Transcript London`s geology overview

London’s geology
overview
• This set of slides will help you to visualise
London’s geological setting and to learn more
about the rock types and how they have
influenced London.
• There are also slides to help you understand
how the London area has been affected by the
Ice Age.
Overview of the London area
(formed from 100 to 66 million years ago)
Chalk
http://commons.wikimedia.org/wiki/File:Geological_map_of_London_
Basin.jpg?uselang=en-gb
London Clay
(formed from 56 to 34 million years ago)
The chalk was originally laid down
horizontally, but was folded during
the formation of the Alps.
The down-fold makes the ‘syncline’
of the London Basin.
The thick line represents a cross-section through
the rocks, depicted in the simplified drawing here:
North Downs
Chilterns
London
London Clay
older rocks
chalk
Overview of the London area
•
This is a more detailed cross-section to compare with the simple drawing on the
previous slide.
London
Chalk
British Geological Survey
Chalk
• Chalk is a fine grained limestone
formed from the calcite shells of
microscopic phytoplankton called
Coccolithophores which pile up
on the sea floor.
• It forms in warm, clear tropical
seas.
• Chalk also contains flint nodules.
• A famous exposure of chalk is the
White Cliffs of Dover.
• It is a porous, sedimentary rock
which can hold water.
• This type of rock is found at 70
metres depth under the Palace of
Westminster.
White Cliffs of Dover
http://commons.wikimedia.org/wiki/File:White_Cliffs_of_Dover_01.JP
G?uselang=en-gb (Immanuel Giel)
coccoliths seen with a
scanning electron microscope
http://commons.wikimedia.org/wiki/File:Coccolith_hg.jpg?uselang=en
-gb (Hannes Grobe)
London Clay
•
•
•
•
•
•
Clay bedrock is a sedimentary
deposit made up mostly of clay
particles.
Clay particles are very small, platelike particles (much smaller than silt
or sand) that are slowly deposited in
calm water.
Clay particles come from chemical
weathering of other rocks and are
transported to the sea by rivers.
Deposits of clay are impermeable
(water does not pass easily through
them).
Various types of clays have long
been used for making ceramics and
bricks.
London Clay is about 45 metres thick
under Westminster, and overlies the
chalk. In places it’s more than 150
metres thick.
A small
exposure
of London
Clay
http://commons.wikimedia.org/wiki/File:Detail_of_cliffs_at_the_Naze
_-_geograph.org.uk_-_1187872.jpg?uselang=en-gb (Simon Huguet)
Clay particles
Seen with a
Scanning
Electron microscope
http://commons.wikimedia.org/wiki/File:Clay_magnified.jpg?uselang=en-gb (USGS)
More about London Clay
• The grey-blue coloured London Clay formed underwater when
sea level was higher than now, and the area was near a warm,
subtropical coastline.
• London Clay is an easy material to tunnel through.
• This helped London to build its underground rail network
quickly in the late 19th century.
• Another advantage of London Clay is that, clay being
impermeable, groundwater does not penetrate into the
tunnels.
• The London Clay is nearer the ground surface north of the
River Thames, and this is why the Tube is much more
extensive on the north side of the river compared with the
south side.
Notice how most of the London Underground in Greater London is on the
north side of the Thames. This is because of the distribution of London Clay.
http://en.wikipedia.org/wiki/London_Underground#mediaviewer/File:
London_Underground_with_Greater_London_map.svg
Geological layers of London
• If you could dig a hole straight down into the
ground starting at St Paul’s Cathedral in London,
this is what you find going downwards:
–
–
–
–
–
A layer of ‘made ground’ which is just an accumulation of older
structures and rubble from earlier times in the City’s history
Layers of sands and gravels (can be several metres thick) dating to
the Ice Age, that were deposited by earlier river channels (geologists
refer to these as ‘superficial deposits’ because they overlie older,
more compacted bedrock)
The London Clay over 30 metres thick under St Paul’s
Layers of sands and clays that are older than the London Clay
The Cretaceous Chalk at about 70 metres depth which extends down
for almost 200 metres (For comparison, Nelson’s Column is 52
metres, and the dome of St Paul’s Cathedral is 111 metres high)
• Some higher areas of Greater London (such as
Greenwich to the south-east and Hampstead to the
north) consist of sandstones that formed after the
London Clay.
http://commons.wikimedia.org/wiki/Category:St._P
aul%27s_Cathedral#mediaviewer/File:St_Paul%27s
_Cathedral_-_geograph.org.uk_-_2952893.jpg
(David Dixon)
Geological layers of the London
• Although the Cretaceous Chalk and the London Clay are the
thick rock layers immediately under London, these aren’t the
only rock layers.
• For instance, older rocks underlie the chalk, and other rock
layers are found between the chalk and the London Clay, and
also between the London Clay and the surface.
• You can review the Geological Timescale and see how it
relates to London’s geology by visiting London Geodiversity
Partnership website
http://www.londongeopartnership.org.uk/londongeology.html
• On the Geological Timescale shown at the above link, the
chalk formed during the Cretaceous Period, and the London
Clay formed during the Eocene Period.
The influence of the Ice Age on London’s geology
• During the Quaternary
Period (the Ice Age), there
were several times when
glacier ice expanded and
then retreated in the British
Isles.
• The most recent British ice
sheet, shown here, reached
its maximum extent around
20,000 years ago.
• The London area was a cold,
treeless tundra landscape at
http://www.bgs.ac.uk/discoveringGeology/geologyOfBritain
this time.
/iceAge/home.html?src=topNav (British Geological Survey)
The influence of the Ice Age on London’s geology
http://commons.wikimedia.org/wiki/File:Svalbard_tundra.jpg (Billy
Lindblom)
This is a photograph from Svalbard in the Arctic Circle, but the London area and
England south of the British Ice sheet would have looked similar to this.
The influence of the Ice Age on London
• An ice sheet reached its
furthest southerly extent
in Britain about 430,000
years ago.
• A rough boundary of this
earlier ice sheet is shown
by the thick black line.
• Notice that its edge came
very close to central
London.
• The ice sheet edge
reached the Finchley area
of North London and
northernmost parts of the
Borough of Havering in
eastern London.
Extent
of ice about
20,000
years ago
Extent
of ice about
430,000
years ago
London
The influence of the Ice Age on London
• When the ice sheet was near London, a huge amount of meltwater from
the edge of the ice caused powerful streams (and the ancient River
Thames) to transport and deposit large amounts of sand and gravel.
• These make the ‘superficial’ layers just beneath the surface in London.
• As the River Thames alternately deposited and then cut down through
this material, gravel terraces formed. (These are like ‘steps’ of gravel at
slightly different elevations in the Thames Valley.) Many parts of London
are built on these various terraces.
• For an idea of how sand and gravel is moved and deposited from glacial
meltwater, go to Swiss Education website photo glossary:
http://www.swisseduc.ch/glaciers/glossary/index-en.html and select
‘outwash plain’ and ‘braided stream’.