Sunday, 6 May 2007

New RIGS sites in South Gloucestershire

Recently the only known exposure of the early Devonian (Downtonian) Thornbury Beds in the outskirts of Thornbury has achieved RIGS status. It is a small roadside stream bank showing the maroon sandstones typical of this formation. They are about 410 million years old and were deposited in desert conditions when 'Britain' was at about 23 degrees South.

A second recently identified RIGS site is near the castle and Parish Church where a low cliff displays the Triassic Dolomitic Conglomerate as a series of clastic flows fining from south to north. Some of the clasts are boulders 1 metre across. You can imagine the power of the flash flood that moved these. It's called 'Thornbury Rock' not to be confused with the sugary stick. It is about 230 million years old and was deposited when 'Britain' was again in desert latitudes at about 23 degrees North.

A third RIGS is in the Tortworth Inlier and is of the Silurian Wenlock Limestone. It's in a ditch 2m x 1m but it does have very good fossils, including stromatolites (algal mats). This limestone is about 425 million years old and was formed in a warm sea when 'Britain' was at about 30 degrees South. This site is on private land and will be visited by members of the Bath Geological Society on their field trip on 12th May.

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