Tuesday 28th April 2015
Blog post 2 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/
Reason 2. Landscape shaping processes are influenced by many different factors. Various tectonic, geological, climatic and ecological factors provide major influences on geomorphological processes and the movement of mass.
While tectonic, geological and climatic factors have been the major factors responsible for the shaping of Wales’s grandest landscapes, ecological factors (plants and animals) also can be an important geomorphological influence. For instance, floodplain vegetation can play a critical role in slowing rates of some geomorphological processes. The photo above was taken after a major flood on the Rheidol River, Ceredigion, and shows how a coherent grass mat on the bank tops and floodplain has helped to protect the underlying loose gravels and sand from widespread erosion (Photo: Stephen Tooth). Floodwaters flowing out of the channel and across the floodplain had begun to peel back the grass mat but the root network helped it to maintain coherence; were the grass and other floodplain vegetation (trees, shrubs) to be extensively removed during larger, more extreme floods, more widespread gravel and sand erosion would ensue, possibly leading to dramatic reshaping of the channel and floodplain. Indeed, studies of rivers in the western United States have noted how loss of riparian vegetation can put the adjacent floodplain in danger of catastrophically eroding during large or extreme floods, a process that can be termed ‘floodplain unravelling’ (Source: Smith, J.D. 2004. The role of riparian shrubs in preventing floodplain unraveling along the Clark Fork of the Columbia River in the Deer Lodge valley, Montana. In Bennett, S.J. and Simon, A. (editors), Riparian vegetation and Fluvial Geomorphology. American Geophysical Union Washington, D.C., Water Science and Application Series v.8, pp.71-85).
By contrast with floodplain vegetation, burrowing animals, such as badgers, moles and rabbits, can significantly increase rates of some geomorphological processes. Across much of Wales, small accumulations of mass moved by these animals as they create setts and tunnels (e.g. molehills) are very common features, but many pass unnoticed in their broader landscape setting. The photos above show the excavated soil associated with a typical badger sett on a Carmarthenshire hillside. The main photo shows three areas of soil associated with sett entrances; in the inset photo, the entrance is approximately 50 cm in diameter and surrounded by loose soil (Photos: Gareth Griffiths). In such cases, animals move mass to the land surface, leaving it subject to further transport downslope by processes such as rainsplash, surface runoff, wind redistribution or soil creep. Recent studies in Wytham Woods in England have recently shown that an individual badger can move up to 4.51 m3 of soil per year. Although the actions of these animals can increase the rates at which mass moves downslope, once stabilised by vegetation or other means, these accumulations of mass can persist as landscape features for decades to centuries (Source: Coombes, M.A. and Viles, H.A. 2015. Population-level zoogeomorphology: the case of the Eurasian badger (Meles meles L.). Physical Geography, v.36, pp.215-238).
Did You Know? While Wales and the rest of the British Isles are generally regarded as tectonically-stable landmasses, tectonic factors have played a crucial part in setting the context for the geomorphological processes that happen in Wales today. In particular, movements associated with ancient, large fault systems (e.g. Menai Strait) mean that northwest Wales experiences many more earthquakes than expected for an area so far from the edges of tectonic plates. Large historical earthquakes include a magnitude 5.4 event whose epicentre was in the northern Llŷn Peninsula, a magnitude 5.1 event in 1990 with an epicentre in the Welsh borderlands, and a magnitude 4.1 event in 2014, the effects of which were felt across Wales and south west England (Source: http://www.museumwales.ac.uk/rhagor/article/2002/). Such earthquakes can contribute to movement of mass, either by gradually elevating or lowering the land surface or by inducing soil creep and landslides, and so alongside geological, climatic and ecological factors, continue to play a role in the shaping of Welsh landscapes.