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 The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. The ...

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. Fossils from Gotland beaches

The lime rocks have been weathered into characteristic

karst Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant ...

ic rock formations known as rauks. Fossils, mainly of

rugose Rugose means "wrinkled". It may refer to: * Rugosa, an extinct order of coral, whose rugose shape earned it the name * Rugose, adjectival form of rugae Species with "rugose" in their names * '' Idiosoma nigrum'', more commonly, a black rugose tr ...

corals and

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, wh ...

s, 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 Irevik (also known as Ireviken or Ihreviken ) in Hangvar ''socken'' north of Visby, is a more than section of the Swedish island Gotland's coastline. History There are several settlements from Stone and Bronze Ages. There are also remains ...

Mulde The Mulde () is a river in Saxony and Saxony-Anhalt, Germany. It is a left tributary of the Elbe and is long. The river is formed by the confluence, near Colditz, of the Zwickauer Mulde (running through Zwickau) and the Freiberger Mulde (wit ...

and

Lau event The Lau event was the last of three relatively minor mass extinctions (the Ireviken, Mulde, and Lau events) during the Silurian period. It had a major effect on the conodont fauna, but barely scathed the graptolites, though they suffered an extinc ...

Stratigraphy

The earlier Paleozoic of the island comprises the following formations, listed from youngest to oldest (i.e. from south to north).

Ludlow Ludlow () is a market town in Shropshire, England. The town is significant in the history of the Welsh Marches and in relation to Wales. It is located south of Shrewsbury and north of Hereford, on the A49 road which bypasses the town. The t ...

* Sundre Formation * Hamra Formation * Burgsvik Formation – terrestrial input; deposited during regression * Eke Formation * Hemse Formation Wenlock * Klinteberg Formation * Halla-Mulde Formation * Fröjel Formation – terrestrial input; deposited during regression and topped with erosional sequence boundary. * Slite Group * Hangvar Formation * Tofta Formation * Högklint Formation * Upper Visby Formation Llandovery * Lower Visby Formation

Quaternary geology

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 Marl is an earthy material rich in carbonate minerals, clays, and silt. When hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae. Marl makes up the lower part o ...

. The reason for this is that limestone is more resistant to erosion than marl.Eliason ''et al''. 2010, p. 11 Another general relief feature is that northwestern margin of the island is higher than the southeastern parts. This higher northwestern coast is in part result of the southeast

tilt Tilt may refer to: Music * Tilt (American band), a punk rock group, formed in 1992 * Tilt (British band), an electronic music group, formed in 1993 * Tilt (Polish band), a rock band, formed in 1979 Albums * ''Tilt'' (Cozy Powell album), 1981 ...

of the sedimentary strata. The northwest coast is straight and contain active

cliff In geography and geology, a cliff is an area of rock which has a general angle defined by the vertical, or nearly vertical. Cliffs are formed by the processes of weathering and erosion, with the effect of gravity. Cliffs are common on coa ...

s. On the other side the eastern coastline is irregular and sinuous. During large Quaternary glaciations Gotland was covered by an

ice sheet In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at L ...

. The effects of Quaternary glacial erosion are not as visible in Gotland as they are in mainland Sweden because of its sedimentary basement. Glaciation did however left thin blankets of fine-grained boulder-clay till. The boulder include rocks of

Archean The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth during the Archea ...

age transported from far-away regions of

Fennoscandia __NOTOC__ Fennoscandia ( Finnish, Swedish and no, Fennoskandia, nocat=1; russian: Фенноскандия, Fennoskandiya) or the Fennoscandian Peninsula is the geographical peninsula in Europe, which includes the Scandinavian and Kola penin ...

. Where the till blanket is absent the bare rock surfaces are exposed. At the end of the last glaciation Gotland was fully summerged in the waters of the proto-Baltic Sea. The

Ancylus ''Ancylus'' is a genus of very small, freshwater, air-breathing limpets. They are aquatic pulmonate gastropod mollusks in the tribe Ancylini within the family Planorbidae, the ram's horn snails and their allies.Bouchet, P.; Rosenberg, G. (2014 ...

and Litorina transgressions left marks in form of beaches. These beaches are now located above current sea level due to

post-glacial rebound Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the removal of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound a ...

and changes in sea level. Compared to the rest of Sweden Gotland host good examples of active coastal processes. There are few traces of river and stream erosion in Gotland. Some drainage in the island occurs through

ic systems, including

cave A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can refer to smaller openings such as se ...

s. The soils of Gotland are thin with calcareous till

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay parti ...

being the main parent material.

Economic geology

The particular geology of Gotland has conditioned many aspects of human life and economic activity. The economic activities influenced by geology include forestry, farming, cement production and quarrying building stones.Eliason ''et al''. 2010, p. 5 Visby ringmur östra delen norrut

During the industrial age demand for Gotland limestone came from

pulp mill A pulp mill is a manufacturing facility that converts wood chips or other plant fiber sources into a thick fiber board which can be shipped to a paper mill for further processing. Pulp can be manufactured using mechanical, semi-chemical, or fu ...

sugar refineries A sugar refinery is a refinery which processes raw sugar from cane or beets into white refined sugar. Many cane sugar mills produce raw sugar, which is sugar that still contains molasses, giving it more colour (and impurities) than the white ...

and iron works. People working in the quarries of Gotland have diminished from about 600 in the mid-1930s to 350 by 2010.Eliason ''et al''. 2010, p. 41 The intended opening of a new limestone quarry in the 2010s in northern Gotland has led to a substantial conflict between environmentalists and those who support the project. In April 2015, the legal process regarding the allowance of the project was put on hold by Swedish courts until the Swedish government takes a decision or, if not, until August 31 of 2015. Exploratory wells have revealed the existence of petroleum oil of Lower Paleozoic age beneath Gotland.

References

Bibliography

External links

Gotland Sandstone
Gotland Geology of Sweden