The chalk cliffs of Dover

Lots of coccolithophores

Photolithographic print from Views of the British Isles, Detroit Publishing Company, c. 1895

“The sea is calm tonight. The tide is full, and the moon lies fair upon the straits…” from “Dover Beach,” by Matthew Arnold, published in 1867.

The White Cliffs of Dover, in southeastern England, are certainly among the most famous exposures of Cretaceous rocks in the world. Close to vertical in many places, and more than 100 meters or 350 feet high, the cliffs are chalk, the sedimentary product of accumulations of microscopic coccoliths, the calcium carbonate plates of coccolithophores. Coccolithophores are floating algae that must have overwhelmed the seas of Europe at times during the Cretaceous.

Coccolithophores. From Monteiro and others, 2016, Why marine phytoplankton calcify: Science Advances vol. 2, no. 7,  licensed under the Creative Commons Attribution-Share Alike 4.0 International license.

The chalk extends in the subsurface and near surface across much of southeastern Britain, and equivalents are found on the continent too. If you were curious about the white material excavated from the trenches in the movie 1917, that was this chalk, an attention to detail that I appreciated in that film. During late Cretaceous time, high-standing sea levels made much of Europe a shallow shelf where the algae apparently thrived. The land areas around the shallow sea were low and small, so there was not much input of clastics – sand, silt, and mud – to the depositional basin. That’s one reason the chalk is so pure.

There are black streaks in the White Cliffs of Dover, formed by strings of flint nodules. Flint is the same as chert, cryptocrystalline quartz, usually black. The source of the silica in flint and chert is debatable and may be from multiple origins, but it’s likely that within the calcareous ooze on the sea floor, made up of all those coccoliths, there were zones where siliceous ooze was produced by, for example, accumulations of sponge spicules or perhaps by algae or other microorganisms such as radiolarians that secreted siliceous shells, just as the coccolithophores secreted calcite plates. An alternative, less likely explanation is that the silica came in later, during the lithification process that turned the sediment into rock, or even later, after the rock had solidified. Either way, the black flint nodules scattered through the white chalk make for a distinctive rock.

In my photomicrograph at right of a flint nodule from the Cliffs of Dover you see support for the biological origin of the flint. The flint includes tiny elongate “sticks” – probably siliceous sponge spicules, the supporting elements that keep soft sponges from collapsing. Spicules had dozens of shapes and geometries, but they are typically broken apart in sediments. A triaxial form is quite common, looking just like a Mercedes logo. Most of the fragments visible in my photo are linear rods, and it would probably take a higher magnification in thin section to find other shapes, but I’m moderately confident that these things are indeed sponge spicules (disclaimer, I’m a mineralogist Jim, not a paleontologist! Happy to be corrected here).

Besides the microscopic algae that make up most of the rock, other fossils are fairly common in the chalk of southeastern Britain, including especially echinoids or sea urchins. Shark’s teeth, brachiopods, bivalves, and crinoid stems are also present.

In the North Sea, the chalk is an important oil and gas reservoir in places, but it also served as a tight seal over older reservoirs, keeping the hydrocarbons in place. As friable as the chalk is, unless it is fractured or has undergone dissolution, the tiny coccoliths that make it up can be very tightly interlocking, reducing the permeability greatly. Permeability is the interconnectedness of the porosity or open space in a rock.

“By forming an external, calcitic (CaCO_­3) skeleton, coccolithophores play a major role in fixing CO₂; they have been the most important long-term CO₂ sink for the last 100 million years.” (Püttmann & Mutterlose, 2021, Paleoecology of Late Cretaceous Coccolithophores: Insights from the shallow-marine record: Paleoceanography and Paleoclimatology, 36, e2020PA004161)

The Cretaceous Period takes its name from these rocks: “creta” is Latin for chalk. The chalk deposits formed around 70 to 90 million years ago. The cliffs themselves are vastly younger, more like the past 500,000 years or less, created mostly by regional uplift and glacial waters flowing through the Strait of Dover (or the Pas de Calais, if you prefer). Multiple catastrophic glacial outburst floods carrying water from the North Sea to the southwest likely contributed to the development of the strait and the cliffs. The timing of the two most significant of those floods is estimated at around 425,000 and 225,000 years ago.

“Coccolithophore” is from Greek words meaning “seed” (for the microscopic seed-like shells), “rock,” and “bearing.”

In this watercolor published in Lyell’s Principles of Geology in 1837, the chalk surrounding the arch of the Weald in southeastern England is gray-white.

If you are interested in more posts with historical aspects, please see the compilation here: Posts on the History of Geology.