Gotland: cliffs, sea stacks and a coral reef from the Silurian
Much of Gotland's bedrock is ancient coral reef from a tropical sea 430 million years ago. On the sea stacks of Fårö, the tilt of the island, its cliffs and what shows when the whole island becomes a relief.
Gotland is an old coral reef. Its bedrock was built up in a warm, shallow sea some 430 million years ago, which is why the island is flat on top, steep to the sea in places and lined with stone pillars along its coasts.
The island is low - its highest point is just over 80 metres above sea level - yet its shape is still clear. Here is what built it, what the famous sea stacks are, and what you see when the whole island becomes a relief.
A reef from the Silurian
During the geological period called the Silurian, what is now Gotland lay in a tropical sea. Reefs of coral and other lime-building organisms grew there, and between them lime mud settled that in time became limestone and marlstone. According to the County Administrative Board of Gotland, much of the island's bedrock is made up of such reefs.
The sea was shallow. It is thought never to have been more than a couple of hundred metres deep, and the layers laid down in it now dip gently towards the southeast. Hoburg limestone from the south of the island has been chosen as Gotland's official provincial stone, and according to the Geological Survey of Sweden it formed at the very edge of such a reef.
The rauks: the hard cores of the reefs
The sea stacks, called raukar in Swedish, are the best-known result. They are remnants of reef cores, hard, unlayered limestone that held out when waves after the ice age wore away the softer rock around them. What remains are pillars one to eight metres high, often standing in groups along the beaches.
Many of the best known are on Fårö. Digerhuvud on Fårö became a nature reserve as early as 1930 and covers 113 hectares, with sea stacks along a long stretch of coast. At Langhammars, in the photo above, they stand close together on a shingle beach.
The tilted island
Because the layers dip gently to the southeast, the island is not equally high everywhere. Along parts of the west coast the limestone drops steeply into the sea in cliffs that give the coastline its sharp edge. To the east the land slopes more gently down to the water.
The highest point is on Lojsta hed, at just over 80 metres. Sources disagree by a metre, so we do not give an exact figure. It is little compared with the mountains, but on an island where almost everything lies low it is enough for the shape to show.
The whole island on one piece
The Gotland piece is not square like the mountain pieces. It shows the whole island, with Fårö and the Fårö strait in the north, and measures 40 x 50 centimetres inside a frame of 43 x 53 centimetres. Up close you can see the land rising towards the west coast. From across the room you recognise the outline of the island at once, because the pale island stands on a matt black base.
It is a good example of how a place does not have to be high to work in relief. The shape just has to be clear. More on that in which places work as 3D topography. The same kind of landscape relief exists for the whole of Sweden.
