maximum, that the oceans invaded the continents and created large shallow epicontinental seas where chalk formed.

Epicontinental seas were widespread in late Cretaceous time—as is chalk today (source).

Huxley's lecture was limited to European chalk. He made no mention of chalk in North America, but that was hardly surprising. At that time even preeminent North American geologists were unwilling to acknowledge that the soft white rock reported from the heart of the continent was indeed chalk.

In 1865, DC Collier—resident of Central City, Colorado, editor of the Daily Miner’s Register, and a man with an interest in geology—was traveling to Atchison, Kansas by Butterfield stage. About 250 miles east of Denver, in western Kansas, they were forced to stop on multiple occasions due to threat of Indian attack. This allowed Collier time to explore the white bluffs nearby.

“With my revolver cocked in my hand … I was able often to go half a mile from coach or camp among the bluffs. On one occasion, in company with a companion I was able to climb to the top of a bluff of pure chalk, so soft that I could cut and carve it with the knife I carried in my belt, and so fine that it covered my clothes as thoroughly as when in my college days a classmate wiped the blackboard with my back.”

Collier collected a mosasaur jaw about four feet long and some vertebrae, just a few of the many fossils he saw in the chalk. Unfortunately his research came to an end when a military escort arrived, allowing them to to continue east. Collier later gave his fossils to Oberlin College, and, at the urging of Professor James D. Dana of Yale, submitted an article to the American Journal of Science about the Kansas chalk. It was published in 1866 (4).

Yet Dana dismissed reports of North American chalk for decades, as did Joseph LeConte of the University of California, also a preeminent geologist of the time. It wasn’t until the third editions of their highly-regarded geology manuals that they revised their descriptions: “in North America [there is] no chalk, excepting in western Kansas, where, 350 miles west of Kansas City, a large bed exists” (Dana, 1880); and “recently good chalk composed of foraminiferal shells, and containing flints, has been found in Texas” (LeConte, 1890).

Fast-forward 150 years:

I was introduced to chalk (the rock) by Huxley, whose intriguing lecture I found in collection of natural history essays. I was especially excited when I later learned that the central part of North America was once covered by an epicontinental sea where coccolithophores thrived, and coccoliths rained down on the seafloor to form chalk. Some of the best exposures are not all that far from where I live, so last month I made a pilgrimage to western Kansas to visit the white bluffs and monuments in the valley of the Smoky Hill River (see To Kansas to See the Chalk).

Late Cretaceous Western Interior Seaway; X marks Kansas (USGS).

Extent of Smoky Hill Chalk, western Kansas (5).

I arrived in the dark, having driven 450 miles with multiple unplanned-but-worthwhile stops. So it wasn’t until the next day that I saw my first chalk. It wasn’t entirely what I expected.

Descriptions of Kansas chalk, especially for public consumption, often reference the famous chalk of Europe (usually the White Cliffs of Dover), as it’s similar in age and composition. But up close, there’s a clear difference. European chalk occurs as thick deposits with few obvious bedding planes, suggesting extremely stable conditions for millions of years. The only visible layers are occasional thin bands of flint. In contrast, Kansas chalk is distinctly layered. Apparently the environment wasn't so stable here.

After my trip, I found a 1982 paper by geologist Donald Hattin about the Smoky Hill chalk. The man must have devoted a huge amount of time and effort to this project, for the resulting description, based on 25 localities, included 600 feet of chalk, much of it laminated (layered). For many localities, Hattin described on the order of 60 to 80 layers! He identified more than 100 bentonite seams—thin beds of altered volcanic ash—and described 23 recognizable marker beds. Much of the layering was subtle, varying mainly in the concentration of fecal pellets—coccolithopore poop.

I loved the photogenic layered chalk!

Lacking Hattin's skill, I contented myself with admiring accumulations of broken shells. I imagined them sloshing around on the mostly flat seafloor, sometimes piling up behind small features.

Broken shells, common in the chalk.

Thin layers of something ... limestone? ... with scattered shells.

