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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 of stratigraphic thickness, being thickest in the south, and overlies a 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 , and shallowed over time as bioherm detritus, and terrestrial sediments, filled the basin. Reef growth started in the Llandovery, when the sea was 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. ==Stratigraphy== The island is composed of the following formations, listed from youngest to oldest (i.e. from south to north). *Sundre Formation - Ludlow (upper Silurian) *Hamra Formation *Burgsvik Formation - terrestrial input; deposited during regression *Eke Formation *Hemse Formation *Klinteburg Formation *Fröjel Formation - terrestrial input; deposited during regression and topped with erosional sequence boundary. *Halla Formation *Slite Group *Lower & Upper Visby Formation *Tofta Formation *Högklint Formation - Silurian〔Eliason ''et al''. 2010, p. 17〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Geology of Gotland」の詳細全文を読む スポンサード リンク
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