Deep Past 4 . Chilling Out
In previous posts I covered the rock foundations of our hills and valleys, so now I will torture the analogy and look at the flooring. This 4th post in the series starts from around 2.5 million years ago which, in geological terms, is a distance from the present day no greater than the gaps in a country bus timetable. Effectively this is the ice ages and their legacy.
It has always been the case that the constant and radical changes to the landscape and the climate over millions of years, made it hard to for a simple punter like me. to sift out simple cause and effect relationships. Now, as get closer to the present, the increasing amount of fine grained detail available makes it even harder to simplify the story. So if any professional paleogeologists are reading this, for the sake of your mental health, stop now.
At the point when when we left the last post, what is now South East England was still connected to Europe. The map below will give you some idea.
|Southern England, 2m years ago.
At first, the climate was pleasant but as the Gods fiddled with the thermostat, there were dramatic cycles of warming and cooling. At times, massive ice sheets covered much of Northern Europe, including Britain, and the land connections to Europe expanded. It would have been like Greenland today, where the ice cap can reach 3000m thick. That is three times thicker than Snowden is high!
At others, between the frozen spells, the climate was positively Mediterranean. The Atlantic would have warmed up and powered sea and air currents which kept temperatures way above what you might expect at our latitude. At those times, as the ice melted, sea levels rose; and low lying land around the Thames Estuary disappeared underwater. We enjoy those effects today. I gather that these cycles were down to the drunken wobble of the Earth in its orbit around the Sun and that scientists are not sure why the effect appears to have less pronounced.
If you took a stroll around in the warmer periods, avoiding the horse-sized sabre-tooth tigers, the animal life would be increasingly recognisable. Species of horses, deer, lions, bears, hyenas and wolves are still with us, albeit often without the armament of their larger predecessors. Others are now extinct but do not appear altogether alien, e.g. Mammoths, Woolly Rhinos and Aurochs. And of course when the glaciers arrived the thin scattering of humankind and most of the wildlife forsook the skiing opportunities and headed for warmer climes.
The glaciers came in waves with warm interstitial periods. In one of these, around a million years ago, early iterations of man homo appeared in England followed in later breaks by neanderthals and our own more direct ancestors. During the glaciations, they all sensibly emigrated elsewhere.
I will cover this in future posts. I have covered the human story separately. Link: Meet the (Pre)eople If the idea of playing with a sabre-tooth pussy appeals to you, you might like this BBC video excursion for Kids. That's my level! See The Sabre Tooth Tiger. If, on the other hand, Aurochs are your thing, See Grow Your Own Auroch
The glaciation that ventured furthest into South East England did its worst around 450,000 years ago. This was no place for casual tobogganing. In places, it must have looked a bit like the Ice Wall in Game of Thrones with some glaciers a thousand metres thick glaciers. The land sank under the weight.
|The Ice Wall of 'The North'.
To the west of London, it never quite breached the Berkshire Downs or the Chilterns. To the north it reached Dunstable, which is why there is no scarp there, and tongues of the ice sheet extended across lower ground, past St Albans to Finchley Road Tube Station and to the Hornchurch Cutting on the Romford – Upminster Branch Line before stopping at the sign warning against trespassing on the tracks. To the east, most of Suffolk and some of Essex would also have been covered.
|The Hornchurch Cutting. Where the Glacier's Gave Up. .
Later glacial excursions never got quite so far. The last one was around 15,000 years ago and while it covered much of North West Britain with thick ice sheets, it didn't get nearly as far south. Notwithstanding, it would have turned the rest of England into a tundra like Northern Canada or Siberia, with a frozen, barren landscape interlaced with vast rivers of meltwater leaving layers of detritus.
|The Limits of the Ice Sheets in the Anglian and Devensian Glaciations
One odd result is that, after it was all over, relieved of the weight of ice, Scotland started to rise up while down here our bit of the Earth's crust still slowly sank. It still is and, together with rising sea levels, this spells bad news for the coastal Thames Basin over the next century or so. It spells goodbye to Thanet and chunks of the Estuary coastline.
So what are the results from this that are visible today? That is, after all, what this blog is interested in. Higher ground first. The greatest legacy was the shape of the land itself. For instance, the glaciers scoured the exposed land surfaces and the soft rocks of the great chalk dome of Southern England.
They also directly changed the pattern of rivers and indirectly lowered the sea levels. For instance, the ancestral Thames probably counted the Severn as a tributary and flowed north of its present course and through the Vale of St Albans before joining the Rhine on its way to the North Sea above the still-existing land connection to Europe. Later, the glaciers diverted it through what is now the Goring Gap and into the valley it now occupies. If you are interested there is a nice and concise Wikipedia article on it. (Link) TheAncestral Thames
Then there is the debris. There are two types of Glacial deposit, both of which can be found easily enough. The stuff dumped directly from a glacier as it melts, mainly rocks, is 'till'. This can include the large 'Sarsen Stones' which have always been used for buildings and boundaries and which were also sometimes used by Neolithic people to create the Stone Circles like Stonehenge.
