Throughout southern England the Middle Jurassic succession is dominated by shallow-water limestones, typically oolitic or bioclastic. These limestones are exposed along the coast and in a few crags inland, and they have been extensively quarried wherever they crop out. More massive beds have been the source of superb building stones, such as the famous Bath Freestone, whereas more thinly bedded units have been split for roofing slates, as in the case of the Stonesfield Slate of Oxfordshire.
Although commonly rich in marine invertebrates such as bivalves, brachiopods and gastropods, echinoderms are generally not as well preserved and, with the exception of the rich echinoid fauna, have largely been overlooked. Intact crinoids in the Middle Jurassic of southern England are rare, reflecting the predominantly high-energy environments in which the limestones were deposited. Any crinoids that lived in these environments disarticulated rapidly once they died, unless some exceptional event intervened.
Two of the most spectacular occurrences of intact Middle Jurassic crinoids in southern England present an interesting contrast in terms of the palaeoecology and taphonomy of the species involved and their history of discovery.
CRINOIDS FROM A CANAL
The earlier discovery, which is also the stratigraphically younger of the two, was made in the early part of the 19th century in a quarry alongside the Kennet and Avon Canal near Bradford-on-Avon, Wiltshire. The Forest Marble Formation, of Late Bathonian age (approximately 163 million years before present), is about 24 m thick in this area and is composed of mostly crossbedded oolitic and bioclastic limestones. However, about 3 m above the base is a 3.5-m-thick band of clay with thin limestone partings, the Bradford Clay.