Rhogo: the heart of the inlier

The expansive common land known locally as the Rhogo (Gilwern Hill and surrounding land) straddles the upland road between Howey and Hundred House. It is a wild, desolate area surrounded by high hills and with extraordinary views.

There is a remarkable archaeological heritage in the area, from the spectacular Castle Bank hillfort to numerous other remains such as standing stones. The acid moorland is also very rich in biodiversity, supporting rare birds such as curlews, and from it drains the Colwyn Brook Special Area of Conservation, and the unspoilt Colwyn Brook Marshes SSSI.

Geology

The Rhogo area exposes many important geological exposures, including unique fossil assemblages and a rich and well-exposed geological sequence through the beginnings of the most violent part of the creation of the volcanic island.

The largest single eruption of the entire history of the inlier created the Llandrindod Tuff Formation: a thick (up to over 100 m) deposit of pale, quartz-rich rhyolite. It appears to have been formed by a classic caldera collapse, one of the most catastrophic of eruption styles in which the entire central region of the volcano collapses, with ash being erupted from a circular ring vent. The lowest levels of the eruption include boulders, but the ash gets finer upwards. The best exposures of this formation are along the side of the road, on the Howey (western) side, but the hard rhyolite forms a prominent crag that can also be seen cutting across hillsides in the distance.

Beneath the Llandrindod Tuff Formation, the rocks were fossiliferous siltstone and sandstone laid down on a normal marine shelf before the start of the eruption. There are several very important exposures of these rocks, known as the Camnant Mudstones Formation, exposed in stream sections and quarries in the Rhogo area.

One, known as Camnant Ravine, has yielded one of the most important early faunas of bivalved molluscs, and also contains a suite of trilobite species that are not known from anywhere else in the inlier but are shared with Shropshire. This is important for showing continuity of the fauna across the ancient coastline, something that then changed dramatically after the eruptions began.

Another site, known as Camnant Cliff, shows the ecological response of the fossil community to the start of volcanic activity, and includes a rare crinoid, Iocrinus pauli, that is known only from the Rhogo area.

The most famous fossil site in the area is the famous Gilwern Hill Quarry, a private site that has produced huge numbers of trilobites, together with new species of starfish, sponges and bryozoans from distinctive green sandstones immediately before the Llandrindod Tuff eruption. Fragments of what may be among the earliest known sea scorpions have also been reported informally. Many of these fossils are yet to be formally described.

The rocks that were laid down after the Llandrindod Tuff Formation include a huge, complex volcanic development that forms the rugged Carneddau range to the southwest. To the north of Gilwern Hill, the volcano started to wane, with the ash being worn away into sandstones and siltstones instead of further volcanic episodes, as the volcanic focus shifted to the south. Those sandstones can also be rich in fossils, with many species unique to the inlier.

One other extraordinary feature stands out in the geological panorama: the superb sill of Castle Bank. This rough crag is an igneous intrusion of slightly younger magma that forced its way between the pre-existing layers of rock. It is composed of dolerite: a coarse-grained equivalent of basalt. Much of the intrusion has been worn away by glaciers and rain, but the remaining part forms a high rocky ridge that made an ideal position for an extensive Iron Age settlement.

Archaeology

Coming soon!

Biodiversity

Coming soon!