Lower Purbeck (Cypris Freestones)
The Purbeck Group comprises the Lulworth Formation (three Members) overlain by the Durlston Formation (two Members). The Cypris Freestones span the lower two members of the Lulworth Fm, the Mupe Member and the Ridgeway Member. The Jurassic/Cretaceous boundary is currently thought to lie within the Middle Mupe Member, thus the Cypris Freestones are partly youngest Jurassic and partly earliest Cretaceous. The Purbeck Group was deposited over a period of some 6 million years (146-140my).
The Cypris Freestones are quarried for building limestone in the area of outcrop from Portesham, through Upwey to Coombe valley/Chalbury Camp and Poxwell. Purbeck limestones from this area are sometimes known as “Ridgeway Stone”. The name is taken from Cypris a family of ostracods (tiny crustaceans) that live in fresh, brackish and sea water up to the present time. Cypris change their form over time, and can be used to compare the order of deposition of the separate beds within the Formation. In most beds of limestone the Cypris can only be seen by using a magnifying glass, but occasionally one bed has larger examples.
The matrix surrounding the fossils is a very fine-grained powdery lime mudstone. Ripple marks are sometimes seen. In historic buildings the Cypris Freestones can be recognised by their white colour and weathering properties. The stone weathers into laminations as if the main body of the stone is more resistant and thin separating layers are softer. In domestic buildings the stone is referred to as ‘rubble’ by the architects, though it has been worked to a rough brick shape, but in churches and more important buildings there are examples of its use as ashlar. JT February 2017