A beach cave collapsed because of a "fault in the rocks", according to a geologist.
The collapse at Holywell cave in Holywell Bay, Newquay, is believed to have happened overnight on Saturday. The cave, also known as St Cuthbert's Cave, sits on the southern cliffs of Kelsey Head. It appeared in the BBC's Poldark series and was known for its multi-coloured rocks.
Geologist Paul Foster called the fault "a double-edged sword" because it created the colourful rocks but also led to the cave's collapse. He warned the collapse might not be over.
Foster said the rocks are about 350 million years old and lie beneath a mountain range. He said "all of the rocks were squashed and folded" and were metamorphosed from mud to slate, which is when the fault developed. He described a fault as a line in the rock where movement has occurred, leaving rocks on the fault line "fractured and broken".
He said: "One side has moved upward, the other side has moved downwards, so it's a joint in the rocks where there has been movement."
"In this case, those breaks have allowed the calcite into the rock which formed all of the minerals on the side of the cave to start with, but it also formed the weakness which has weakened the rocks and caused it to collapse, so it's sort of a double-edged sword."
The area was cordoned off on Sunday, and lifeguard warning signs remained on Wednesday. Foster said the area is still "very dangerous".
"There are two main hazards, the first being screed pile has lots of gaps in it, and you could fall down into one of those gaps. The second is the collapse might not be over and there might be further collapses at the top around the coast path."
Lee Griffin, surf instructor and vice chair of the Holywell Bay Surf Life Saving Club, said everyone was "really gutted about it". He added: "It's quite upsetting for a lot of people, it's part of the beach, it's part of the DNA of Holywell Bay."
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Holywell Cliff Fall Caused by Rock Fault