Age: early Devonian
The Rhynie Chert is early Devonian in age, some 400 million years old. It is therefore one of the oldest hot spring systems in the world, and preserves one of the earliest terrestrial ecosystems (a hot spring is a surface seepage from a natural circulation of underground water, which is heated by high subsurface temperatures).
During the early Devonian Scotland made up a part of a large continent called Laurussia. This continent is also known as Euramerica, because it incorporated what is now Europe, North America and Greenland. It was located south of the equator in a tropical to subtropical environment. Sedimentation on this continent was in a fairly arid environment, and so many of the sandstones deposited by streams and rivers were stained red by the oxidation of iron to haematite (in hot dry environments iron reacts with oxygen to form haematite, it is this haematite that gives the sandstones their red colour). This process gave Laurussia the name the "Old Red Sandstone Continent".
The Old Red Sandstones in the vicinity of Rhynie were deposited in an alluvial fan environment (an environment where there is an accumulation of sediment deposited from fresh running water with a downstream-broadening shape). These sandstones lie on top of a basement of heavily metamorphosed volcanic rocks, and surround the Rhynie Chert.
The Rhynie Chert was formed in a sedimentary basin, bounded
on the western side by a major fault zone. This low-angle extensional
fault system was the pathway by which siliceous fluids fed the
hot springs. The location of the fault system can therefore indicate
the location of the heat source for the springs,
believed to have been to the SE of the basin. Hot spring
activity would have deposited siliceous sinters (a variety of
opal found in geysers and hot springs) in the basin, as a result
of repeated subsidence. These sinters were interbedded with shales
and sandstones to form a unit of approximately 35m thick near
the top of the succession.