Orcadian Lakes
Encyclopedia
The Orcadian Lakes are a series of lake
s which existed during the Devonian
period in the region which is now northern Scotland
, Orkney and Shetland. The sedimentary rocks they left behind have been studied since the 1830's. They contain a huge variety of very well preserved fish
fossil
s, which give a notable insight into the evolution of fish during this period.
and igneous rock. The lakes varied in depth and extent from time to time, sometimes lapping against the side of the hills and sometimes retreating so that river flood plains were able to form.. Stromatolites can be found at Stromness, indicating that the lake was sometimes saline. Locally Scolecodonts
(marine microfossils) have been found in the Eday Marl suggesting at least a temporary connection to the sea. The hills were bare of vegetation. The land was not colonised by large plants as it is today. Erosion would therefore be rather rapid and probably seasonal, as would be lake productivity. This is reflected in the deep lake carbonates, which show very fine laminations, interpreted as varves.
has yielded the most extensive fish fauna and at least one example of all the following groups have been found there.
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
s which existed during the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
period in the region which is now northern Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
, Orkney and Shetland. The sedimentary rocks they left behind have been studied since the 1830's. They contain a huge variety of very well preserved fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s, which give a notable insight into the evolution of fish during this period.
Landscape
The landscape consisted of rounded hills formed of older metamorphicMetamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
and igneous rock. The lakes varied in depth and extent from time to time, sometimes lapping against the side of the hills and sometimes retreating so that river flood plains were able to form.. Stromatolites can be found at Stromness, indicating that the lake was sometimes saline. Locally Scolecodonts
Scolecodonts
A scolecodont is the jaw of a polychaete annelid, a common type of fossil-producing segmented worm useful in invertebrate paleontology. Scolecodonts are common and diverse microfossils, which range from the Cambrian period to the present...
(marine microfossils) have been found in the Eday Marl suggesting at least a temporary connection to the sea. The hills were bare of vegetation. The land was not colonised by large plants as it is today. Erosion would therefore be rather rapid and probably seasonal, as would be lake productivity. This is reflected in the deep lake carbonates, which show very fine laminations, interpreted as varves.
Fauna
Fish living of the edge of the lake would float out into the centre, then sink and be preserved in the anoxic conditions prevailing at depth. The Achanarras quarry near ThursoThurso
-Facilities:Offices of the Highland Council are located in the town, as is the main campus of North Highland College, formerly Thurso College. This is one of several partner colleges which constitute the UHI Millennium Institute, and offers several certificate, diploma and degree courses from...
has yielded the most extensive fish fauna and at least one example of all the following groups have been found there.
- placoderms
- acanthodians
- actinopterygians
- porolepiforms
- osteolepiforms
- dipnoans
Flora
Very few plant remains have been found and none in situ. There is evidence of algal and bacterial activitySee also
- List of fossil sites (with link directory)
- Geology of ScotlandGeology of ScotlandThe geology of Scotland is unusually varied for a country of its size, with a large number of differing geological features. There are three main geographical sub-divisions: the Highlands and Islands is a diverse area which lies to the north and west of the Highland Boundary Fault; the Central...
- Geology of OrkneyGeology of OrkneyThe geology of the Orkney archipelago in northern Scotland is dominated by the Devonian age Old Red Sandstone . In the southwestern part of Mainland, this sequence can be seen to rest unconformably on a Moinian type metamorphic basement....