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.