Devonian Lakes and Hot Springs
After the storm of colliding continents came a period of calm. At this time in geological history, the evolving Britain sat some 10° south of the equator, and Laurentia was joined, as one landmass, to North America and Greenland. Much of Scotland was made up of high mountainous areas of alpine proportions, with a few isolated freshwater basins fringing this super-continent. It was in one of these large lakes and its associated rivers and deltas that the Orkney Islands were born.
The mountains surrounding the lake, which has now come to be called Lake Orcadie, were eroding rapidly as rivers cut through the barren high ground. Enormous volumes of mud and sand were transported along these river systems to the shores of the lake which - teemed with primitive life-forms of the most bizarre construction. Armour-plated fish, now long extinct, and the precursors of the species we see on the fishmonger's slab today, were found in profusion in Lake Orcadie. Evidence for great shoals of these monsters is provided in particular rock horizons, called 'fish beds'. These special layers are finely banded, indicating a very slow rate of sediment accumulation in the deepest part of the lake, with each layer representing the sediment input for one year. Like counting the rings of a tree stump, the length of time represented by a particular fish bed can be readily established.
Some of the earliest insects and most primitive plants to grow on land are also preserved in rocks of a similar age to the lake sediments of Lake Orcadie. A unique deposit of chert, found near the village of Rhynie, Aberdeenshire, preserves the fossilised remains of some of the earliest land plants recorded from anywhere in the world. The deposit is an excellent example of the preservation of life-forms in an instant of geological time. An ancient marsh plant community grew close to a number of hot springs, which overflowed from time to time, flooding the adjacent wetland with silica-rich boiling water.
This deadly cocktail cooled quickly, trapping plants and insects in a Pompeii-like act of instantaneous preservation. This primitive time capsule has been much studied by scientists around the world and, as a result, a great deal has been learned about the early life-forms which inhabited planet Earth 400 million years ago. The Rhynie Chert locality was recently gifted to Scottish Natural Heritage ensuring that this world-renowned place will be preserved for many future generations of geologists to study.