Features of Glacial Erosion The Nant Ffrancon Valley, Snowdonia
Hanging Valley – a small tributary valley high above the floor of the Nant Ffrancon valley trough.
There are a number of hanging valleys on the NE slopes where the Nant Ffrancon valley itself was eroded more deeply as ice flowed in from a number of sources.
Truncated spur – steep/rocky section of the trough where the pre-glacial interlocking spurs of the river valley have been eroded by a powerful glacier.
Roche Moutonnée – asymmetrical outcrop of resistant rock – smoothed and steepened by glacial erosion.
The up-valley side has been smoothed by abrasion as the increased pressure causes melting and basal sliding.
The leeward side is a jagged steep slope formed by plucking of pre-weakened bedrock. As the ice flowed over it, the pressure was reduced and re-freezing occurred resulting in plucking along joints and bedding planes.
Ribbon Lake - Llyn Ogwen, Snowdonia This occupies an area of the trough floor that was over deepened by extending and compressing flow.
Striations – these grooves on the exposed rock of the Roche moutonnée were created by abrasion caused by debris embedded in the base of the glacier.
U-shaped valley (trough) The Nant Ffrancon Valley The once u-shaped valley has been distorted by moraine deposits and a collection of scree from weathering and mass movement.
This armchair shaped hollow situated on the hillside above the Nant Ffrancon valley is a pre-glacial hollow. It was formed due to enlargement by plucking and abrasion as ice moved in a rotational manner under gravity. Ice from here then fed the Nant Ffrancon valley below. Cwm Idwal - Corrie
Steep, frost shattered back walls, subjected to plucking during glaciation
Diffluent col – area cut by ice spreading out over a col from its valley into an adjoining one. Here marks the point where the headwall was breached by ice flowing in from the Llandberis valley to the Nant Ffrancon valley.
Arête - a knife shaped ridge between two corries.
Arêtes – are formed as two corries are enlarged back to back. As this happens the ridge between them narrows.
Pyramidal Peak - formed where three or more corries intersect back to back, forming a sharp, pointed hilltop between them Mount Snowdon
small tributary valley above the floor of the main valley (due to differential erosion) (e.g. Cwm Dyli, N Wales)
- steep-sided valley with flat-floor (e.g. Nant Ffrancon, N Wales) - steep, rocky section of the side of a trough where the tips of pre-glacial interlocking spurs have been removed by glacial erosion (nr trough end in Nant Ffrancon Valley) - resistant rock outcrop with gentle sloping smooth up-valley side and jagged lee-ward side. (e.g. in Nant Ffrancon Valley, N Wales) Hanging Valley U-shaped valley (trough) Truncated spur Roche Moutonnée
- grooves on rock created by abrasion due to debris embedded in base of glacier - Sharp, pointed hilltop between three or more corries (e.g. Snowdon, N Wales) - large armchair shaped hollow enlarged as ice moves in a rotational movement under gravity (e.g. Cwm Idwal, Glaslyn, North Wales) - Knife shaped ridge between two corries (e.g. Grib Goch, N Wales) Striations Corrie (Cirque) Arête Pyramidal Peak Ribbon Lake - long narrow lake occupying an area of the trough floor which has been overdeepened (e.g. Llyn Ogwen, Snowdonia)
Diffluent Col - a low pass at a valley side cut by ice spreading out over a col from its valley into an adjoining one.