Broken Shell Limestone (BSL), also known as Burr (lead authors JT/WGT)
The Purbeck Group comprises the Lulworth Formation (three Members) overlain by the Durlston Formation (two Members). The Broken Shell Limestone Bed (aka Burr) lies at the base of the upper member of the Durlston FM, the Peveril Point Member (Early Cretaceous, Berriasian). The Purbeck Marble lies a little above. The Purbeck Group was deposited over a period of some 6 million years (146-140my).
The BSL has a distinctive texture resulting from dissolution of small bivalve shells, leaving densely packed shell moulds cemented by sparite.
The BSL has been quarried on the lower part of the steep north-facing slope of the Purbeck anticline, where the beds are folded so tightly that the outcrop is located on the hill above the Marble quarries dug by the Romans. The outcrop of both Marbles and BSL runs from Peveril Point through the valley to Worbarrow Tout, and both would have been quarried at least from Norman times onwards.
At Crack Lane (a quarry near Swanage opened in the late 19th century and working until 1960) the Broken Shell Limestone can be seen to be in several thick beds suitable for cutting as ashlar. The lowest 2 metres are of massive, cream-coloured stone, consisting of the broken bivalve shells (Neomiodon, Unio). The shell debris is disturbed, as if by storm action in shallow water. The middle section in Crack Lane is again nearly 2 metres of thinner beds, often cross-bedded, with the same shell debris, and an upper section is 2 metres of massive stone. The shell debris is cemented with sparite (crystalline calcite), which makes it a strong and weight-bearing stone. Crack Lane is one of five quarries in the Isle of Purbeck that have been designated as Regionally Important Geological Sites (RIGS), and the DIGS group has prepared an illustrative CD which was funded by Purbeck District Council’s Purbeck Keystone Project in 2009. This CD may be purchased from DIGS, through the Dorset Wildlife Trust.
Burr vs. Quarr
Despite being about 110 million years older, Burr is similar in appearance to the Eocene -Oligocene Quarr Stone of the Bembridge Limestones (Isle of Wight) and it is often hard to decide between them when examining a weathered building block encrusted with fungal/algal growth. Both are mouldic bivalve biosparite grainstones. This table may assist. (see right)
Text JT/WGT March 2017