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Geologická expozice university v Kodani

16.12.2007  |  126× přečteno      vytisknout článek

Článek je v angličtině

 

 

The Prequaternary Geology of Denmark

The exhibition treating the geological development in Denmark prior to the Quaternary Ice ages deals with a time interval of about 1,500 million years from the formation of the basement in the roots of Precambrian mountain chains to the last time the sea withdrew a few million years ago during the late Neogene. About 500 million years ago the Danish area was positioned roughly as far south of the equator as it is now positioned north of it. From that time onwards the continental plate on which the Danish area is located drifted slowly towards and across equator, and thence moved northwards to obtain its present position. This journey through 500 million years and amounting to more than 12,000 km obviously caused dramatic climatic changes in the Danish area through time, which are a main topic in the exhibition. In addition are shown examples of rocks, fossils and mineral resources from the various geological time periods represented in the Danish area.

     
dimonds 
The exhibition the Geology of Denmark 
Quartz crystal, "Bornholm diamond", Cyrtograptus shale, Island of Bornholm
 

Diamonds on Bornholm
Quartz crystal ”Bornholm diamond”, Cyrtograptus shale, Island of Bornholm.
In the Silurian Cyrtograptus shale on the island of Bornholm, which is around 425 mio. years old, are found large chalk nodules - up to 1 metre large. The chalk was deposited by water that seeped through the layers a long time after their depositing. The nodules contain open cracks in their interior which are covered by calcite crystals and among them are seen glass clear quartz crystals which are called "Bornholm diamonds". These quartz crystals have been used in the manufacture of jewellery in the years around 1800. It is claimed that a diamond cutting establishment was built in 1791 on the south coast of Bornholm. Most of the chalk nodules were fished out of the sea off the south coast of Bornholm - on the charts of the day there is a mention of a diamond reef. A total of around 2000 "diamonds" were extracted. Each diamond fetched between 24 "skilling" and 4 rix-dollars, which corresponded to between a day's and monthly wages of a worker at that time. The chalk nodules on the bottom of the sea are now protected. 

     

trilobit
The exhibition the Geology of Denmark
Trilobite, Megistaspis polyphemus (Brøgger, 1882), Komstad Chalk, Bornholm.

trilobit1
Trilobite, Ogygiocaris(Brünnich, 1781)

 

30,000 species
Trilobites are an extinct group of arthropods that lived in the Earth's seas from around 525 ma. until 250 ma. Around 30,000 species are known. Most of them lived on the sea floor as present-day crustaceans. Trilobites are used in the dating of the layers in which they are found. E.g. Megistaspis polyphemus is an index fossil for a zone with the same name in the oldest part of the Middle Ordovician (approx. 470-472 ma.) and is known from contemporary layers in Denmark, Sweden, Norway, Estonia and Russia. The extension of the individual trilobite groups was also typically restricted to specific continents and is used for reconstructing the geography of the past (thus Megistaspis polyphemus is only known from the continent Baltica - nowadays are e.g. kangaroos only known from Australia and giraffes only from Africa). Trilobites are often very common in layers from the Cambrian and Ordovician and more than 100 species are known from Bornholm.

     
halichondria  

Halichondria are common fossils in the Danish white chalk, which is 65-70 ma. The chalk sludge was deposited in a deep sea that covered almost all of North West Europe. The depth of the past North Sea might have been more than 500 metres. On the sea floor was a rich fauna including halichondria. In spite of the name they are very primitive animals which are still thriving in modern seas. They fed on filtering the sea water for food particles; the water was led through the many holes and channels which are seen in the fossil. The skeleton was formed by silicon dioxide needles and many halichondria have subsequently been fossilized by the filling of flint (which also consists of SiO2, silicon dioxide) deposited by percolating water. Previously it was thought that this was the primary mechanism for the formation of flint, but it is now known that most flint layers have been deposited around open burrows, made by crustaceans.

     
tand fra dinosaur  

Tooth from meat-eating dinosaur, related to Dromaeosaurus, the Jydegård Formation, Island of Bornholm

Dinosaurs are an extinct group of reptiles that lived on land 259-65 million years ago. During this period the Danish area was generally covered by the sea and moreover layers of this age are deeply buried in most of the country, so chances of finding fossil dinosaurs are slim. But on the island of Bornholm, which at that time was situated in the coastal area, we have been successful - the tooth represents a meat-eating dinosaur that lived in the Lower Cretaceous 140-135 million years ago. After its death the skeleton was scattered and the tooth ended up in a lagoon on south Bornholm. Fossil sharks, crocodiles, fish, lizards, turtles, bivalves and snails have also been found in the same layers. Subsequently several small teeth from meat-eating dinosaurs have been found in the slightly older Rabække Formation on Bornholm, but they have not yet been described. In Lower Cretaceous the Danish area lay somewhat more southerly on the globe, approximately corresponding to the present-day position of Italy and thus the climate was warmer than now.

ZDROJ: Universita Kodaň


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