Monday 30th March 2015
Blog post 1 of 10 about the geomorphology of Wales. Click on images to view larger versions in separate windows. Parallel blog in Welsh at http://hywelgriffiths.blogspot.co.uk/
Inspired by a trip into the mountains of Snowdonia in snowy January, Hywel Griffiths (http://hywelgriffiths.blogspot.co.uk/) and I decided to initiate a Welsh translation of the ‘10 Reasons Why Geomorphology is Important’ brochure (see earlier blog post). A request for sponsorship from the British Society for Geomorphology (BSG) for the translation, design and printing is pending. If successful, this will enable us to target a wider cross-section of the public with the message. To highlight this initiative, in the remaining 10 months of 2015 Hywel and I will be providing examples of Welsh landforms and landscapes that illustrate the 10 reasons, in both Welsh and English. These examples will help to demonstrate how many geomorphological issues are represented right here in Wales. In particular, although some aspects of present and future environmental change pose threats to Welsh landscapes, past environmental changes – including some extreme events – have been responsible for the shaping of some of the country’s most treasured scenery.
Reason 1. Landscapes are shaped by movements of mass. Landforms are shaped by geomorphological processes, which essentially involve the movement of mass ‒ rock, sediment, water ‒ across the Earth’s surface.
The mid Wales uplands have been subject to a long, complex and imperfectly understood history of tectonic uplift, rock weathering, periodic erosion by ice sheets and glaciers, and various slope and valley-floor geomorphological processes. This view of the upper Elan valley (flow from upper left to lower right) in the Cambrian Mountains near the Ceredigion/Powys border, shows how the associated movements of mass have resulted in generally subdued upland topography. Many mountain tops and hillslopes are characterised by rounded, smoothed forms, while valley floors have partially infilled with a mixture of weathered and transported rock (gravel, sand, silt, clay) and peat. Rivers like the Elan are gradually reworking some of the valley-floor deposits, transporting them farther downstream. As can be seen below, within this subdued upland topography, local examples of more spectacular movements of mass can be found (Photo: Stephen Tooth).
Nant Cwm Du, Cambrian Mountains, Ceredigion. This northward draining, left-bank tributary of the Afon Ystwyth (flowing left to right at the bottom of the photograph) is incised into large postglacial landslide deposits. Numerous movements of mass are visible here. Sometime after the valley glacier receded after the Last Glacial Maximum, leaving unsupported valley sides, a landslide moved sediments across the valley floor, forming the large landform in the middle of the photograph (on which trees have grown and some ruined buildings can be seen) and creating the ‘cwm’ at the top of the photograph. Over time, the Nant Cwm Du has eroded this landform, creating a deep and narrow valley and moving boulders, cobbles, pebbles and finer sediment into the main Afon Ystwyth. Within the Nant Cwm Du valley, large floods have transported cobbles and boulders and deposited them as boulder berms on the small floodplain (an example is visible on the left bank of the stream, immediately upstream of the small track). These smaller-scale landforms can provide valuable information on the timing and size of historical flood events (see Foulds, S.A., Griffiths, H.M., Macklin, M.G. and Brewer, P.A. 2014. Geomorphological records of extreme floods and their relationship to decadal-scale climate change. Geomorphology, v.216, pp. 193-207). Other movements of mass are indicated by scree slopes on the steep back valley wall and by small terracettes (or steps) on the valley sides above the banks of the Afon Ystwyth that indicate soil creep (Photo: Hywel Griffiths).
Did You Know? In the UK context, Wales is characterised by high ‘relative relief’, defined as the difference between the highest and lowest elevation for a given area. For instance, the highest point in Wales is the summit of Snowdon in Snowdonia National Park (1085 m), but this mountain is surrounded by deep, steep valleys that descend sharply to narrow coastal lowlands. Other parts of Wales possess similarly high relative relief. For nearly two centuries, though, most attention has focused on the identification of the high-level erosional surfaces (‘denudation surfaces’) that characterise much of Wales. New approaches using satellite data and computational mapping methods show that the country as a whole is characterised by four widespread denudation surfaces at 560–500, 430–370, 245–155 and 100–45 m above sea level. Identification of these surfaces is an important first step towards improved understanding of the geomorphological history of Wales (Source: Rowberry, M.D. 2012. A comparison of three terrain parameters that may be used to identify denudation surfaces within a GIS: A case study from Wales, United Kingdom. Computers and Geosciences, v.43, pp.147-158).