Area: 13.1 hectares.
This is a new site.
The site is within an Area of Outstanding Natural Beauty and Heritage Coast. Geological interest extends into the adjoining Loe Pool SSSI.
Description and Reasons for Notification:
Porthleven Cliffs East comprises a 1.5 kilometre stretch of coast immediately to the east of Porthleven, 3 kilometres south-west of Helston. The cliffs and beach are of particular interest for their geology and coastal morphology.
Porthleven beach is an integral part of the classic coastal landform which includes Loe Bar to the east. The beach system is an essential member of a suite of major beaches formed and maintained by predominantly south-west wave regimes. The beach comprises mainly flint shingle and coarse sand. Inputs from adjacent cliffs are small, and overall the beach is in deficit.
The geology interest of Porthleven Cliffs includes both intrusive igneous rocks and complex Variscan structures. The igneous interest is confined to the north-western half of the site. Although generally regarded as the type area of the Mylor Series sediments of West Cornwall, this section of coast also exhibits many small high-level intrusive greenstone bodies. The main interest lies in their contact relationships which indicate intrusion into wet sediments and thus were penecontemporaneous with sedimentation, unlike some of the larger intrusive bodies in south-west England.
The Variscan structures extend along the entire section of cliff. This locality provides an outstanding illustration of the interplay of the three main regional phases of deformation which affect South Cornwall. There are three generations of folds and associated cleavages cut by a series of steep northeast-southwest extensional faults. The section lies at right angles to the dominant north-easterly strike of the structure. Exposures of thin-bedded Mylor slates show an eastwards transition to more proximal Gramscatho turbidites. The stratigraphy is cut up into a number of fault-bound packages which probably contain repetitions of parts of the sequence. Within some slices there is a suggestion of fining up from slate to greywacke. The ‘Carrick Thrust’, which supposedly marks the Mylor-Gramscatho boundary, is not evident in the exposures and if it exists, it has been cut out by normal faulting. First generation folds are especially well represented in the striped Mylor slates. Folds and thrusts marking the secondregional deformation phase are intermittently developed throughout the section but are more prominent towards Loe Bar. Recumbent third generation folds overprint the earlier deformation phases giving rise to spectacular fold interferance patterns. The latter phase is associated with the emplacement of the Cornubian batholith.