Then there are the layers of gravel and sand from the beds of the outwash streams of meltwater. Together, these are known as 'drift', varieties of which form a surface layer across many parts of our area beyond the chalk escarpments. This can be easy to spot if you are going through a cutting along a footpath or road.
|Drift / Boulder Clay over Chalk
In particular in the lowlands to the North of the Chilterns and Downs, this drift will often take the form of ‘boulder clay’. This, as the name suggests, is clay with more rocks and pebbles embedded in it. Some of them will have been carried by the glaciers from as far away as Scandinavia before being dumped as the retreated. In places, the layer is quite thick in comparison with the thinner chalky soils on the Downs to the West, which is one reason (there are others) why the former have a lot more woodlands while the latter are more open and offered as 'gallops' for horses.
The pic below was taken at College Lake Nature Reserve near Tring, which is worth a visit. The 'base' level here is the chalk surface that survived the glaciers intact. The next layer up is a relic of the chalk that was broken up and spread over the ground in slow moving rivers of slurry. The third layer is the pebbles and sandy silt carried by the meltwater rivers in later summers. Finally, the relatively recently created topsoil is easily identified.
|College Lake Cutting 2023
If you are interested in seeing this stuff with someone who can explain it all, you could try and visit the Sand Pit in Buckingham. See: Bucks Geology
Another memento of the frozen tundra is many of the 'dry' or river-less valleys that are common in the chalk hills. Water will normally sink through exposed chalk, but when the ground is frozen it can flow on the surface and cut valleys in the way that rivers normally do. Later, when the ground warms up, those streams sink through the porous chalk and disappear. Good examples are common enough and can be readily identified from an OS Map. They include Lilley Bottom, which is featured on one of the cycle routes here.
Here are the notes. Giro de Lilley Bottom
|Ivinghoe / Chilterns. A Dry Valley in Chalk
In the higher Chilterns in particular, you will also see fields of clay soil littered with flints. These were once embedded in the old chalk layers which were eroded or dissolved away, leaving the harder flints behind. They are easy to spot on the higher ground, even when crops are growing. You will see a lot of the hard, weatherproof flint used in local buildings. There is more on the chalk generally and flint in particular in this post. Link: The Chalk Hills
|Chalk & Flint in a ploughed field / Chilterns
Meanwhile, when the Glaciers retreated and the sea levels rose, Doggerland would be drowned once again and the Thames Basin would be inundated. Salty seawater might sometimes have reached Newbury, all the while depositing more layers of clay, mud and gravel. When the river was cutting its valley, as rivers do, it once again left behind traces of its former river beds, banks and beaches, in much the same way as long lost seas begat the ghost beaches found beneath Ruislip and referred to in the previous post.
In the last post, I referred to the relics of former shorelines in the Thames Basin, left behind as the river cut deeper and sea levels fell. They form terraces up the side of the valley. The highest of these are the oldest. So, for instance, you can identify terraces rising from Maidenhead on the present Thames, right up to Nettlebed, some 160m higher up the adjacent hills. This is illustrated in the pic below, I appreciate that the labels are difficult to read on a phone in particular, but you will get the idea.
|Thames Basin Terraces. Maidenhead to Nettlebed
There are lots of these terraces. When you stand on Trafalgar Square, you are on one and, looking up, you can see the National Gallery, sitting on a higher and therefore older one. Some isolated spots survived the erosion and now form the remaining higher ground, giving us Highgate, Parliament and Richmond Hills etc etc.
|The National Gallery on Trafalgar Square
Anyway, if your priority to understand why things look the way they do, it doesn’t really matter. It is enough to understand that the glaciers came and went many times. Even though they never quite got past the scarp of the chalk hills, they covered the Eastern parts of Herts and bequeathed the rough shape of the landscape while turning most of the area into a barren tundra riddled by outwash streams and lakes. A lot of the surface soil today is derived from the muddy mess or 'loess' that they left in their trail.
The Thames Basin ended up being a prime recipient of that muddy mess, the bulk of which we now call 'London Clay'. The layer of this blue-grey gunk can be a 40m+ thick and it is a sod in so many ways. For the farmer it makes ploughing difficult. For the city dweller it can damage building foundations because it swells when wet and shrinks again as it dries out. This is why your palatial London residence probably needs subsidence insurance.
The next post in the series Link : Meet the P(re)eople