Modification of peripheral mountain ranges by former ice sheets: The Brecon Beacons, Southern UK

Abstract

The Brecon Beacons in southern Wales is a large upland area (900 km 2) close to the periphery of the former British–Irish Ice Sheet. The geomorphology of the Brecon Beacons highlands and surrounding areas was mapped using satellite imagery and a digital elevation model (DEM). A fragmentary pattern of glacial lineations is inferred to represent ice build-up in the mountains. A more coherent set of glacial lineations formed by southward ice flow across the area from the Welsh Ice Cap represents recession from the Last Glacial Maximum (LGM). During this phase the Brecon Beacons acted as an obstacle, deflecting the ice around higher terrain. Most of the higher terrain in the Brecon Beacons therefore appears to have escaped significant glacial erosion during both phases of ice-flow due to cold-based ice or low basal ice velocities beneath a local ice cap. Landforms previously interpreted as cirques in fact have v-shaped profiles and lack evidence of major over-deepening. They are interpreted to have been formed primarily by processes other than glacial erosion. The overall impact of glaciation in the Brecon Beacons appears to be limited primarily to the time when the area was over-run by the Welsh Ice Cap and the influence of local glaciation has been minimal. This study has two main implications for landscape evolution in mountain ranges that lie at the periphery of former ice sheets. First, due to the presence of cold-based ice or low basal ice velocities beneath a local ice cap it may be possible to find fragmentary traces of pre-LGM ice flow. Second, the pattern of glacial lineations in and around the Brecon Beacons indicates that peripheral mountain ranges may have deflected ice flow at the LGM, resulting in low basal ice flow velocities and possible frozen-bed conditions over higher terrain which consequently resulted in ineffective glacial erosion within the mountains themselves.