Area: 9.6 hectares.
This is a new site.
The site is listed as of national importance in the Geological Conservation Review, under the name Porthcew.
The site is within an Area of Outstanding Natural Beauty.
Description and Reasons for Notification:
The roof zone of the Tregonning-Godolphin granite, of Permo-Carboniferous age, is exposed at Porthcew.
The granite contact is stepped along joints in the Mylor metasediments, which here form a roof pendant, and the contact demonstrates emplacement of the granite by stoping and subsidence. Dislodged blocks of metasediment occur as xenoliths within the granite and display all stages of digestion and tourmalinisation.
The granite is a lithium-mica variety, with the development, just below the roof, of a spectacular leucogranite-aplite-pegmatite sheet complex. The sheet complex reflects the build- up of a volatile phase trapped below the impermeable roof.
The interest of the site complements that of Megiliggar rocks and together the two sites provide superb exposures through a complex granite roof zone.