Geology of Gotland

Gotland is made up of a sequence of sedimentary rocks of a Silurian age, dipping to the south-east. The main Silurian succession of limestones and shales comprises thirteen units spanning 200–500 m (660–1,640 ft) of stratigraphic thickness, being thickest in the south, and overlies a 75–125 m (246–410 ft) thick Ordovician sequence.Precambrian shield rocks that underlie these sediments are found 400 to 500 meters sea level. Sedimentary rocks cropping out in Gotland were deposited in a shallow, hot and salty sea, on the edge of an equatorial continent. The water depth never exceeded 175–200 m (574–656 ft), and shallowed over time as bioherm detritus, and terrestrial sediments, filled the basin. Reef growth started in the Llandovery, when the sea was 50–100 m (160–330 ft) deep, and reefs continued to dominate the sedimentary record. Some sandstones are present in the youngest rocks towards the south of the island, which represent sand bars deposited very close to the shore line.

The lime rocks have been weathered into characteristic karstic rock formations known as rauks. Fossils, mainly of rugose corals and brachiopods, are abundant throughout the island; palæo-sea-stacks are preserved in places.

The rocks of Gotland display signals of global extinction events, which take their name from parishes on the island: the Ireviken, Mulde and Lau events.

The island is composed of the following formations, listed from youngest to oldest (i.e. from south to north).

Gotland has mostly a subdued relief composed of flat erosion surfaces. Higher areas usually correspond to those of more-less pure limestone while lower areas have commonly a geology of marl. The reason for this is that limestone is more resistant to erosion than marl. Another general relief feature is that northwestern margin of the island is higher than the southeastern parts.

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