Geosites Project

The GCR site at Rhynie is arguably the most important Old Red Sandstone site in Great Britain and one of the most important in the world. In addition to its status as a unique Early Devonian faunal and floral lagerstatte, it is also one of the earliest-known occurrences of the surface expression of a hydrothermal spring system. The strata include cherts that contain an exceptionally well-preserved, silicified, internationally important early land-plant and arthropod assemblage, the whole in-situ biota providing an insight into plant and arthropod evolution and the development of terrestrial ecosystems. The fossiliferous horizons are placed into a context of evolving geological environments at the margins of a small, subsiding half-graben. Initial alluvial-fan deposition was followed by the eruption of andesitic lavas, marking the local onset of volcanicity. Subsequent sedimentation was on an alluvial plain with ephemeral lakes and ponds. The climate throughout was semi-arid and seasonally wet. As vulcanicity waned, the basin-margin faults acted as conduits for silica-rich, mineralized fluids. These emerged on the alluvial plain as hot springs and preserved the biota in siliceous sinters, now represented by the Rhynie and Windyfield cherts.
(From ORS volume September 2004)