Gotland is made up of a sequence of

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s of a Silurian age, dipping to the south-east. The main Silurian succession of

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

s 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. T ...

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 A reef knoll is a land-based landform that comprises an immense pile of calcareous material that accumulated on a previously existing ancient sea floor. At the time of its accumulation it may have had enough structure from organisms such as sponges ...

detritus, and terrestrial sediments, filled the basin. Reef growth started in the

Llandovery Llandovery (; cy, Llanymddyfri ) is a market town and community in Carmarthenshire, Wales. It lies on the River Tywi and at the junction of the A40 and A483 roads, about north-east of Carmarthen, north of Swansea and west of Brecon. Hi ...

, when the sea was deep, and reefs continued to dominate the sedimentary record. Some

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

s 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 karstic rock formations known as

rauk A rauk is a column-like landform in Sweden and Norway, often equivalent to a stack. Rauks often occur in groups called "raukfält" or "rauk fields". The limestone rauks of Gotland in the Baltic Sea are among the best known examples. Sweden Rau ...

s. 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 trap ...

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and ...

s 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, w ...

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,

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 (with ...

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 extin ...

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 ...

Sundre Formation Sundre is a town in central Alberta, Canada that is surrounded by Mountain View County. It is approximately northwest of Calgary on the Cowboy Trail in the foothills of the Canadian Rockies. Sundre takes its name from a town in Norway, the ...

* 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

* Lower Visby Formation

Quaternary geology

Gotland has mostly a subdued relief composed of flat

erosion surface In geology and geomorphology, an erosion surface is a surface of rock (geology), rock or regolith that was formed by erosion and not by construction (e.g. lava flows, sediment deposition) nor fault (geology), fault displacement. Erosional surfaces ...

s. 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.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 co ...

s. On the other side the eastern coastline is irregular and sinuous. During large

Quaternary glaciation The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial and interglacial periods during the Quaternary period that began 2.58 Ma (million years ago) and is ongoing. Although geologists describ ...

s 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 La ...

. The effects of Quaternary

glacial erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust, and then sediment transport, tra ...

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 image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...

. 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 Arc ...

age transported from far-away regions of Fennoscandia. 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 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 ...

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 karstic 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 sea ...

s. The soils of Gotland are thin with calcareous

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 par ...

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 mills, sugar refineries and

iron works An ironworks or iron works is an industrial plant where iron is smelted and where heavy iron and steel products are made. The term is both singular and plural, i.e. the singular of ''ironworks'' is ''ironworks''. Ironworks succeeded bloomer ...

. 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 Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...

Stratigraphy

Quaternary geology

Economic geology

References

Bibliography

Further reading

External links