The Darens: Ice Age Landslips in the Olchon Valley – the tenth in a series about top Earth Heritage sites in Herefordshire and Worcestershire.

Dick Bryant writes:

This post is about a unique area of geomorphology in the Olchon Valley where the effects of landslips on mountain slopes during the last Ice Age are clearly apparent. Here, in the far west of Herefordshire, the Welsh Black Mountains intrude into England.  This relatively small area contains several spectacular landscape features, including the Darens, described here, and the Cat’s Back – described elsewhere in the EHT website as another ‘Top Earth Heritage Site’.

Fig.1. Location map of Red Daren and Black Daren, west of Longtown, Herefordshire. The yellow line marks the English / Welsh border.

If you’ve ever imagined what a big landslip might look like, then a visit to the Darens is well worthwhile. There is of course now no danger, as the slips happened at least 10,000 years ago, yet the resulting cliffs, ridges, and humps and bumps are still clearly visible. There is evidence of at least two landslips at the Darens. One is at Red Daren, and immediately adjacent, another much bigger one at Black Daren*. In between the two Darens is a shallow embayment, which doesn’t appear to have been affected by the landslipping. The key geomorphological features of the area are depicted in figure 2.

 *Rather perversely, this is the only ‘daren’ (= edge) place name anywhere in the Black Mountains which is spelt on the OS map with two ‘r’s. For consistency, all darens are spelt with one ’r’ in this current article..

Fig. 2. The main geomorphological elements at Red Daren and Black Daren.

Red Daren

It is best to start with Red Daren, because although the key features are less extensive than at Black Daren, they present a good opportunity to appreciate the underlying geology, which is critical to the formation of the landslips. There is a clear and consistent tripartite lithological arrangement of the Lower Devonian rocks throughout the Olchon Valley, typified by those at Red Daren (fig.3)  In the upper part of the slopes, the main cliffs are made up of the competent sandstones of the Senni Formation. These form the ‘headscarp’ of the  landslip.  Immediately below these sandstones, lies the Fynnon Formation, which is a chemically-formed limestone created by the evaporation of moisture in the soil layers during a phase of semi-desertic conditions within the Devonian. Although this Formation is only a few metres thick, it is makes a very useful marker horizon throughout the Olchon Valley.

Fig. 3. Key features of the Red Daren landslip.

Finally, below the limestone are the beds of the Freshwater West Formation (formerly known as the St Maughan’s Formation), which is composed of sandstones and siltstones. Generally, it is a weaker rock than the Senni beds, and its upper horizon appears to provide the main plane of slope failure for many of the landslips in the Black Mountains. The majority of modern-day springs and seepages still emanate from this formation, just below the Fynnon beds. Here at Red Daren, the Freshwater West beds are largely masked by the landslip, which forms a prominent bench, with hummocky ground further downslope.  This bench may in part rest on a structural geological feature, but since this is the only bench of its type in this part of the Olchon Valley, it strongly suggests it is primarily the result of the landslip.

At the foot of the cliff, resting on the bench is an unusual talus (scree) deposit. (ridge D in fig. 2). This is made up of two contrasting materials: large angular blocks of sandstone derived from the Senni beds, and fist and boulder-sized limestone nodules which have come from the Fynnon Formation. In places, running along the outer edge of the talus, a small but distinct ridge can be discerned which represents the furthest limit reached by the talus rocks rolling downslope, a process which still occurs from time to time as the cliff above continually weathers. This ridge is a feature which geologists sometimes rather grandly call a ‘protalus rampart’; there is a much bigger and more complex one at Black Daren.

Black Daren

This large Daren has something of the air of a secret site. Approaching from the east above Longtown, ahead is the seemingly featureless wall of the Black Mountains, and it is only when one drives up the mountain road to the car park (SO 2975 2995), that a much more complex topography is gradually revealed.

Fig. 4. The upper and middle sections of the Black Daren landslip. Image taken from Red Daren, looking south-east.

Look at the photograph in figure 4. Starting at the top, on the right (west) lies the Black Mountain ridge at c. 600m above sea level, and as at Red Daren, is composed of the resistant sandstones of the Senni Formation. The prominent cliff (headscarp), marks the source of much of the material involved in the landslip. Below this, as the great mass of rock moved downhill, it left behind roughly parallel ridges which in some cases are backtilted, such that their steepest slopes face uphill. These are known as ‘counterscarps’ or ‘antiscarps’, and are evidence of some rotational movement within the landslip. Further downhill, the moving mass appears to have become more chaotic, leaving behind a large area of humps and hollows. The boulders here are angular, large, and shattered, and now largely covered in vegetation.

