• Jon Gutmanis

Elusive geology - The PTF

As a lifelong field geologist (whenever I could escape the desk and screen…..) I have been spending some of my lockdown time doing geological mapping of my local foreshore at Falmouth. Low tide reveals a continuous exposure of Devonian marine metasediments which underwent complex Variscan orogenic deformation. These rocks were some 8 to 12km below ground during the early Carboniferous in the roots of a Himalayan-scale mountain belt – the Variscides. Now back at surface via uplift and the slow grind of erosion, that gives you a sense of unimaginable time for starters.

The foreshore exposure west of Gyllingvase beach, Falmouth, Cornwall

Image via Google Earth


And mystery. Because however hard we try its very hard for geologists to reconstruct in detail the ‘nuts‘n’bolts’ of the Falmouth foreshore geological evolution over 400 million years. That may sound like an apology and its not what the engineers and drillers on our team want to hear when money has to be spent, but it is what it is.

Geology is an inexact science, generally, lurking somewhere between science and art in one view of life. Just as the detective has to reconstruct a murder scene (remember the old verbal puzzle games about a body in a locked room, no keys, rope high on ceiling, pool of water on floor)...... so does the geologist with his imagination, built on his training and tempered with a dash of sense-checking from his peers. For some it can be a recipe for a rampant imagination…..but anyway this is the beauty of geology.

As well as going back to refresh my mapping skills I admit to an ulterior motive for the foreshore mapping. Along with our partners, Geothermal Engineering Ltd, we have drilled two deep wells, the deepest being 5km into Cornwall at United Downs looking to develop a viable commercial use of geothermal energy for heat and power. Success relies on finding a connected volume of fault related fracturing in the high heat flow granite that will sustain water flow rates between the injector and the producer wells, thus allow us to go ‘heat-mining’. Temperatures at that depth are ~190C.

That fractured volume, named the Porthtowan Fault Zone (PTF) relates to a long-lived fault zone which crosses Cornwall from the North coast at Porthtowan to the South coast at Falmouth. There is evidence for the fault in the cliffs of the Porthtowan area but tracing the structure from United Downs down to the coast at Falmouth is more difficult, and at Falmouth itself the evidence is elusive, like we cant prove.... ‘whodunnit’. There are scraps of evidence which taken together could well add up to ‘proof’ but I am still on the trail of the suspect.

Ariel view of Porthtowan on the North coast of Cornwall

Image via Geothermal Engineering Ltd/United Downs Geothermal


At the end of the day it might be a case of extending the evidence with some 'educated imagination' using better exposed analog structures..

But then geologists drilling for oil and gas on the basis of geophysical data have always done that – with loads of success.

So, on with the mapping!


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