Thus ends my story of chalk. But I want to add Huxley’s final words, because they're more fitting than anything I could write about this ghost rock. It “has become luminous, and its clear rays, penetrating the abyss of the remote past, have brought within our ken some stages of the evolution of the earth. And in the shifting ‘without haste, but without rest’ of the land and sea, as in the endless variation of the forms assumed by living beings, we have observed nothing but the natural product of the forces originally possessed by the substance of the universe.”

Notes

(1) Huxley was a strong proponent of scientific education for adults, perhaps because he was largely self-educated.

(2) Most “chalk” now used for writing, marking boards, decorating sidewalks, etc. is made of gypsum (calcium sulfate).

(3) At the time of Huxley’s lecture, it was unknown whether coccoliths were produced by organisms or through chemical precipitation. Huxley himself wasn’t sure, finding “the nature of [coccoliths] extremely puzzling and problematical.” However, by the time the lecture was published in Macmillan’s Magazine, later in 1868, he was convinced they were products of “independent organisms” as he explained in an endnote.

(4) Calvin (1895) described earlier reports of chalk-like rock from the central part of North America, such as “prairie chalk” in 1841, and “chalky limestone” from the surveys of Meek and Hayden in the 1850s.

(5)  The Smoky Hill Chalk is sometimes called the Niobrara Chalk or simply the Chalk.

Sources

Calvin, S. 1895. Composition and origin of the Iowa Chalk. Reports of the Iowa Geological Survey 3: 213-236. Available online.

Collier, DC. 1866. Notes on chalk and Cretaceous deposits in eastern Colorado. American Journal of Science, 2nd series. 41: 401-403. [The chalk Collier described was in Kansas.]

Diffendahl Jr., RF. 2017. Great Plains geology. University of Nebraska Press.

Hattin, DE. 1982. Stratigraphy and depositional environment of Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of the type area, western Kansas. Kansas Geological Survey Bulletin 225.

Huxley, T. H. 1868. On a piece of chalk (lecture given to the workingmen of Norwich). Published later in 1868 in Macmillan's Magazine, and as a book by Scribner in 1967. Available online.

To Kansas to See the Chalk

Where's Toto?!

I recently vacationed in Kansas, to the surprise of everyone I’ve told (they think Kansas is flat and covered with corn). It was a wonderful trip. I visited the late Cretaceous chalk, famous for abundant marine fossils and weirdly-beautiful sculpted rock made of microscopic shells. In my imagination, I was able to experience Kansas as it was 85 million years ago—at the bottom of a sea.

Kansas chalk formed during a time of peak sea level. The oceans didn’t just flood the coasts; they also invaded the continents creating shallow seas (vs. deep ones in ocean basins). One of these epicontinental seas—the Western Interior Seaway—stretched from the Arctic Ocean to the Gulf of Mexico across a broad swath of central North America, including Kansas.

At its peak, the Western Interior Seaway was 2000 miles long and 600 miles at its widest. Added X marks western Kansas (USGS).

The climate was quite warm, and there was no ice to speak of—no polar ice caps or sheets, no alpine glaciers. But melted ice alone doesn’t explain the very high sea levels, estimated to be 100 to 170 m (300 to 500 ft) higher than now. If all of today’s ice were to melt, sea level would rise only about 70 m (Miller 2009).

It’s likely that the ongoing breakup of the supercontinent Pangaea also contributed to rising sea level, but how is debated. Perhaps seawater was displaced by the underwater mountain chains that developed along rifts, or perhaps the higher seafloor spreading rates of the late Cretaceous produced hotter expanding crust, which displaced seawater (Miller 2009).

By Late Cretaceous, today's continents were recognizable but hadn’t yet moved to current positions (source).

The Western Interior Seaway was not especially deep, but it was big. Between 87 and 82 million years ago, when it was at or near its peak, the seafloor that would become western Kansas was located far from land, out of reach of terrestrial debris—sand, silt and clay. Instead, biological sediments accumulated, mainly tiny exquisite coccoliths—wheel-shaped pieces of planktonic armor.

