Thursday 28th May 2015
Blog post 3 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 3. Landscape processes operate at many different scales. The tectonic, geological, climatic and ecological factors that influence geomorphological processes and movement of mass change with different time and space scales.
Landscapes contain a nested series of landforms of different spatial scale (size), the development of which occurs across a range of temporal scales (time), as illustrated in the area around Solva, Pembrokeshire. The aerial photograph provides an overview of the landscape, showing how low relief, upland surfaces (marked by fields) are dissected by a deep estuarine valley, and end abruptly at a rugged coastline characterised by bays, headlands and offshore islands. At low tide, sand and gravel is widely exposed along the estuary and in pocket beaches (Source: imagery from Google Earth; the image is about 1.5 km across and oriented with south towards the top). The arrows indicate the directions in which the ground-level photographs shown below were taken.
Ground-level photographs illustrating a range of landforms at different scales: top left) the largest scale landforms such as the upland surfaces and valley have developed over very long timescales (many hundreds of thousands to tens of millions of years) as a result of weathering and erosion. During extended intervals of extreme cold in the past, glacial ice and/or large volumes of glacial meltwater likely occupied the valley many times, contributing to punctuated episodes of more rapid deepening and widening; top right) other large scale landforms such as bays, headlands and islands also have developed over relatively long timescales (likely hundreds of thousands to a few million years) as result of weathering, mass failure (e.g. landslides, rockfall) and wave action. At present, wave action is a potent agent of erosion, especially during extreme storm events, but would have been reduced in importance during the extended colder intervals in the past, owing to global falls in sea level. During the last glacial maximum about 20 thousand years ago, sea levels were about 120 m lower and the shoreline would have been located farther south, far from the mouth of the present-day estuary; bottom left) medium-scale landforms such as gravel beach ridges develop over intermediate timescales (years to decades) in response to high-energy wave events, especially those generated during extreme storm events. Major reworking of the gravel ridge occurs when overtopping waves alter the height of the crest and perhaps cause the ridge to migrate a short distance inland. Occasional floods along the river help to maintain a breach in the gravel barrier, and rework some of the gravel seaward; bottom right) small-scale landforms such as sand ripples develop over short timescales (minutes to hours) in response to the vagaries of river and tidal currents. Such landforms form and reform repeatedly, although never replicate exactly the same patterns of crests and troughs (Photos: Stephen Tooth).
Did You Know? One of Wales’s most dynamic landscapes is the estuary of the River Severn. The tidal range in the estuary is the second largest in the world, being as much as 50 feet (approximately 15 m). Tides are funnelled from the Bristol Channel into the estuary, which narrows and shallows rapidly upvalley, commonly creating a large (up to 3 m high) surge wave known as the Severn Bore. The bore normally takes 2 to 2.5 hours to travel around 34 km upvalley to Gloucester. River flows combine with the daily tide and wave action to move large volumes of sediment into, around and through the estuary, and give rise to a range of dynamic landforms over multiple scales, ranging from mud ripples to rock cliffs. Although difficult to quantify precisely, it is estimated that the Severn and its tributaries supply about 1 million tonnes per year of sediment to the estuary, with erosion of intertidal mudflats and rock cliffs possibly supplying another 2.5 and 1.3 million tonnes per year, respectively. This annual total of up to 4.8 million tonnes of sediment is about 160 times the weight of reinforcing steel in the Second Severn Crossing, the M4 motorway bridge that roughly marks the lower limit of the river and the head of the estuary. (Sources: The Severn Bore: a Natural Wonder of the World; Detailed history of the M4 Second Severn Crossing; Severn Estuary Shoreline Management Plan Review: Appendix C).