Porthcew
Also within
Cornwall County Cornwall Scenic Area King Charles Way - Penzance to St Mawes National TrailSouth West Coast Path National Trail
Condition (Natural England)
| Unit | Feature | Condition | Area (ha) | Assessed |
|---|---|---|---|---|
| SW591269 | — | Favourable | 1.79 | 2010-01-29 |
| SW592268 | — | Favourable | 1.79 | 2010-01-29 |
| SW593269 | — | Favourable | 8.16 | 2024-02-21 |
Geology & landscape
Geological Period: Carboniferous · Rock Type: Granite
Landscape processes: Volcanic / Igneous
Description
Porthcew SSSI Description 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.