Emiliania huxleyi (source), a ubiquitous modern coccolithophore. huxleyi honors TH Huxley, who in 1868 presented the now-famous lecture On a Piece of Chalk to the workingmen of Norwich.

Coccolithophores are a type of plankton. Upon death, their calcareous armor falls apart into component coccoliths. They were so abundant during the late Cretaceous that their coccoliths rained down on seafloors in massive quantities, forming thick layers of limey muck. With time and pressure, the muck became rock. However, coccoliths are not that easily compressed, and the resulting rock is soft—chalk.

The most famous late Cretaceous chalk formed in the European Epicontinental Sea. This is the chalk of the White Cliffs of Dover and the Champagne province in the Parisian Basin of France. It’s so distinctive and well known that it’s often referred to simply as “the Chalk”—and is also the eponym for its time, the Cretaceous Period (creta is Latin for chalk).

White Cliffs of Dover (source).

Given the extent of the Western Interior Seaway, one would expect chalk to crop out elsewhere in North America. Indeed it does, at scattered locations from western Canada to Texas and Alabama, but never as charmingly as in Kansas, where curious chalk pinnacles and monuments (1) rise from the prairie.

Above and below: Monument Rocks, also known as the Chalk Pyramids.

The chalk of western Kansas is the Smoky Hill Member of the Niobrara Formation, named for the valley of the Smoky Hill River where it was first described. Its constituent sediments date from 87 to 82 million years ago, when conditions in the Western Interior Seaway were just right for chalk. For five million years, coccoliths rained down. Occasionally dead creatures sank to the bottom, where they were preserved as fossils when the limey muck eventually turned to rock.

The rock record indicates that by 80 million years ago the Western Interior Seaway had shrunk enough to end chalk formation. Atop the Smoky Hill Chalk is the Pierre Shale, indicating that dry land was nearby; terrestrial detritus dominated seafloor sediments. The change probably was related to uplift to the west, the beginnings of the major episode of mountain-building that created the Rocky Mountains and tilted the Great Plains—the Laramide Orogeny. By 60 million years ago, the Western Interior Seaway was gone.

For millions of years, the chalk lay buried under shales, siltstones and sandstones—lithified detritus eroded off the mountains to the west. Then for reasons still debated, streams shifted from deposition to erosion. They became sculptors, cutting and carving sedimentary rocks to make drainages decorated with photogenic bluffs, cliffs, pinnacles, monuments and hoodoos—among the highlights of my trip!

More photos of Monument Rocks:

Chalk hoodoo and badlands, courtesy Kansas Geological Survey.

The Smoky Hill Chalk varies in composition and hardness. Sometimes a layer of “tough chalk” forms caps that protect underlying softer rock from erosion, creating ledges, pinnacles and monuments (Hattin 1982). But tough is relative. Geologically speaking, this protection doesn’t last long, and the life of a monument is almost as fleeting as ours (2).

The topmost layer is a protective cap of tough chalk.

Above and below—this cap is coming apart. How soon will the monument fall?

Fallen chalk is quickly weathered and eroded, appearing to melt into the ground.

The Kansas chalk is most famous for its fossils—oysters, clams, fish small and huge, sharks, monstrous marine reptiles, birds, dinosaurs and more. But my fossil hunting was limited to gazing at masses of microscopic coccoliths and noting occasional thin layers of shell fragments. To see the more charismatic fauna, I visited several local museums.

Famous fish-within-a-fish fossil, 14 feet long; Sternberg Museum of Natural History in Hays.

Vi Fick’s unique fossil artwork is on display at the Fick Museum in Oakley (click on image to examine trees made of fish backbones and small shells).

The week before my trip, I ran into two friends—retired active adventurous types. “Any travel plans?” I asked. “We’re going to Tibet and China, and then to Everest Base Camp” she replied. “How about you?” “I’m going to Kansas” I said. There was a pause “ … Do you have family there?” “No” I said. “I want to see the chalk, the late Cretaceous chalk that’s the same age and composition as the White Cliffs of Dover, the chalk that’s so rich in fossils that it made western Kansas famous!” There was another pause, “ … sounds … interesting” he said. And it was. In fact, the trip was so interesting that I can't possibly fit everything into one post. More to follow.

