Orcadian Lakes

The Orcadian Lakes are a series of lakes which existed during the Devonian period in the region which is now northern Scotland and Orkney. The sedimentary rocks they left behind have been studied since the 1830's. They contain a huge variety of very well preserved fish, which give a notable insight into the evolution of fish during this period.

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Landscape

The landscape consisted of rounded hills formed of older metamorphic 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 (1). Stromatolites can be found at Stromness. These are indicative of marine conditions and so the lakes must have been periodically connected to the sea. The hills were bare of vegetation(1). The land was not colonised by large plants as it is today. Erosion would therefore be rather rapid and probably seasonal. This is reflected in the sediments which show very fine laminations, known as varves. (2)

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 Thurso has yielded the most extensive fish fauna and at least one example of all the following groups have been found there(2).

• 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 activity(4)

References

1. Stephen Jay Gould. The Book of Life. Ebury Hutchison 1993
2. TREWIN and DAVIDSON Journal of the Geological Society.1999; 156: 535-548
3. http://www.snh.org.uk/publications/on-line/geology/orkneyShetland/life.asp
4. R. O. Muir and J. M. Ridgway, Mineralium Deposita, Volume 10 Number 3, pp 205-215

See also

• List of fossil sites (with link directory)
• Geology of Scotland