More than 400 000 years ago, a vast glacial lake in the North Sea basin spilled over its banks and triggered a catastrophic flood, unleashing water at roughly the rate at which Lake Erie surges over Niagara Falls today. The water quickly overwhelmed a ridge of Cretaceous period sedimentary rocks, mostly white chalk, between what is today southeastern England and northern France. For the first time, the island of Great Britain separated from the European continent.
In April a team of geologists presented exciting new evidence from the floor of the Dover Strait that paints a clearer picture than ever of the great floods that isolated Britain from the rest of Europe. Much of the coverage of the Nature Communications study plugged the flooding episodes as the “original Brexit.” Unfortunately, the initial flurry of articles largely ignored the profound implications of those floods and Britain’s on-and-off isolation since then. Indeed, the new study offers fascinating insights on the historical flow of water in western Europe, the importance of chalk, and the ability of the biota, including human, to colonize the British islands.
The formation and evolution of the Dover Strait, the narrowest region of the English Channel, has captivated geoscientists for almost two centuries. In Reliquiæ Diluvianæ (1823), William Buckland, England’s first academic geologist, proposed that in the recent geological past the rivers of the southern North Sea region must have passed through land that is now the strait. The idea was developed further by Thomas Belt in a letter in Nature in 1874, and it was discussed in correspondence between Alfred Wallace and Charles Darwin.
But early geologists had trouble devising a credible mechanism for the breach of the Weald–Artois ridge, an upward flexure associated with the uplift of the Weald of southeast England and the Paris basin in France. The landform, which continues to rise to this day, separated the North Sea from the English Channel for millions of years. In 1985 Alec Smith proposed that the ridge could have been breached by a catastrophic flood, although he failed to offer a plausible explanation of the mechanism that would have initiated the flood event.
In 1988 I demonstrated that the flood arose from a massive glacial lake in the southern North Sea basin. That lake would have formed in the Middle Pleistocene, when glacial ice advanced from northern Britain and Scandinavia, blocking European rivers’ exit to the North Atlantic. It was initially considered a single event about 430 000 years ago, but later investigations confirmed that a second major lake formed during a younger glaciation more than 250 000 years later.
Despite confirmation of the broad mechanism for the catastrophic formation of the breach (see Physics Today, September 2007, page 24), details of the nature of the erosional breakthrough of the Weald–Artois ridge remained elusive. Today the region of the Dover Strait where the ridge was breached experiences substantial erosion, making evidence difficult to recover, and it sits below some of the busiest sea-lanes in the world.
In the new Nature article, Sanjeev Gupta, Jenny Collier, and colleagues offer startling new insights that they gathered by collecting and interpreting high-resolution sonar-derived bathymetry and seismic reflection data from the strait floor. The study shows that opening of the strait occurred at least twice, triggering major erosional episodes. The authors also demonstrate an assemblage of isolated, sediment-infilled depressions that are deeply incised into bedrock. Those substantial features appear to be massive potholes or plunge pools, thought to have been formed by cascading, highly turbulent water. They support a model of initial erosion of the Dover Strait by lake overspill, plunge-pool erosion by waterfalls, and subsequent dam breaching. The valley on the floor of the strait today, the Lobourg Channel, originated from the second major flooding event.
Combining the new insights with previous research, we can now work out a clear picture of why and how the breach of the Weald–Artois ridge occurred. One important contributing factor is the ridge’s continual uplift, which over time has resulted in a series of cracks oriented east–west that weaken the rocks and make them susceptible to erosion. In addition, the ridge is composed primarily of a very pure form of calcium carbonate: chalk, which famously forms the White Cliffs of Dover (and the cliffs on the French side as well). Chalk is highly susceptible to fragmentation under a freezing and thawing process that occurs annually in periglacial regions. Those processes would have combined to “prepare” the rolling chalk landscape for rapid destruction once the southern North Sea lake water arrived.
Once the water invaded, at least two processes hastened the opening of the strait. Erosion by turbulent water created pothole-like depressions similar in form, if not scale, to those at the foot of Niagara Falls. The second process, one that’s often overlooked, is carbonate dissolution. Calcium carbonate is soluble in cold water. In cold climates, and glaciated areas in particular, dissolution of carbonate bedrock is a major erosional factor.
In eastern England there are substantial steep-sided valleys that owe their existence to subglacial meltwater discharge under high hydrostatic pressure. Those valleys, which reach depths exceeding 100 m, are particularly prominent in areas with chalk bedrock. It’s very possible that high-velocity water, combined with dissolution of rock, figured prominently in the formation of the Dover–Calais seaway passage once the water had spilled over the ridge.
What is now the Dover Strait formed during a cold glacial interval, but it was the rising sea levels of the intervening temperate periods that caused the sea to invade the gap and isolate Britain. Cliff erosion widened the Dover Strait before another glacial period arrived, again exposing the shelf areas as land. As sea level fell, the rivers of the coastal areas extended their courses across the newly exposed shelf. To reach the sea, those major rivers—the Rhine, Thames, Meuse, and Scheldt—had to pass through the strait to join the Channel River, which flowed along the floor of what’s now the English Channel toward Cap Finistère (in present-day France) and the Atlantic Ocean.
Since the first flood that formed the Dover Strait, isolation of Britain from the rest of Europe has happened periodically during eras of high, interglacial sea level, like today. The separation inhibited the free exchange across the divide of plants and animals—including humans, who were absent from the British Isles during the Last Interglacial period 125 000 years ago. By contrast, during the intervening cold periods, when sea level was as much as 120 m lower than today, Britain was connected both by the emergent channel and by the southern North Sea. That connection allowed free passage of animals and plants, albeit under cold-climate conditions. Britain was last connected in this way until about 8000 years ago in the North Sea, and we can expect it to be reconnected during future glacial periods.
There is clearly much more to do to understand the details of the Dover Strait’s formation. Unfortunately that will require extremely expensive, highly risky drilling to recover the sediments filling the deep depressions that Gupta and colleagues identify. For now, their research goes a long way to offering us valuable new insights into the opening of the symbolic seaway.
Philip Gibbard is a Quaternary geologist at the University of Cambridge. His interests include using multidisciplinary methods to study Pleistocene/Neogene geology, sedimentation and stratigraphy, and changing paleogeography and paleoenvironments.