Gotland – The Nordic Treasure Island
The Swedish island of Gotland in the Baltic Sea, with unique scenery and traditions, has been occupied since prehistory and has plenty to offer the visiting geologist, since the bedrock geology of the island has had a great influence of the cultural and economic history of the island community. Quarrying of limestone and sandstone has since medieval times been important for trade and commerce. Even today, the main sources of income to the island are tourism, agriculture and aggregate and concrete production from locally mined limestone. Every year around 300,000 tourists visit Gotland, with the aim of exploring the landscape and interesting geological sites.
Around 450 million years ago Gotland comprised a shallow sea lying on the edge of a continent close to the equator. In these warm waters carbonates and fine clastic sediments accumulated among the coral reefs and bioherms. Today, Gotland is made up of several units of Lower Palaeozoic rocks building up a sedimentary succession several hundred metres thick, which dips to the south-east.
Growth of coral reefs started in the Early Silurian, when the sea was between 50 and 100m deep. Comparable reefs can be seen on seismic sections southwards in the Baltic Sea and in Skagerrak. Fossils are commonly found throughout the island, and Gotland is a paradise for fossil-hunters. Walking along the beaches on the southern shores of the island you cannot fail to find nicely preserved specimens. Most typical and commonest among the Silurian marine fossils are corals, stromatoporoids, bryozoans and brachiopods, but you will also find trilobites, cephalopods and echinoderms.
The scenic "rauks"
The landscape on Gotland takes its shape from the Silurian limestone covering most of the island. In several places along the coast, the limestone rocks have been weathered into scenic karst formations know as "rauks", which is a Swedish term for a stone column or sea-stack created by natural erosion.
On Gotland the rauks are generally well-cemented reef limestone which has been eroded into unusual shapes, often resembling figures and strange constructions. In geological terms the rauks are quite young features, as they started to form after the ice receded from this area after the last ice age, some 12,000 years ago. These rauks are found in great numbers around the coast of Gotland, and many of them are packed with fossils.
The rauks have given rise to several local myths, and some of the rocks have names like ”The Old Patron”, ”The Virgin” and ”The Man of Hoburg”. The myths of the rauks have often been created to explain religious disputes or to understand historical events and people.
The weathering of the carbonate rocks of Gotland has created numerous caves and sink-holes where the surface-water escapes into the ground. The Lummelunda underground cave system (Lummellundagrottan), which was discovered by three schoolboys in 1948-55, is one of Gotland’s most visited attractions. In the cave there are large natural cave rooms with artworks made from fossils and stalactites. Special guided tours are available for further exploration through alleys and lakes. At Lummelunda there also is a long, attractive stony beach and excellent walking areas by the sea and along Lummelundsån River.
The lonely oil pump
On our way to the north of the island there is an unusual tourist attraction near Risugns; a lonely oil-pump bearing the name Gotlandsolja AB. Exploration for hydrocarbons on Gotland began in the mid-1930’s, when the company Skånska Cementgjuteriet drilled for gas in Cambrian sandstones at File Haidar in northern Gotland. No gas was found in the target reservoir, but traces of oil were discovered in the upper part of the Ordovician limestone. In the 1960’s, the Geological Survey of Sweden carried out hydrocarbon prospecting on the island, resulting in discoveries of traces of gas and oil in Cambrian sandstones, and observations of oil impregnations in Ordovician limestones and Silurian sandstones.
In 1969, OPAB (a subsidiary of Swedish oil company Svenska) took over the prospecting, and in 1974 oil was discovered in Upper Ordovician carbonate mounds. Between 1972 and 1987 the company drilled 241 wells and acquired more than 2,500 line-km of seismic data. When the price of oil fell in 1986, OPAB left the island, and Gotlandsolja AB took over until activities ceased in 1992. The total hydrocarbon production during the 20-year period from 1972 to 1992 was around 100,000 m³ (630 Mb) of oil. It is said that the oil was of such a quality that it could be used directly for firing in the local power plant at Visby. Today, the only visible sign of the Gotland oil-age is the lonely pump at Risugns.
Tjelvar's ship of stone
Gotland is not rich in black gold, but the island certainly exhibits other important and very interesting remains. Archaeological and historical sites are very common in Gotland. Among these are the more than 600 small, historical limestone quarries and a couple of hundred burners. Several of the burners are now restored and preserved, and at some of them you can get a demonstration of how limestone was burned for lime production in the past.
Among other historical remains, of particular interest are "shipsettings" of raised limestone or erratic blocks, which have been arranged in the form of a ship. The shipsettings on Gotland are from two different periods. The youngest are from the Viking Age or the later part of the Iron Age, while older ones are from the late Bronze Age. On Gotland there are about 350 such shipsettings preserved, more than anywhere else in the north. The longest shipsetting on the island of Gotland lies at Gnisvärd near Tofta and is more than 45m long.
Near Boge on the east side of Gotland you can find an interesting shipsetting called "The Grave of Tjelvar". According to Gotland folklore, the first person living on the island, about 6,000 years ago, was called Tjelvar. The setting of stones in the shape of a ship is supposed to be his grave. This shipsetting is 18m long and 5m wide.
A historical treasure island
The old city of Visby is surrounded by an impressive fortress wall, dating from the time of the Hanseatic League, and today, medieval Visby is a UNESCO World Heritage Program site. In addition to the famous city wall, Visby has winding cobblestone streets and alleys and a large number of ancient stone and wooden houses, churches and ruins. Gotland is famous for its around 100 medieval churches, exhibiting Romanesque and Gothic architecture. Most of the churches are restored and still in active use. During the summer season, Visby is a typical tourist town bustling with energy, while the autumn, winter and spring seasons offer peace, quiet and a unique poetic atmosphere. In Visby and elsewhere on Gotland you find abundant galleries and stores selling local arts and handicrafts.
Gotland is an archaeologist's paradise. Reminders of Viking and Medieval times are found almost everywhere on the island. Among the more famous remains are hoards of coins of gold, silver and copper, and more than 700 such coin-treasures have been found. A few years ago a farmer, Björn Engström, found the world's largest haul of Viking treasure on the north-eastern part of the island. The loot included coins, necklaces and other jewelry, which altogether contained 65kg of silver and 20 kg of bronze. He was given 2.1 million Swedish kronor (more than US $300,000) as a reward. Without doubt, there are several treasures left to be found, but unfortunately, illegal treasure-hunters with metal-detectors secretly exploring the fields at night have become some of a problem, as they destroy unique and valuable historical sites.
For geotourists seeking scenic landscapes and a relaxed atmosphere, mixed with a strong taste of history, Gotland is the place to visit. For fossil-collectors the island is a true treasure chest. If you visit the island in May you will escape most of the heavy tourist traffic, and in addition have the chance to enjoy the beauty of the millions of wild orchids that thrive and bloom in the carbonate-rich soil.