Geology
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 of stratigraphic thickness, being thickest in the south, and overlies a 75- to 125-m thick Ordovician sequence. It was deposited in a shallow, hot and salty sea, on the edge of an equatorial continent. The water depth never exceeded 175–200 m, 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 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.
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