Notes

(1) It’s impossible to find a definition for “monument” as used in this context. But it’s a handy term. Many of these features don’t have the narrow pointed top characteristic of pinnacles. So like the locals, I use “monument”.

(2) Chalk monuments are short-lived even on a human timescale. Several famous ones have fallen in the last 20 years, e.g. the Sphinx and the tallest part of Castle Rock.

Sources

Buchanan, RC, and McCauley, JR. 2010. Roadside Kansas; a traveler’s guide to its geology and landmarks, 2nd ed. University Press of Kansas (for the Kansas Geological Survey).

Diffendal, Jr, RF. 2017. Great Plains geology. University of Nebraska Press.

Everhart, MJ. 2005. Oceans of Kansas: a natural history of the western interior sea. Indian University Press.

Hattin, DE. 1982. Stratigraphy and depositional environment of Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of the type area, Western Kansas. Kansas Geological Survey Bulletin 225. Available online.

Hull, PM. 2017. Emergence of modern marine ecosystems (Primer). Current Biology Magazine 27:R466-R469. doi: 10.1016/j.cub.2017.04.041.

Huxley, TH. 1868. On a piece of chalk (a lecture given to the workingmen of Norwich). Macmillan’s Magazine. Available online.

Kauffman, EG. 1977. Geological and biological overview: Western Interior Cretaceous Basin. The Mountain Geologist 14: 75-99.

Liggett, GA. 2001. Dinosaurs to dung beetles: expeditions through time. Hays, KS: Sternberg Museum of Natural History.

McPhee, J. 2010. “Season of the chalk” in Silk Parachute. Farrar, Straus & Giroux.

Miller, KG. 2009. Sea level change, last 250 million years. in Encyclopedia of Paleoclimatology and Ancient Environments (pp.879-887). Springer. PDF

Prothero, DR. 2018. “Chalk” in Story of the Earth in 25 rocks; tales of important geological puzzles and the people who solved them. Columbia University Press.

Boxelder News & Other Exciting Changes

It’s time for the monthly report about my boxelder by the warehouse, so let’s take an early morning walk to see what’s up. Heading west on the dirt road to the Laramie River, we immediately see changes since last month’s report. The old railroad ties piled up near the end of the warehouse are now gone. The Wyoming Department of Transportation has covered the new road adjacent to the dirt road with ground-up asphalt, salvaged from the old bridge across the railroad tracks (recently torn down). More relevant to the topic at hand—tree-following—cottonwoods bordering Murdoch's distribution center the north, and along the river to the west, are now yellow-orange or have lost their leaves entirely.

But around the corner ...

… we can see even from a distance that the boxelder is still green. That nook must afford a fair amount of protection.

Not totally green however.

Leaves close to the walls appear to be the richest green, while others are in various stages of turning yellow. Maybe not obvious at first, a lot leaves have fallen leaving barren petioles (leaf stems).

Two leaves (boxelder leaves are compound).

Lower right—petioles with no leaves.

Geeking-out on botanical terminology.

Nearby, the usual trailer is parked in its usual place. A close-up shows where this boxelder grows in the greater scheme of things.

Our home.

Next, let’s head west across the field toward the river … but wait!

Sirens wail and lights flash on the Snowy Range Road. A firetruck shows up. Then the sirens stop. But the flashing lights continue, for longer than we care to watch.

In the field between the warehouse and the river, we see more cleanup. More piles of old ties have been removed, along with miscellaneous discards and debris. Now there's a huge patch of bare dirt decorated with frosted bulldozer tracks. And right in the middle …

 … we spot a small shrub.

It's a ragged little lilic bush that the earth-movers, debris-loaders and garbage-haulers left standing, somehow avoiding it with their huge machines!

I smiled and walked home in a sunny mood, happy to be a tree-follower.

For more tree-following news, check out the latest virtual gathering, kindly hosted by The Squirrelbasket. And for you tree-aficionados: New members are always welcome!