Lastly, below the mountain road and the car park, the finer materials of the landslip are spread out in the form of an apron (see fig.2), whose outer edge is now marked by a distinct line of trees and bushes. The whole landslip can be regarded as a good example of Rock Slope Failure (RSF), in this case largely reflecting forward (translational) movement, but with some rotational elements.

There are also a number of noteworthy features which add complexity to the basic structure of the landslip. First, immediately below the headscarp, are a series of large coalescent talus cones, composed of blocky material which has fallen off the cliff above. Clearly, these cones developed after the occurrence of the landslip. Nowadays they are mainly stable, but one can still observe occasional minor falls of rock and soil which add to the talus.

Another interesting feature is the ‘spur’, which is a solid rock ridge standing proud of the main headscarp, and running parallel to it. A pertinent question here is whether the spur is related to the landslip and consequently is a result of substantial lateral movement. A related point is to ascertain the origin of the valley between the two.  However, although the sandstone beds forming the spur show some signs of minor cambering at the top, otherwise they seem to be largely intact and remain quasi-horizontal in line with the regional dip. Moreover the position and elevation of the Fynnon limestone beds on the western side of the spur are consistent with those at Red Daren, again suggesting that no substantial downslope movement has occurred to the spur. This would imply that the valley has an erosional origin, perhaps starting out as a tension feature which has subsequently been widened out by weathering fluvial processes, and the landslip. It may also have an ancient glacial origin, such as originating as a former marginal meltwater channel, which has been heavily modified. Clearly, further evidence is needed to resolve this geomorphological quandary.

Fig. 5. The pronival ridge at Black Daren, looking north. This ridge is likely to have formed at the foot of a semi-permanent snowbank or static ice body lying below the  cliffs to the left of the picture.

Finally, there is the intriguing problem of the nature of the most prominent depositional ridge (A in fig. 2) and also clearly seen in the two photographs ( figs. 4 and 5). It is largely composed of rocks of similar character to material found in the talus cones. In part, the ridge may have originated as a counterscarp created  in the original landslip, but there is no obvious evidence of back-tilting.  It also seems to extend southward beyond the confines of the main landslip. A likely explanation is that it was built up during the severe conditions of the last Ice Age (the Devensian) when a large semi-permanent snow bank (or static ice body) developed in the lee of the Black Daren. Rather like the way in which cirques in Britain are preferentially orientated, the cliff similarly faces north and east where it receives the least solar insolation, and this would have allowed snow to remain all year. Frost shattered debris would have slid down and moved through the snowbank, accelerating rates of erosion and leading to the accumulation of rocks and debris at the foot of the snowbank , as depicted in figure 6. Collectively, the processes involved are known as ‘nivation’. So, although this ridge can be described morphologically as another protalus rampart, as also seen at Red Daren, in terms of its likely origin it is a ‘pronival ridge’ (literally, ‘in front of a snow bank’).

Figure 6 (left) Diagram illustrating the formation of a pronival ridge / protalus rampart at the foot of a semi-permanent snow bank.  Figure 7 (right): The extent of the Welsh ice sheet (dashed line) in the Late Devensian. The area of the Darens are shown as a grey square in the middle of the map.

It is interesting to note that, at the maximum of the last glaciation (called the Late Devensian) about 22,000 years ago, two large glaciers flowed eastwards from the Welsh mountains. The Wye glacier pressed around the north of the Black Mountains and then spread out over Herefordshire, and the Usk glacier flowed to the south, also not over-riding the Black Mountains (fig. 7). Valleys in the  Mountains, and particularly the Olchon Valley, appears to have remained unglaciated at this particular time, except possibly for one or two minor cirques. Nonetheless, these valleys have certainly been glaciated at some time in the past before the last glaciation, since they tend to have U-shaped profiles, and this has led in places to the over steepening of the valley side slopes.

During the Late Devensian, the Darens, along with the rest of the nearby area, would have experienced an extremely  cold climate which resulting in the existence of permafrost and considerable solifluction (soil flow) on the mountain slopes. A good example of this exists on either side of the Darens. Smooth concave-upwards slope profiles reflecting solifluction stand in stark contrast to the tumbled nature of the landslips (figure 8). The direct effects of this cold environment on the landslips would have been exercised through increased pore water pressures in winter in the finer grained rocks (in this case, the Freshwater West beds) and the creation of elevated ground water tables over permafrost in summer. The likelihood of the occurrence of landslips would have been considerable. The overall situation at the Darens at this time can be summarised as having a strongly increased potential for slope failure already made likely by the lithological set-up and the over steepened slopes.

Fig. 8. In the foreground, smooth soliflucted slopes form the hillside immediately south of Black Daren, and also on Hatterrall Hill in the distance. This is in marked contrast to the broken landscape of the landslips.