The hottest day of the year so far gave us a bright blue sky and deep blue sea as a
backdrop to our geology fieldtrip to Porthleven/Loe Bar on Saturday 12 th July 2025.
Dr Sam Hughes, aka the Cornwall Geologist, specialist in structural geology led the
trip for 26 RGSC members. We walked from Chyvarloe National Trust car park down
to Loe Bar beach and then walked south along the beach to study the outcrops. With
low tide, and low sand levels this year, there was plenty to see. Sam used a series of
rapidly-drawn diagrams on his whiteboard to explain how huge scale plate collisions
and the resultant mountain building gave rise to the spectacular and complex fold
structures in the rocks along the beach. It is not surprising that the folds are complex.
The original Devonian 380-million-year-old) sediments, which are a mixture of
sandstones deposited rapidly in the ocean as turbidite sequences on a continental
slope and shale deposited much more slowly in intervening quiet periods have
certainly had a hard time since their formation. Although the sediments have only
been subject to low grade metamorphism, they have been intensely folded under
great strain in two phases of plate collision. Some are now lying sideways (rumbent),
with cleavage formed by their recrystallised and aligned mineral constituents that is
difficult to distinguish from bedding. Not only di the plate collision cause folding,
whole sections of the crust were faulted and thrust over the top of the crust below.
Loe pool marks the boundary of one of these thrust fault units, with the Portscatho
Formation that we looked at to the south and the Mylor Formation to the north. After
the collision, a period of relaxation caused a low angle normal (detachment) fault that
can be seen in cliffs towards the south of the beach and enabled the intrusion of the
granite batholith elsewhere in Cornwall and West Devon. Finally, came the
Northwest-Southeast cross-course faulting, visible in the cliffs here and important in
geothermal energy and lithium brines in Cornwall. So, the fieldtrip provided an
amazing synopsis of the key structural geology events in Cornwall all in a short walk.
The group learned a lot and thanked Sam very much for such clear explanations.
You may well already be familiar with Sam’s brilliant ‘Cornwall Geologist’ pictures
and geology explanations on Instagram. Sam has recently set up his own company
to provide guided geowalks, talks, teaching and consultancy. Please check out
https://www.ornwallgeologist.co.uk/ to find out more and Royal Geological Society of
Cornwall Field Trip to Loe Bar | Cornwall Geologist to see a post about our RGSC
fieldtrip.
Frances Wall, July 25
Photos Frances Wall
Sam Hughes at work on his whiteboard, RGSC group photo, Geology in the sunshine, Complex folds in lighter grey sandstone and dark grey shale
