The boundary between the Silurian and the Devonian lies within the Raglan Mudstone Formation, indicating that the fluvial conditions persisted into the Devonian. The top of the Raglan Mudstone Formation contains the concretionary limestones of the Bishops Frome Limestone. These represent fossilised calcium carbonate rich soils which are called calcretes. Calcretes are normally found in modern day semi-arid areas since they require mean annual temperatures of between 16-20 Degrees Centigrade with an annual rainfall of 100-500mm.
This would suggest that the climate at this time was semi-arid with a markedly seasonal rainfall. By mid Devonian times England and Scotland would have formed a large alluvial plain (Figure Eleven), which was located approximately 15 – 20 Degrees South of the Equator.
a. Environmental Conditions during the mid-Devonian
b. Position of the continents during the mid-Devonian (375 million years ago)