Deep Past 5 . 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 floor coverings. 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. 

The debates about the constant and radical changes to the landscape and the climate over millions of years have always made it hard for a non-specialist 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. So if any professional paleogeologists are reading this, stop now, for the sake of your mental health.  

At the point 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 with dramatic cycles of warming and cooling as the Earth wobbled drunkenly in its orbit around the Sun. 

At times, massive ice sheets covered much of Northern Europe, including Britain, and the land connections to Europe expanded as the glaciers were built with the water!  It would have been like Greenland today, where the ice cap can reach 3000m thick. In comparison, that is ten times Walbury Hill, the highest point in South East England. 

If you find it difficult to believe that the planet behaves like a docker on a Friday night, check this: Milankovitch Cycles .

Between the frozen spells, the climate was positively Mediterranean. The Atlantic warmed up and powered sea and air currents which kept temperatures way above what you might expect at our latitude.  During those times, as the ice melted, sea levels rose and lower-lying land around the Thames Estuary disappeared underwater. 

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. 

Woolly Rhino

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 glaciers came in waves with long, warm periods in between. 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 swapped their skis for surfboards and emigrated south for the season. 

The first five posts in this series cover the making of the landscape before humans turned up. From Post 6 (Welcome Homo) I introduce man as an influence on what you can now see. 

The glaciation that ventured furthest into South East England was the 'Anglian' which 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 literally sank under the weight. 

The Ice Wall of 'The North'.

To the west of London, the Anglian glaciers 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. 

Later glacial excursions never got quite so far. The last one, the Devensian, 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 which cover swathes of our countryside today.   The Limits of 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 our bit of the Earth's crust started sinking. It still is and, together with rising sea levels, this spells bad news for the coastal Thames Basin over the next few centuries. It's probably goodbye to Thanet, the marshes in the Thames estuary marshes and chunks of the Suffolk and Essex coastlines. 

So what results from all this 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, especially North of the Chilterns and Downs, cutting valleys and leaving the outcrops. The pic below comes from Swanscombe Heritage Park and shows what the view from the Thames Estuary might have looked in the Anglian glaciation.

The Thames in the Anglian glaciation

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 drained North Wales. Their combined waters flowed north of its present course, creating the Vale of St Albans. It then merged with the Rhine, entering the North Sea above the still-existing land connection to Europe. Later, when the glaciers reached the Aylesbury Vale and blocked its path, it was diverted through what is now the Goring Gap and into the valley it now wriggles about in. If you are interested there is a nice and concise Wikipedia article on it. 

(Link) The Ancestral Thames 

Then there is the debris. There are two types of Glacial deposits, both of which can be found easily enough. The rocks and stones dumped directly from a glacier as it melts 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, sand and pebbles 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. The glaciers carried some of them from as far away as Scandinavia before  dumping them as they 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 Chilterns have more woodlands while the latter are more open.

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. This was covered in my post on 'The Chalk'. 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 flint used in local buildings. They are weatherproof and hard, but the latter makes them difficult to work with. 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

When the Glaciers retreated and the sea levels rose, Doggerland was drowned once again and the Thames Basin was inundated. Salty seawater might have reached Newbury, all the while depositing more layers of clay, mud and gravel. When the river was cutting its valley, it once again left high and dry, 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. Now, 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 as outcrops 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 is 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 Suffolk and bequeathed the rough shape of the landscape while turning most of the rest 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 compressed into what we now call 'London Clay'. The layer of this blue-grey gunk can be  40m+  thick. 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. 

Ouch!

On the plus side, it proved easier to tunnel through than rock or finer soil, This facilitated the creation of the Tube network. South East London has fewer tube lines partly because it was swampier in some places and sandier in others.

So far I have tried to describe the bones of our landscape, with rocky continents forming and disintegrating in volcanic hellscapes, oceans rising to cover most of the planet and then shrinking away and. most recently, massive ice sheets cloaking the land land before beating a retreat. Sometimes boiling hot and sometimes cold, sometimes dry and sometimes wet. A battered planet with no one there to bear witness to it. 

The fact is that I haven't made much reference to mankind here simply because, so far, they have made no impact on the landscape. In my next two posts I aim to make up for that. The earlies humans left few mementoes beyond some bones, lots of flint tools in the Museums and the odd bit of graffiti. But after the glaciers retreated Man becomes an very active creator of the scenery. 

The next post in the series : Welcome Homo