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SB's Garden

Warning: This page contains a discussion geological in nature

SB has been wondering about what seemed to him as the anomalous rocks sitting in his garden. Now SB is a man known to take offence at rocks so an explanation needed to be found.

Old Gits are not afraid to examine their rocks in public and that is why at a recent meeting at headquarters we found ourselves looking at a bag of old chippings. CH tried manfully to offer an explanation but was less than satisfied with his offering. He is a fretful sort of bloke and some might say obsessional. The problem niggled away until walking along the road to St Thomas he had a "Damascus" moment.

Fortunately to put the matter to rights CH remembered that many old buildings in Exeter were built of Permian Breccia and Sampford Courtenay sits on the extreme west edge of the same deposits just 3 miles east of the Sticklepath Fault. We already knew that but the clue was the breccia. Fretfulness gave way to obsession and now the need was to look for evidence.

Click on the map below to see a geological map of the area. There is no key but the brown/orange tongue running from East Devon to Jacobstow is the Permian deposit filling an ancient valley. This is known as the Crediton Trough and is known to contain gold deposits.

map1thumb

Click on the one below for more detail. The orange is the Permian, the pale yellow is alluvium, the bright yellow is Carboniferous sandstone and the rest is Carboniferous mudstone. The green is a Permian igneous intrusion.

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I couldn't find a local quarry for breccia and a drive through Sampford Courtenay by Google Street View showed that it was not used locally for building. However, looking at a few cob constructions breakdown products of breccia can be seen.

evidence1 evidence 2
Permian materials used on bottom of roadside wall
Angular fragments can be seen in the cob wall

Cob would have been made from local raw materials purely and simply because your local mud is as good as anyone else's.

I cannot continue without a description of Permian Breccia. Sorry. To avoid it click here. That was a trick. There is no escape.

Between 250 and 299 million years ago Britain was part of a super-continent called Pangaea. It wasn't called Pangaea then because there was no one around to call it anything. It was situated just north of the equator.

The rainforests of the Carboniferous had disappeared, leaving behind vast regions of arid desert. Dinosaurs had not been invented but reptiles, who could better cope with these dryer conditions, rose to dominance. The Permian Period ended with the largest mass extinction in Earth's history,when three quarters of known species of land animals and 96% of marine animals became extinct. Groups such as the trilobites disappeared forever.

SB's garden was situated near a mountain range in a desert. The next oldest period in the geological table is Carboniferous so that is the likely age of the mountains although it is possible the constituent rocks were older - possibly Devonian. The carboniferous rocks were mostly very fine grained mudstones. Dartmoor had been emplaced during the Carboniferous with the associated mineralisation. Therefore the mountain fragments would include quartz, tourmaline and other odds and sods.

To see the formula of the varieties of Tourmaline click here. Now you know why I didn't want to get drawn on its makeup.

Anyway the thing is that mountains are good things for capturing water. Tropical downpours would create flash floods washing debris down onto the desert plain where it would quickly fan out - losing energy and dropping its load. The angular nature shows the rocks did not travel far from their source. A bit like people from Totnes.

If you look at the cliffs of Dawlish or Exmouth you can see that the rock is far from uniform in nature. Some parts are more consolidated than others. The less well consolidated deposits would have broken down with exposure to air and colonisation of plants leading to more organic rich soils. The red soils of Devon are what is left from the erosion of Permian sandstones.

My hypothesis is that this is what we are now observing at SB's garden. His little house on the Permian prairie situated on the furthest edge of an alluvial fan is built on soil containing debris which elsewhere would have made up quite strong consolidated rock.

Breccia

New Red Sandstone Permian Breccia