Sunday 25th June 2000

Members from Severnside, South West and Wessex including some S260 students met Dave Green at the old Railway cutting to study the folded Devonian and Carboniferous strata exposed there. The cutting was ideal to practice as you could work your way up the succession. Measurement of strikes and dips was also easier as the dips are very clear.

Dave patiently explained and re-explained the purpose of mapping and the use of a compass clinometer. Debbie’s paper protractor in an audio tape box with a weighted hand suspended from the 180 degree point proved to be an inexpensive alternative but she needed a compass to measure the strike.

We used a 2.5 inches to the mile base map with the grid lines at 100m intervals. This made it easy to orient ourselves and see where there were changes in the strata going west along the railway cutting.

The first exposure was at the unconformable junction between the Devonian Lower Old Red Sandstone and the Upper Red Sandstone. There is a gap in the Geological record with no evidence of the Middle Old Red Sandstone. The older Brownstones Formation comprises competent red coloured sandstones with cross bedding and interbedded friable shales which are more eroded. Above the conformity is the quartz conglomerate which also had interbedded layers of shale. The conglomerate was texturally and mineralogically immature, giving evidence of a high energy, perhaps flash flood type environment of deposition. The rounded pebbles were probably reworked as texturally the great variety of sizes and types of clasts indicate that this had been deposited quite close to its source. The strike/dip for both was 186/60W.

The next exposure in a quarry was the Devonian, Tintern Sandstone, a yellowy colour with a different texture to the Brownstones Sandstone. We measured strikes and dips in several places which to the eye seemed to be dipping differently. However this was an illusion as the results were very similar with 195/65W, 198/69W and 200/60W.

An exposure of the Lower Limestone Shale brought us into the Carboniferous with crinoids in a competent limestone. There were also some calcareous sandstones and sandy limestones (identified by the amount of fizz with HC1 and the porosity). Dip and strike again similar to the other exposures.

Near the disused railway tunnel was a small outcrop. It was less easy to measure the dip but the consensus was 195/65W. The rock appeared crystalline and sugary, no fizz with HCl, scratched by a steel hammer (hardness 5.5) a copper coin (hardness 3.5) left a mark on the rock. Therefore there was no calcium carbonate and the hardness was between 3.5 and 5.5 on Mohs scale. The colour was grey with some veining and clearly not igneous. By process of deduction we realised this was a dolomite with the Ca having been replaced by Mg.

As many of you have been through this process on tutor led groups at Summer School I will stop now! I found it really good to have the challenge of trying to confirm the earlier mappers and to justify why we were saying a rock was a specific type. Throughout the rest of the day we identified the Lower Dolomite Formation, Crease Limestone Formation, a minor unconformity, the Whitehead Limestone Formation and Drybrook Sandstone Formation. We also managed to have a Roast lunch at the pub and I’ve organised a trip with Dave to The Forest of Dean next March having called at Clearwell Caves, a Colliery and a Medieval Castle Youth Hostel on the way home to Somerset (Monday felt like a day of rest at work!).

Thanks Jan for organising the trip and Dave for explaining things so well.

Sheila Alderman OUGS Wessex