Deep Past 2. Coming Up For Air

Dragging your bloodstained carcass out of the apocalyptic hole left by the second life-eradicating asteroid impact and into the 'Tertiary' Period, you find a planet that is very slowly becoming more recognisable, mercifully shorn of giant reptiles and with mammals, birds and leafy trees. 

Apparently, this is no longer woke geological terminology. I should be giving the period the correct geo-pronoun, which is (I think) the Danian Age in the Palaeocene epoch in the Paleogene Period in the Cenozoic Epoch of the Phanerozoic Aeon. (Point being, I am merrily skipping through around 5 of these Epochs and  20 Ages in this post, and I want you to know just how much grit and gyp I am sparing you!)

There was no quick fix for the planet's hangover from the asteroid impact, and it also continued to suffer from the dramatic fluctuations that had afflicted the climate, periodically morphing from icehouse to greenhouse.  

Tectonic plates and the continents atop them continued to tootle around the globe, merging and splitting. Yo-yoing sea levels engulfed and uncovered the land. At times they were as much as 200m higher than today. Rinse and repeat. 

New life emerged from the ashes, including the small and hungry but rarely cuddly ancestors of modern mammals and the forebears of the trees and plants we see today. A few of the dinosaurs survived, mainly the ones that could fly and evolved into birds. (You knew that, didn't you? And every time that seagull eyes up your chips, you see the Velociraptor in its eyes, don't you?)

The tables are turned?

By this time, the bit of the planet's crust carrying what would become England had drifted 1,000 kilometres northwards and arrived at very roughly the same latitude as it is today, although longitudinally much closer to North America.  It was still a very different shape, as you can see from the pic below, in which the outline of modern Britain is rather indistinctly outlined in yellow.  

It was a warm, watery and very humid world, where the levels of carbon dioxide made the levels in our current atmosphere look modest.  What is now England was an archipelago, maybe not unlike the modern Caribbean, but as you can see from the graphic, most of what is now the Southeast remained at the bottom of the sea, where the thick layers of limestone and chalk bequeathed during the preceding Jurassic and Cretaceous periods became even thicker. 

Britain from the Paleocopter

As time passed, temperatures dropped, sea levels fell, and gradually those layers started to emerge as land, with palm trees, crocodiles and large, carnivorous, flightless birds. 

....and from a tree?

Ridges rising in the middle of the emerging Atlantic Ocean were pushing the Eurasian and American tectonic plates apart, rearranging and reconfiguring the continent. Africa bashed into Eurasia, creating a mountain range which would once have been as high as the Himalayas. After aeons of erosion, it became the Alps. This caused a crumpling of the distant layers of limestone and chalk in our corner of the world; the Downs are effectively the foothills of those mountains, and the nearly vertical beds of chalk on the Isle of Wight and the Downs show just how powerful the pressure was.  

Elsewhere, proto-America pulled away from proto-Europe. That continues today.  I have seen the spreading gap in Iceland. See below. 

Iceland: The boundary between two tectonic plates

Locally, this is when southeast England began to emerge. The previously submerged layers of limestone and chalk were lifted up and tilted, creating the escarpments and dip slopes of the North Wessex Downs and Chilterns. The Thames Valley follows a syncline (or downward fold) between them and the North Downs in Kent and Surrey. 

The forces of erosion then got to work. Rain is acidic enough to slowly erode soft rocks made of alkaline calcium carbonate, and the rivers did what rivers do, cutting valleys and carrying away lots of the soft rock and depositing it as layers of clay and mud in the valleys. 

The Thames Valley and estuary also changed a lot over time. At times it was an outlet for a river and at others a more expansive inlet of the sea.  Every time the sea and river levels fell, they left behind the ghosts of earlier shorelines. The gravel from those ancient river and estuary beaches is now plundered for building aggregates, leaving the exhausted pits and quarries as lakes, such as those strung along the Colne Valley between Rickmansworth and West Drayton. 

The London Basin. Warm. Inviting. With Crocs.

This pattern is broadly reflected in the lie of the land today, as you can see from the diagrammatic map below. The blue in the Northwest is mudstone and limestone, which is the oldest layer. Then came the thin dark green belt of sandstone, and finally the light green belt is the white chalk. (I love it so much; it gets its own post later!) Finally, the gravel and mud, coloured brown here, collected in the Thames Basin, which not only received a lot of the stuff brought down by the river system but also spent a lot of time as an inlet of the sea. 

The Bedrocks of SE England today

Even on higher ground, you can find traces of the old seabed and coastlines in some surprising places. More on this in the next post.  Entertainingly, while building a tunnel for HS2 in Ruislip, a stretch of what was once a tropical beach was discovered 30 m below the surface. Maybe Ruislip has some mystical memory of its past, as witnessed by Leslie Thomas' 1974 novel (below) and the 'Tropic' nightclub at Wealdstone's football ground at Grosvenor Vale. 

A few things in passing. 

First, the sea inlet or estuary referred to as the Thames Basin didn't actually contain the Thames for much of this time. The folding of the rock layers created the syncline, or trough, which the river subsequently occupied. The pattern of rivers has changed a lot; if you are interested, check out Ancient English River Systems (NB. We are somewhere around the Miocene here.)  As you can see, the Thames is just a pale shadow of a mighty ancestor which once flowed further north, joining the Rhine on the way to the North Sea. 

Secondly, what is now England was still joined to France. The first Brexit was a long time coming.

At the end of this period, there are still a lot of changes to come, but South East England is taking shape.  It has found the parking spot which it now occupies, and even though the sculpting of the hills has just begun and the valleys are still being cut, the building blocks are in place.  

The next post takes me closer, but not much closer, to the time at which the first vaguely human hitchhikers, homo but not sapiens, arrived. But first, I want to write a bit more about the stuff that is usually under our boots today, namely the mud and the chalk. 

The next post in the series: Link Mud