The Group's aim is to identify, survey, protect and promote geological and geomorphological sites in the former County of Avon - the modern unitary authorities of Bath and North East Somerset, Bristol, North Somerset and South Gloucestershire. RIGS are selected for their educational, research, historical and aesthetic value.

Sunday, 4 November 2012

Geo-gardening - Sunday 21st October 2012

John Toller was joined by a very small group from WEGA to clean up the RIG site at Itchington which the group had agreed to maintain. The site demonstrates a small but excellent exposure of the "Bristol Time Gap". This shows the angular unconformity which separates the almost horizontal Triassic Mercian Mudstone Group, locally Dolomitic Conglomerate, from the older, underlying and tilted Carboniferous Limestone, which is known locally as Clifton Down Limestone. This was formed approximately 334 to 341 million years ago in the Carboniferous Period. 

The Carboniferous Limestone was deposited in warm shallow seas covering much of the country. This was followed by the Coal Measures and then thick layers of sandstone lain down by river deltas until the start of the Permian Period. No Permian rocks are present here, but during that period the continents were all brought together to form one huge landmass, called Pangaea.

The Carboniferous rocks here were folded and compressed to form mountains, then weathered and eroded down to produce more sediments. The layers overlying the limestone here are products of that erosion during the Triassic Period, resting on the worn down limestone surface. The environment by then had changed to mainly desert, resulting in characteristic reddish sands, clays and debris as seen here, contrasting with the hard grey limestone. The time gap between the two layers is at least 50 million years, enough time for a whole mountain range to be created and worn down.  

The rock face had become covered in a fine variety of plants and saplings which were threatening to completely engulf it. The many ash sapling were especially unwelcome as the strong roots can quickly penetrate the joints and fissures in the rock to break down the exposure. After a couple of hours determined clearance the unconformity was looking very much more obvious, as John is pointing out in the photo. Next year it will all need doing again or the ash will take hold, so keep the clippers handy! 

To admire this site, take the turning  from the A38 at the Grovesend road junction near Thornbury signposted to The Slad and Itchington. Keep travelling until you are about to go under the M5 and see the exposure on the right. Close by you can also see a beautifully flat natural bedding plane of limestone forming a bank. The exposures are fenced off and there is always a danger of falling rocks, so keep at a safe distance. 

Photo credit: Sandi Shallcross

Sandi Shallcross