Devonian

Upper Old Red Sandstone

This geological system has rocks from two distinctly different environments, one being marine and the other desert. Only a small part of the desert environment is represented by the rocks in South Gloucestershire. To date it has not been possible to correlate the marine and continental facies because the marine sequences have been identified by the presence of invertebrate fossils and the continental rocks are identified by early types of fish.

The outcrops in South Gloucestershire were deposited in the Upper Devonian age, so a long period of time is absent, either due to non-deposition in this area or erosion.

The time gap is seen in the Buckover Road cutting on the A38 ST667907. Here the beds of rock are tilted due to later earth movements, but the Silurian rocks are at the same angle as the Devonian rocks that are above them. We know from comparison with other areas where Old Red Sandstone occurs, that some beds are missing, so the gap between the Silurian rocks and the Devonian is termed an unconformity.

The lowest of the Old Red Sandstone age rocks at Buckover is a Quartz Conglomerate. This rock was formed from coarse boulders eroded from a landmass which lay to the north at the time. Great rivers brought these boulders into the desert where they were eventually cemented to form rock. Environmental conditions changed from time to time, such that finer sediments were carried by the rivers and deposited as silt and mud. The thin bands of siltstone and mudstone lie above the Quartz Conglomerate. Above this sequence is another massive bed of Quartz Conglomerate.

The top bed in the succession, the Tintern Sandstone is mostly a sandstone, but also includes coarser beds of conglomerate. This shows slight changes in the sediments. The succession is disturbed by a fault, which can be seen in the road cutting.

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