Area: 4.42 hectares.
Description and Reasons for Notification:
A Geological Conservation Review site.
The quarry shows high-level massive and pillowed, metamorphosed alkali dolerites associated with pelagic sediments of Lower Carboniferous age. Their main significance relates to the tectonic evolution of central south-west England, in that the greenstones are interpreted as the volcanic remnant of a small ocean basin. This was originally situated to the south of the present outcrop, but with associated pelagic basinal sediments was thrust northwards as the Greystone Nappe. The sinuous basal thrust is exposed in the quarry.
The site also includes a series of imbricate rocks (duplexes) contained within a Lower Carboniferous sequence of dolerite sills, radiolaria-rich cherts and shales. These thrust structures are exposed in 3-dimensions, and provide the best inland exposures of such structures in southern Britain. The relationships between the imbricates, floor and roof thrusts, and side-wall ramps, are seen here. The Lower Carboniferous sequence containing these structures is overthrust by the Upper Devonian Lezant Slate. The thrust contact is clearly exposed.