Area: 9.4 hectares.
A new geological site which includes four Geological Conservation Review sites: Pentonwarra Point, Trevone Bay, Marble Cliff and Porthmissen Bridge. Within Area of Outstanding Natural Beauty.
Description and Reasons for Notification:
This is the type locality of the Pentonwarra Point Goniatite Band, which has yielded a rich late Givetian goniatite fauna (terebratum Zone), within the Trevose Slates. This locality, in black slates, is of interest in illustrating a palaeoecological contrast to goniatite-bearing beds of similar age in Devon. This is the best fossiliferous locality in the Trevose Slates and it provides a useful horizon for correlation in Devonian-aged rocks in the south- west.
The eastern side of Trevone Bay shows a small, high-level, differentiated intrusive dolerite that in terms of its chemistry and mineralogy is different to all other Hercynian greenstones. This is one of the better exposures of this unusual type, which has limited occurrence in Cornwall, and were termed ‘proterobases’ in the early literature. Its main interest lies in the fact that this body represents the crystallization of an incompatible element-rich hydrous basaltic magma, as shown by the presence of primary, high temperature titaniferous brown amphibole, biotite, and abundant apatite.
Marble Cliff has an extensive exposure through a sequence of alternating shales and turbiditic limestones, which have yielded conodonts indicating an early Frasnian age (hermanni–cristatus Zone to lower asymmetricus Zone). This is an extremely important locality as it provides a detailed section through an Upper Devonian basinal facies which is unrivalled in Europe.
Porthmissen Bridge is an important geological site illustrating a number of principles concerning the geometry and development of folds, and affords excellent three dimensional perspectives. The structures occur in the overturned limb of a major north facing recumbent fold in which a parasitic fold is developed. The small isthmus of Porthmissen Bridge comprises the steep short limb of the parasitic fold, in which the beds become almost vertical. The main lithology (Upper Devonian Marble Cliff Limestone Formation) is a fine-grained, dark grey argillite with a series of limestone layers up to l m thick. Each limestone is sufficiently separated from its immediate neighbour to have behaved as an independent layer during deformation. Three dimensional exposures allow inspection of the variation of fold profiles along the fold axis and confirm the general non-cylindrical nature of buckle folding. Other interesting relationships between the cleavage and the folds can be used to assess the mechanisms of fold and cleavage formation.