New geological sites (RIGS )

The rocks surrounding the Helford have a complex history dating back 300-400 million years (nothing compared with the NHS or other government budgets in terms of numbers!!). The oldest rocks were formed in an ocean basin, quite unlike those of the same Devonian age in the rest of Britain, which were laid down under Sahara desert conditions. This is why they have been long recognised as of national importance, and designated Sites of Special Scientific Interest (SSSI), giving them statutory protection. These SSSIs, surrounding the Outer Helford, stretch from Rosemullion Head through Gillan to Porthallow and beyond. There are other sites which although not meriting national status, have regional significance in the understanding of how Southwest Britain developed after that ocean was bulldozed close to and piled onto Cornwall. The Helford area is surprisingly rich in such Regionally Important Geological Sites (RIGS) or County Geological Sites.

Four sites have already been identified and designated. Two concern the events associated with the emplacement of Cornwall’s famous granites. The lamprophyre rock at the western end of Prisk Beach is a precursor to the great granite intrusions. In the winter of 2004/5 the rocks adjacent to it were involved in a landslip, emphasising its importance as a local navigation landmark. This event highlights an important difference between the rocks on either side of the Helford. On the north side the rocks dip into the Helford and can easily slip into its waters down the bedding planes, whereas on the south side they slope into the hillsides, and erosion has a harder time of it.

At last year’s HVMCA event at Goongillings Farm, some interesting old surficial mine workings, associated with the iron-rich lode which was formed subsequent to the granite and exploited by the Brogden Mine on the other side of Polwheveral Creek, were explored. These workings are in remarkable condition and have been designated as an important conservation site. Lesser horseshoe bats have also found them of interest as hibernation sites (hibernacula). Access is therefore strictly controlled and by permission of the owner.

The other two sites concern processes that happened after the Helford River itself was incised into the underlying rocks. The section at Nelly’s Cove, just north of Porthallow, is a classic locality for understanding the development of the cliffs around the Helford River. Here you can see from the Coastal Footpath a former raised beach and cliff buried by Ice Age Head, a very common occurrence around the Helford. The importance of this site was recognised and sketched as early as 1839 by Henry De La Beche who went on to become the first Director of the British Geological Survey.

The stonework of the Old Schoolhouse and Quay Cottage, Durgan, shows significant erosion due to salt weathering and has also been designated to highlight a little-recognised process that affects our coasts and estuaries. Importantly, because the cottages are known to have been built in 1876, the rate of salt weathering can be measured. This would make an ideal hands-on project for National Science Week next year. Any takers?

Peter Ealey

Extract from HVMCA newsletter No.31 Autumn 2005