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How the Channel was formed

Was England once joined to France?
Fault lines along the Channel
The last Ice Age
South of the ice sheets
The climate warms up
Sea level rises
Changes in the coastline
Sinking land
Building the Channel Tunnel

Channel ferry

If you dropped St.Paul's Cathedral in the deepest part of the straits between Dover and Calais, the dome would stick out of the water!


Was England once joined to France?

English Channel - geology either side
MAP: Geology in Kent and Nord / Pas de Calais (simplified)

In the 18th century, the first amateur geologists began mapping the rocks either side of the Channel .

The chalk cliffs and other similarities suggested that - perhaps - England might once have been joined to France.

But what made the 22 mile (30km) gap between Dover and Calais?
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Fault-lines along the Channel
Geological faults along the line of today's Channel are several million years old. They mark where part of the Earth's crust has dropped. The faults had to be mapped in detail before building the Channel Tunnel (see below).

Dover Strait - rift walley and faults
Fault lines cause occasional earthquakes

Geologists call this a "rift valley".

This happened several million years ago, but the crust is still moving along these lines of weakness.

There was a major earthquake in 1580, and again in 1995 (4.9 on the Richter scale).

This lower land is more likely to be eroded by rivers and the sea - and that's what happened in the last Ice Age...
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Changing sea-levels and the last Ice Age
25,000 years ago, the climate of Northern Europe started to grow colder.

Winter snows in the far north did not melt in the summer, so huge ice sheets started to pile up.

25,000 years ago - very low sea level in Ice Age
MAP 1: 25,000 years: When sea-level were much lower , the rivers flowed across grassy plains where the sea used to be.

Ice sheets blocked the North Sea
By 18,000 years ago, it was the height of the Ice Age.

Ice sheets 1,500 metres thick covered northern Britain and much of the Continent.

Sea levels fell considerably because so much water was trapped in the growing ice sheets - Map 1 shows that rivers of Nord - Pas de Calais flowed either towards the Rhine or the Seine.
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18,000 years ago - land bridge
MAP 2: 18,000 years ago, a "land bridge" emerged as sea level fell. Then ice sheets blocked the North Sea, creating a giant pond which overflowed, gouging deeper valleys through the "land bridge"

Land bridge
To the south, a "land bridge" between England and France emerged as the sea level fell.

There was no-one around to walk across - in the middle of the last major Ice Age, early man had retreated to the south of France.

Climate south of the ice sheets
Kent and Nord-Pas de Calais had a very cold, tundra climate. In the brief summer, meltwater from surface snow flooded rivers just as in the Russian Arctic today.

These seasonal rivers cut deep valleys into the frozen sub-soil of the chalk hills. Back to top

Ice sheets block the North Sea
When the ice sheets blocked right across the North Sea, water from the river Rhine ponded up in a lake between the land bridge and the ice. Eventually the water poured out through the lowest gap in the hills (see map 2) - gouging out deeper valleys which after the Ice Age became the Straits of Dover.
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10,000 years ago - dry land bridge between England and France
MAP 3: 10,000 years ago, the climate had warmed up dramatically. Rivers again flow to the North Sea - sea levels slowly rise towards today's level.

The climate warms up
10,000 years ago the ice sheets were retreating and melting. Rivers again flowed out to the North Sea. Sea levels were rising, but still about 50 metres below today's levels.

Early man and animals returned to northern France, and crossed to England over the "land bridge" - see map 3. New forests and grasslands covered the low land, and plentiful game attracted Stone Age hunting groups.
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8,500 years ago - flooding the Straits of Dover
MAP 4: 8,500 years ago - sea level rises, flooding through the gaps in the hills, joining the North Sea and the Atlantic.

Sea level rises
As the world warmed up again, the ice continued to melt and sea level rose. 8,500 years ago, the rising sea flooded up the river valleys through the hills joining England to France.

Eventually it broke through where deeper valleys had been gouged out in the Ice Age -see map 4. Swift currents flowing between the Atlantic and the North Sea soon eroded the islands, leaving the stumps as sandbanks in the channel.
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Changes in the coastline
Since then, the cliffs on either side of the narrow straits are still being worn away. Each year's winter storms leave the channel a few metres wider. But the sea has deposited all the eroded rock elsewhere along the coast.

The present-day coast is marked with yellow dots on Map 4. It shows areas like the marshes between Calais and Dunkerque, and Romney Marsh on the Kent side, where sand, shingle and mud have accumulated to make new land.

Cap Blanc Nez  Cap Gris Nez   Northern France - sand dune coast
1. Cap Blanc-Nez and sandy beach at Wissant - the gleaming white chalk shows the cliffs are being eroded
2. Cap Gris-Nez the other side of Wissant - sandstone and clay-chalk cliffs
3. Lowland coasts are lined with sand-dunes.

Sinking land
In the Ice Age, land weighed down under 1500m of ice sank by over 100m. Land round the edges (like north France and southern England) rose in compensation - like a see-saw.

When the ice melted, the fairly rigid land mass took time to return to its original levels - so much so, that Kent and Nord-PasdeCalais are still slowly sinking back at rate of a few cm every 100 years. This causes long-run worries for sea-flooding. Much of the low-lying coast on either side is protected by seawalls, which can be breached if there's a storm at high tide. Where the land behind is former marshland, it may be 0,5m below mean sealevel.

Building the Channel Tunnel
1980's engineers used new technology developed for deep-sea oil exploration to check exactly what was under the sea bed - finding evidence that confirmed how the Straits of Dover were formed after the last Ice Age.

English Channel - geology cross-section Channel Tunnel route - geological features
1. Cross-section of the geology as known today - showing the present tunnel, which follows the Lower Chalk most of the way across, except for the under-land tunnel on the French side. The fault-lines run along the Channel, wherea block of the earth's crust has dropped several million years ago (see diagram above).
2. They found hazardous "buried valleys" on the sea bed, filled with mud and sand rather than solid rock. If the tunnellers had hit one of these, their tunnel would have collapsed and flooded.

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Weblink: Find out about the latest research -"How Britain was almost joined to France", at


Places to visit:
Transmanche Museum - Cap Blanc-Nez
'Entre 2 Caps' - district overlooking the channel straits
Boulonnais Regional nature park -

2 caps map
Click map for info on places to visit

Related background information
Medieval Flanders - reclaiming low areas from the sea
Channel ferries - history of crossing the channel.
Chennel Tunnel - why it took so long to build
First crossings - pioneers crossing the Straits of Dover
Early man - in Nord Pas de Calais




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