Saturday 29th August 2015
Blog post 6 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 6. Landscapes are archives of the past. Landscapes contain histories of their development that potentially can be deciphered and reconstructed from study of the associated landforms and sediments.
Many Welsh landscapes bear the unmistakable signature of activity by ice sheets, ice streams or valley glaciers during past intervals of extreme cold. Much evidence is derived from erosional landforms, many of which represent large-scale landscape elements. The armchair-shaped hollow of Cadair Idris in Gwynedd (top left) is a classic cirque (‘cwm’) resulting from glacial erosion in the Welsh uplands, while parabolic (‘U-shaped’) valleys such as those near Lake Vyrnwy (top right) and Llanwrtyd Wells (bottom left) in Powys reveal significant widening and deepening by ice that was conveyed from the uplands to the lowlands. The present-day rivers that occupy the valley floors are too small to have carved these large features by themselves and are said to be ‘underfit’. Scoured, striated and streamlined bedrock surfaces are smaller-scale erosional landforms that can provide indications of the nature and direction of past ice movement, and can be found around Cadair Idris (bottom right) and in many other parts of Wales. Other evidence for past glacial activity is derived from sediments and depositional landforms. For instance, low relief, poorly sorted sedimentary deposits that range from mud through to boulders are known as ‘diamicton’ (also referred to as ‘till’ or ‘head’), and can be found in many Welsh landscapes, commonly being well exposed in coastal cliffs. Elevated (up to ~10 m tall), arcuate ridges of poorly sorted sediment commonly represent ‘terminal moraines’ and, where present, have been used to reconstruct the former extent of glaciers in places such as the Nant Ffrancon valley, Snowdonia (Photos: Stephen Tooth).
Besides the landforms and sediments resulting from past glacial activity, landscape histories also can be reconstructed from other forms of evidence. For instance, Borth Bog (Cors Fochno) in Ceredigion is one of the largest surviving raised estuarine bogs in western Europe, and is important not only ecologically but also as an archive of past landscape and wider environmental changes, including the frequency of extreme events (Photo: Stephen Tooth). The lower photo shows part of a core recovered from the margins of the bog, and reveals the transition from grey estuarine clays (lower part of core on the right) to brown peat (upper part of core on the left) that occurred about 6000 years ago in response to rising sea levels (Photo: Henry Lamb). Using a 4 m long core from the bog, variations in the sand content and the bromine deposited from sea spray have been used to reconstruct a decadal-resolution record of late Holocene storminess. Twelve episodes of enhanced storm activity have been identified during the last 4500 years, and these extreme events have been related to changes in ocean and air temperatures and their influences on the intensity of westerly airflow and atmospheric circulation (Source: Orme, L.C., Davies, S.J. and Duller, G.A.T., 2015. Reconstructed centennial variability of Late Holocene storminess from Cors Fochno, Wales, UK. Journal of Quaternary Science, v.30, pp.478-488).
Did You Know? Although there is an impressive array of erosional and depositional evidence for multiple episodes of past glacial activity across Wales, the nature, timing and extent of that activity remains a topic of considerable debate. Even for the last glacial cycle, which peaked at around 20-25 000 years ago, many uncertainties surround the thickness, direction of movement, and extent of ice in places like Snowdonia, the Cambrian Mountains, the Brecon Beacons, and the Gower Peninsula. Ongoing accumulation of field evidence, coupled with increasingly sophisticated numerical models for ice sheet growth and decay (see Numerical Modelling of the Last British Celtic Ice Sheet and Reconstructing the Last Ice Cap Over Wales), should help to resolve some of these uncertainties in the years to come. Past episodes of deglaciation have also left their mark across many parts of Wales; with the decay of ice at the end of glacial cycles, large volumes of meltwater were generated, which contributed to the formation of many lakes in low-lying terrain, as well as the carving of interconnected valley networks, such as in the Cardigan-Fishguard region in South Ceredigion and Pembrokeshire. Following the peak of the last glacial cycle, the global decay of ice sheets and glaciers also resulted in a substantial (~120 m) rise of sea level, drowning many parts of the Welsh coastline and forcing the transition from estuarine to wetland environments in places like Borth Bog. (Source: Glasser, N.F., Etienne, J.L., Hambrey, M.J., Davies, J.R., Waters, R.A. and Wilby, P.R., 2004. Glacial meltwater erosion and sedimentation as evidence for multiple glaciations in west Wales. Boreas, v.33, pp.224–237).