London: its geological journey and heritage

Key questions

What is the geology in and around London?
How has the Ice Age affected the area of London?
What is the evidence for change found in London?

In the last lesson, we looked at the Earth’s geological history and the Ice Age in a general way – now we can apply this to understanding how the area that is now London has changed through time, and how this ‘geological journey’ has shaped the city we know today.

The London area, and Britain generally, has had an extraordinary geological journey. Going back 520 million years, the land mass that is now England and Wales was in the Southern Hemisphere, at about 60°S! The land mass that is now Scotland was also in the Southern Hemisphere, but was nearer the equator and separated from England by a large ocean.

The process of plate tectonics caused the English land mass (called ‘Avalonia’) to move slowly northwards over hundreds of millions of years. By 410 million years ago the English and Scottish landmasses had joined in a collision that through up a great mountain chain, of which the Scottish Highlands are just a small remnant.

As the British land mass continued its northward journey, the area that is now London was sometimes dry land and sometimes submerged by the sea. When dry land, it has at various times been desert, tropical forest, and arctic tundra.

The evidence for these great changes can be seen in the rocks that underlie London and that, in places, are exposed at the surface. While the area of central London was never covered by an ice sheet during the Ice Age, the landscape was strongly influenced by this ice – cold winds from the nearby ice sheet created a cold and dry tundra landscape, and meltwater from the margins of the Ice sheet delivered large quantities of sand and gravel to the London area.

Starter

The starting point to understanding London’s geological history is to be aware of how, as part of the English land mass, the area has gradually moved northwards over hundreds of millions of years. Over this timescale, England has moved through different climate zones, and this has resulted in the formation of different types of ‘sedimentary’ rocks.

The other two major factors are changes in sea level (also resulting in different kinds of sedimentary rocks) and continental collisions which have caused rock layers to be folded and faulted (which means broken along fractures). In parts of Britain far from London there are also ‘igneous’ and ‘metamorphic’ rocks near the surface that resulted from volcanoes and other tectonic forces.

Finally, and most recently, London’s geology has been influenced by the Ice Age.

Watch this video clip (1.5 minutes long) of Earth’s history in the last 600 million years. As you watch, look out for the following changes:

Ice over the south polar area 600 million years ago and 300 million years ago
The northward movement of many land areas with a collision from 430 million years ago (English and Scottish landmasses join)
The ‘super continent of Pangaea’ about 220 million years ago (England a desert)
High sea level 90 million years ago (most of England underwater)
The presence of a northern ice sheet 50,000 years ago (England connected to France)
Discussion questions:

How do you think changes in the London area’s latitude over hundreds of millions of years will have affected its climate? 
How would changes in distance from the sea have affected its climate?
How can rocks, and the fossils they contain, give us evidence of these changes?
For a review of different types of rock (igneous, metamorphic, sedimentary) you can study this Geological Society weblink.

Main Activity

The rocks near the surface in the London area are sedimentary and relatively young compared with rocks in other parts of the British Isles. Visit the British Geological Survey website, follow this weblink to view a summary image of rocks across Britain.

There are two main layers of ‘bedrock’ beneath London: these are chalk (of Cretaceous age) and London Clay (of Palaeogene age). The chalk is older and the London Clay is younger (and sits above the chalk). Both of these rocks formed underwater, but the chalk formed in a more offshore sea environment.

When the African and Eurasian plates collided to form the Alps, immense forces affected rocks as far away as England, and the layer of chalk rock that had formed across much of England became uplifted and folded leading to the familiar landscape of chalk hills that exists today. The area of London is in a down-fold, what geologists call a ‘syncline’. The chalk beneath London rises to the surface in the Chilterns to the north and the North Downs to the south of London.

These two layers of rock have played an important role in London’s growth: the porous chalk rock provides a source of groundwater which has been heavily used for industry, and some of London’s drinking water supply today comes from boreholes that bring up water from the chalk ‘aquifer’ beneath. London is also fortunate to sit on such a thick layer of clay because clay is an easy material to tunnel through. Most of London’s Tube network tunnels go through the London Clay (and the network itself has largely been shaped by where the clay is). Clay has also long been used for brick making.

On top of the bedrock lies ‘superficial deposits’. In contrast with the solid bedrock beneath, these are loose deposits of sand and gravel that have been spread across the land by ancient rivers during the Ice Age. These deposits also provide a resource to people as sources of ‘aggregate’ for uses such as road making and cement production.

Download and go through the London’s geology overview PowerPoint slides to review the points above and to learn more about London’s geological setting.

Following from this, download and complete the Describing London’s geology task sheet using the ‘Geology of Britain viewer’ on the British Geological Survey’s website.
(The BGS also provides a similar visualisation as a free iOS app called iGeology.)

Plenary

Just as different environments over time have caused different types of rocks to be formed across the British Isles, these past environments also had their own distinct types of plants and animals. Some of the plants and animals that once lived in the London area have gone extinct. Others still exist today, but in places very far away from London.

The evidence is in the form of fossils found in London’s geological layers. Download and look through the Fossil evidence of London’s past environments PowerPoint slides.

Create a short (single sided) written summary of the environmental changes that the London area has experienced since Cretaceous times (when the chalk formed) up to the present day.
In your summary, also note down some of the different types of fossils (both marine and land-based) that have been found in the London area.
Another aspect of London’s geology today is the amazing range of different types of rocks that make up London’s buildings, much of which were quarried far away and brought to London. For example, some of London’s most iconic buildings (such as St Paul’s Cathedral) were built with Portland stone – a fine, white limestone from the Isle of Portland (Dorset) which had to be shipped up the English Channel.

You can visit this link on the British Geological Survey website to learn more about the geology of London’s buildings and this link to purchase ‘Holiday Geology Guides’ that explain the geology of different areas of London. Included in this series are: Westminster, Trafalgar Square, St Paul’s, The Tower, and Greenwich.