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Geology Walk to the Devil’s Quoits – The Strange Tale of Two Pebbles

John B writes:

December 17 2020, a gloriously sunny morning, saw a group of six of us, including Lesley Dunlop our excellent Geology specialist who had provided us with her customary helpful background notes, take a somewhat muddy walk around Dix Pit to examine the Devil’s Quoits, an impressive ceremonial circle of Bronze Age standing stones.  

A gloriously sunny morning at Dix Pit.

Some well-established Equisetum and Phragmites caused passing interest as we walked, the former having been used as a pot scrub in the past.

There are only three or four of the original stones remaining, the rest having been used either as building stone – some even in Stanton Harcourt church – or cleared away when the area was excavated for gravel and even turned into an airport runway during the last war.  Replacement stones were provided by the gravel company from their current workings when the circle was reconstructed by archaeologists more recently.  Since these massive stones consist of gravels which have been cemented by iron oxides to form extremely hard concretions, they were of no use to the gravel company anyway.

Close up examination of the surface of the stones revealed a fascinating array of lichens.  Oh Peter, we were in need of your expertise!  All we amateurs could discern was that on the sunny side there were completely different species from the shady side and also that those on the original stones were far more mature and varied.  Not surprising, since they had had several thousand years to develop after all.

But what about the pebbles of the sub-title?

Most of the gravels which make up the standing stones consist of cemented, small angular fragments of local Cotswold material which was deposited by outwash streams flowing from the north at the end of the last Ice Age.  Together these make up the economically important gravels of the Thames terraces complex.  However, Lesley pointed out two small pebbles which differed remarkably from them in both composition and form.  These were made of white quartz and were well rounded.  The nearest source for these is the Bunter Pebble Beds which were deposited in the Permian about 250M years ago, well to the north of our area.  They had subsequently been transported south by the ice sheets and outwash streams.  However, the original source of the Bunter pebbles themselves is far to the south in what is now northern France.  Therefore, these little quartz pebbles were eroded from pre-existing rock in northern France, rolled and rounded on their original journey north and deposited 250M years ago in the Midlands.  From there they were eroded once again and transported to their present location.  Mind-boggling!  Fascinating!

You can tell why I, for one, am looking forward to yet more insight into the wonders of our geological past on Lesley’s next field excursion to Dry Sandford Pit.

John Baker