Deep Past 2. Coming Up For Air

Let's drag our bloodstained carcass out of the apocalyptic hole left by the second life-eradicating asteroid impact into the 'Tertiary' Period. Life without giant reptiles on a planet that is very slowly becoming more recognisable, with mammals, birds and leafy trees. 

Apparently this isn't geo-politically woke terminology now and I should be giving the period the correct geo-pronoun which is (I think) the Danian Age in the Paleocene epoch in the Paleogene Period in the Cenozoic Epoch of the Phanerozoic Eon. (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!)

No quack remedy was going to cure the Planet's hangover from the Asteroid impact and the dramatic fluctuations that had afflicted it so far, will continue. The climate periodically morphing from icehouse to greenhouse. Tectonic plates and the continents atop them continued to tootle around the globe. Yo-yoing sea levels engulfed and uncovered land but were usually as much as 200m higher than today. Rinse and repeat.  From our perspective, these changes were slow, but they were constant and profound. (Perhaps until now, when the pace of climatic change has picked up a bit but aren't too worried because our collective head is safely buried in the sand). .  

New life emerged from the post-extinction ashes, including the small, hungry and rarely cuddly ancestors of modern mammals and also the forebears of the trees and plants we see today. A few of the dinosaurs survived, mainly the ones who 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).

The tables are turned? 

England had come towards the end of a 1,000 kilometre trip north and arrived at very roughly the same latitude as it is today, but longitudinally, was much closer to North America. It was still a very different shape, as you can see from the pic below.  It was a warm and watery world, the original greenhouse, with levels of carbon dioxide levels that make our current efforts to pollute the atmosphere look modest. For a lot of the time, most of what later became South East England remained at the bottom of the sea, where the thick layers of limestone and chalk were becoming even thicker. 

Britain from the Paleocopter 

Over time, 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, displacing a lot of seawater!. The layers of limestone and chalk began to crumple fold as tectonic forces rearranged and reconfiguring the continent. Some emphasise the traumatic effect of the African plate bashing into Eurasia, an event that created the Alps. But it is more likely that there were multiple causes, one of which might be the stretching as proto- America pulled away from proto-Europe.  (They are still at it! I have seen the spreading gap in Iceland. See below. And there is a triple junction in the Azores!)

Iceland :
The boundary between two tectonic plates

Locally, this is when our bit of the world began to emerge as the previously submerged layers of limestone and chalk in particular were lifted up and tilted, creating the escarpments and dip slopes of the Downs, Chilterns and in a downward fold, the Thames Basin. This is a classic example of how relatively gentle folding in bedrock can create a regional landscape! I love 'The Chalk' so much I am going to write a separate post on it later. 

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 in the process. Some of it would have ended up as the layers of clay muck in the valleys. 

The Thames Valley has also changed a lot over time, oscillating between being a river outlet and a much more expansive sea inlet.  When the sea and river level fell, it left behind the ghosts of earlier shorelines and beaches in the form of layers of gravel, mud and sand you get on any shoreline. These can be identified now and each has a name. (You really, really don't want to know them; too much detail and not useful). But you can see part of their legacy where they are being plundered for building aggregates and the exhausted pits and quarries, now water-filled, 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 North West is mudstone and limestone. The light green is chalk and the thin dark green belt is sandstones which preceded it and which is more resistant to erosion. Finally the gravel and mud, coloured brown, collecting in the Thames Basin which not only received a lot of the stuff brought down by the river system, but was also frequently submerged as an inlet of the sea. 

The Bedrocks of SE England today

Even on higher ground, you can find traces of the old sea bed 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 tropical beach was discovered 30m below the surface. Maybe Ruislip has some mystical memory of its past, as witnessed by Leslie's Thomas 1974 novel (below) and the Tropic nightclub at Wealdstone's Football ground at Grosvenor Vale. (This blog covers contemporary London after all!) .

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. It was a creation of the folding of the rocks, which created a trough which the river subsequently occupied. The pattern of rivers has changed a lot; 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 its former self which once flowed further to the 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 to be 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 shape of the hills and valleys and the surface coverings will change, the building blocks are in place. The sculpting of the hills has begun and the valleys are being cut. 

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.   

The next post in the series : Link The Mud


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