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Guernsey becomes an island

The Channel Islands were not always islands.   Guernsey's major rocks were born deep beneath volcanoes then crushed, twisted and cracked deep in the earth. The earliest rocks visible today are up to 2,600,000,000 years old and some of the oldest form the cliffs of the South Coast.

Around 400 to 500 million years ago, Guernsey was at the foot of a mountain range in almost desert-like conditions. Sandstone rocks in the north of Alderney are made from material that washed down from these mountains. Rocks of this age and from the age of the dinosaurs have been completely eroded away in Guernsey, so their fossils are not found here.  When dinosaurs walked the earth 300 to 65 million years ago we were close to the coast of the Atlantic Ocean which was only just forming at that time.  Some 5 to 10 million years ago Guernsey was on the sea bed, which explains why the top of the island around the airport and the top of Sark is so flat. Since then, this part of the Earth's crust has risen upwards, pushing the old sea bed 90 metres above the current sea level. Guernsey became an island, and both sea and weather sculpted the land into the shapes we see today. Click here to watch an animation about Guernsey becoming an island.

During the past two to three million years the world has been gripped by ice ages. A fall in temperature across the globe resulted in great sheets of ice spreading out from the Arctic. At times, ice covered most of Britain and was up to 1.5 kilometres thick.  This is fifteen times the height of Guernsey's cliffs. As the ice sheets grew, sea levels fell, leaving the Channel Islands as no more than hills in an arctic plain. Guernsey would have looked more like Finland than the place we know today. From time to time, the world warmed, the ice retreated and the seas came back again. Guernsey was at times as warm as Spain is now and the tooth of an extinct elephant has been found in St Peter Port.

It should be remembered that the ice ages have not ended. We now live in an 'interglacial', a warm period between ice ages. If sea level continues to rise, it will make the islands smaller. One day in the far future it may fall once more meaning we will once again be connected to France.

Places to visit to see evidence of sea level change:

*  Castel Church is on top of an ancient cliff and the bottom of Rectory Hill was a beach 180,000 years ago when the sea was 30m higher than today.

* At Rocquaine you can still see the beach and cliffs from 150,000 years ago when the sea was 18m higher than today.

*  Beside the causeway to Lihou Island you can see an ancient beach from 100,000 years ago when the sea was 8m higher than today.

* The steep valleys of the islands were carved by rushing meltwater during thaws in the icy winters. These even extend under the sea!

 

Please get in touch with our Access & Learning Manager for more information:

01481 747264

museums@gov.gg