From tropical sea to ice age

Tropical sea to ice age

444 million years BCE 419 million years BCE SILURIAN DEVONIAN

359 million years BCE CARBONIFEROUS

299 million years BCE

Gotland's history begins long before Europe or any other of today’s continents saw the light of day. Four-hundred-thirty million years ago, during the Silurian period, there were coral reefs in a tropical ocean along the coast of an ancient continent in the southern hemisphere. There, organisms with calcareous shells lived. When they died and sank towards the bottom, they eventually formed Gotland's bedded limestone. The limestone skeleton of the coral reefs became the hard reef limestone.

“Huge amounts of sediment eventually became part of the continent's hardened bedrock. Naturally, this took many millions of years. Gotland's Silurian bedrock with reef limestone is among the world's best preserved,” says geologist Sara Eliason, who works at Gotlands Museum Fornsalen.

Colliding continents and dinosaurs

It took several hundred million years for the continents to collide, be torn apart and move to their present-day location. The reef limestone land areas moved to the current latitudes – at a rate of a few centimetres per year. In the end, the ancient Scandinavian Peninsula continent and the sediment layers became part of the Baltic area's bedrock. Gotland, as well as Öland and parts of the Baltics, belong to these ancient reefs and seabeds.

Lots of periods, such as the Devonian, Carboniferous, Permian, Triassic, Jurassic and Cretaceous, passed. Different plant and animal species came and went. When the dinosaurs roamed the Earth, large parts of southern Sweden were coastal areas. There were both gigantic marine animals and large land dinosaurs. In Skåne, fossils of these animals have been found, while Gotland has no sediment layers preserved from this time.

Two million years ago, the continents were much like they are today. Lots of mammals evolved and the vegetation was reminiscent of today's flora. And time went on.

When the last Ice Age ended about 14,000 years ago, Gotland was first covered by water. The ice withdrew and Gotland slowly rose above the water's surface. Eventually people found their way here, but that's a different story, a Stone Age story.

How rauks are formed

The bedrock of Gotland is largely made up of hard reef limestone. Between the fossil reefs lie compressed, bedded limestone and muddy marl. The sea slowly eroded away the softer parts of the bedrock, but the harder reef limestone fared better. Limestone stacks – rauks – remained. There are rauks elsewhere in the world, but Gotland is home to some of the most magnificent.

Fossils – historical remains

Fossils are the remains of dead animal or plant parts preserved in sediment and certain types of rock.

“Gotland is one of the best places in the world for those looking for fossils. In both the reef limestone and the bedded limestone you will find many different kinds of animals that lived on and around the reefs, such as corals, shells, octopuses, bryozoans, trilobites, brachiopods and sea lilies," Sara Eliason explains.

Travelled from north to south and in time

All the layers of limestone that constitute Gotland were formed during different times. It can be said that Gotland's limestone bedrock consists of layers stretched “diagonally” over Gotland.

“If you go from north to south on Gotland, you can see how nature and the surroundings change significantly. This is because there are different layers of limestone bedrock. Put simplistically, you could say that you are traversing millions of years when you move over the different layers of bedrock. Each layer also has its own unique composition of fossils, which are remnants from a certain period. So there are some fossils that you only find in a certain part of Gotland," says Sara Eliason.

Footnote: For this article, we have received expert help from Sara Eliason, geologist at Gotlands Museum Fornsalen. Please visit the museum for thrilling tours of Gotland’s history. Facts about Gotland stone

• Limestone is a common building material for Gotland houses. When the stone is sawn or sanded, it turns white and its fossils are no longer visible. • If you find greystone on Gotland, it has been dragged here by the ice sheet. • The reddish limestone from Hoburgen is called "Hoburg marble". It is full of fossils and is the provincial stone of Gotland. • Sandstone, which consists of compressed sand, is only found on southernmost Gotland.

201 million years BCE 145 million years BCE 66 million years BCE 2,6 million years BCE