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
Text courtesy of Julian Ravest
Gilwern Hill, (Rhogo), is an archaeologically rich area with many prominent man-made features from the bronze and iron ages to the medieval and modern periods. Here some selected features are presented which tell a story of occupation over four millennia.
While there is no direct evidence, the distribution of surviving pre-historic sites strongly suggest an ancient routeway across the Common following the general line of the modern Howey to Hundred House road. This line follows the “natural” east-west route which would have been important for trade from earliest times.
An early indication of the importance of the Common is illustrated by the line of bronze age burial cairns across the modern road at the top of Rhogo. There is, additionally, a small standing stone close to the modern road in the same line. Together these comprise a clear statement that the traveller is crossing a boundary. Each of the cairns are clearly visible on the sky line on the approach road. They meet the criteria often associated with the location of Bronze Age cairns of having views over the associated countryside and, the obverse, of being highly visible.
More evidence of Bronze Age activities on the Common is the stone row comprising one impressive upright stone and three recumbent stones.
As the road leaves the Common beneath the towering bulk of the Castle Bank Hillfort, it is overlooked by three cairns on the lip of a small plateau beneath the hillfort. Two of these cairns are double, each being made of two individual overlapping cairns. They appear on the skyline when viewed from the road.
The Iron Age Castle Bank Hillfort in the single most impressive pre-historic feature on the Common. Its northern end is complex with a second, outer bank, apparently added later to the main bank that encloses the hillfort, is clearly designed to project power over the road beneath.
The hillfort appears to have been built in three phases and has some 50 platforms within it, indicative of it being the major population centre for the area. There are no records of Roman occupation on the Common though it is likely that there was continued occupation of the Hill Fort by the indigenous people after the Roman conquest of Wales.
The medieval occupation of the Common is illustrated by three farmsteads. Two have earthworks which deflect surface water from the farm house. The buildings themselves were small, probably only two rooms. Near the road are four later encroachments onto the Common, three of which are abandoned. Each consist of a farmstead surrounded by small enclosures and are likely the results of claims of “squatter’s rights”
The extent of the unenclosed Common is now less than in previous times as adjacent farms have enclosed and “improved” areas at the edge of the extant Common. Additionally, many archaeological features exist in the now enclosed fields around it and more probably exist on the Common awaiting discovery. There are, for example a cluster of pre-historic sites to the north near the high point of Gilwern Hill.
Biodiversity
Coming soon!