A year or so ago an old Porthlevener asked me if I knew about the Moonstone. (I think that he might even have called it an asteroid at the time.) When I said I hadn’t he told me I couldn’t miss it, this rock that had supposedly fallen from space, if I went to cliffs outside the village at low tide. Of course I had to see it for myself and he was right, the rock really is unmissable! It isn’t hard to see why it has piqued people’s interest and imagination for centuries, all trying to explain where it might have come from. It is known locally as the Porthleven Moonstone or the Giant’s Rock but to geologists it is famous, a scientific enigma, and they call it the ‘Porthleven Erratic’.
And, all jesting aside, it really is rather special!
In a way it is such a mystery that it might as well have fallen from outer space!
The Porthleven Erratic
“Most remarkable of the Cornish erratics is the large smoothly worn boulder which lies on the shore north of Porthleven . . .”
– R. M. Barton, The Geology of Cornwall, 1969
An ‘erratic’ is basically a geological term for a rock that is out of place, one that shouldn’t be where it is. This is usually a boulder that has been transported somehow, perhaps by a glacier or an ice sheet, a long way from its original source and deposited in a quite different location. The Porthleven Moonstone is the perfect example of this.
This stone giant came to rest many thousands of years ago on a rocky platform known as the Pargodonnel Rocks between Tregear Point and Breageside and not only is it like no other rock in Cornwall, it is also like no other rock in the whole of the UK.
It is a 50 ton lump of Garnetiferous Gneiss and where it came from and how it got to Porthleven is a mystery – and has been the source of some debate for many years.
Garnetiferous Gneiss is a type of metamorphic rock, characterised by a concentration of garnet crystals. If you peer closely at the boulder’s unusually smooth surface, beneath the barnacles and salt, it is possible to make out the tiny sparkling crystals of this semiprecious, red stone and this is what gives the Moonstone its distinctive reddish-orange colour.
‘Borne of Floating Ice’
“Porthleven has long been famous for the large stranded erratic known as the `Giant’s Rock’. Its exact origin and mode of emplacement have been much debated but never satisfactorily solved . . .” – S. Campbell, Quaternary of South-West England, vol 14, 1998
Suggesting that a rock 6ft high and 12ft long rock, weighing an estimated 50 tons, floated to Cornwall sounds like something from one of our legends of our wonderful saints and the various unlikely ways that they were transported to our shores, but in all likelihood that is exactly how the Porthleven erratic arrived here.
Although it is agreed that the Moonstone was deposited on its rocky platform at some point during the Pleistocene period, a geological time epoch that lasted from about 2.58 million to 11,700 years ago, it is what was involved in the boulder’s transportation that is up for debate. The question of whether it travelled by glacier, an ice sheet or on a giant ice burg has been the cause of much study and controversy for more the 100 years.
The problem seems to be (from my interpretations of the reports I have read) that some geologists believe that the Porthleven erratic is proof that glacial ice did extend much further into Cornwall than has been commonly agreed upon, while others believe that the boulder arrived floating on an “ice raft” of some kind and was stranded on the coast after a melt.
An iceberg carrying our boulder to Cornwall tends to be the most popular and many believe most logical idea but there are still mysterious problems with this theory.
With so much frozen water trapped in the ice sheets the sea levels during this time were lower and some geologists claiming that the water would have been too low to support this mode of transport. Then there is the fact that the coastline was actually much further out than it is today. We know, for example that St Michael’s Mount was an inland rocky outcrop surrounded by forest before sea levels changed around 1700BC, hence its Cornish name, Carreck Los yn Cos meaning ‘grey rock in the woods’.
So, did the Porthleven erratic really float that far inland?
It seems no one really knows.
“The Giant’s Rock is the most impressive and intriguing of the large erratics found around the south and west coasts of Britain. Despite having attracted scientific interest for nearly a century, its exact origin and mode of emplacement are still unknown and it remains the subject of much controversy. Although some workers have maintained that the 50-ton erratic was emplaced directly by glacier ice, most believe that it was delivered to its present location on floating ice. One recent interpretation invokes Greenland as a possible source and a disintegrating Laurentide ice sheet as a transport mechanism.” – S. Campbell, Quaternary of South-West England, vol 14, 1998
A Legendary Boulder
The Moonstone was first studied in any detail by two geologists, J. S. Flett and J. B. Hill, who published a book called ‘The Geology of the Lizard and Meneage’ in 1912. It was they who were the first to conclude that the boulder was unique in the British Isles and they also noted that according to local people, and the evidence they had seen, the boulder never move. Even in the heaviest and wildest of seas, it does not budge from its position. Even during the epic storm of December 1989, when Porthleven was famously battered by tremendous waves, it did not move a single inch on its sea swamped platform. The implication is that where the rock now sits is where it came to rest all those thousands of years ago.
I like calling it the Moonstone because of how it was introduced to me – as something that fell to earth from space but the stone’s more common local name is the the Giant’s Rock. This idea of course connects it to so many Cornish legends, many of which involve giants hurling boulders about – which is supposedly, according to legend, how this one came to lie where it does.
It is a running theme in Cornwall – if something is seemingly unexplainable we blame it on a mythical event, usually a giant or King Arthur!
The stone does strike you as strange when you get close to it, even I with very little geological knowledge could see that it was entirely different from anything I had seen before; and to touch it almost feels like metal. Too solid somehow, even surrounded by our impregnatable granite.
There can be little doubt that our ancestors would have been aware of the strangeness of this stone too and it is interesting to wonder what stories they might have told about where it had come from or who had put it there.
One thing is clear however, while we all puzzle and scratch our heads, while the tides roll endlessly in and out and while the world continues to turn, the Moonstone stands still and silent as a sentinel, quietly keeping its secrets.
To Find the Porthleven Erratic:
PLEASE NOTE: You can only get to the stone at very low tide. Do not attempt to visit it on a rising tide, you do not want to get cut off.
Walk out of Porthleven past the Ship Inn, up Mount Pleasant Road, bear left onto Ocean View Terrace and follow this to join the footpath. Walk to the Memorial for Lost Mariners (see below) and just past the white cross there is a steep, narrow path down the cliff. From here you can climb down onto the rocky platform and walk out to the stone. You really can’t miss it! And if you can’t see it the tide is in and you should come back another time!
Further Reading:
Memorial for Lost Mariners – Porthleven
Walking Opportunities:
Circular walks around Porthleven
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