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Glaciation in the Nant Ffrancon Valley
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Glaciation in the Nant Ffrancon Valley

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Glacial features in the Nant Ffrancon Valley, Snowdonia.

Glacial features in the Nant Ffrancon Valley, Snowdonia.

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  • 1. Features of Glacial Erosion The Nant Ffrancon Valley, Snowdonia
  • 2. Hanging Valley – a small tributary valley high above the floor of the Nant Ffrancon valley trough.
  • 3. 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.
  • 4. 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.
  • 5. Roche Moutonnée – asymmetrical outcrop of resistant rock – smoothed and steepened by glacial erosion.
  • 6. The up-valley side has been smoothed by abrasion as the increased pressure causes melting and basal sliding.
  • 7. 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.
  • 8.  
  • 9. Ribbon Lake - Llyn Ogwen, Snowdonia This occupies an area of the trough floor that was over deepened by extending and compressing flow.
  • 10. 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.
  • 11. 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.
  • 12. 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
  • 13. Steep, frost shattered back walls, subjected to plucking during glaciation
  • 14.  
  • 15. 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.
  • 16. Arête - a knife shaped ridge between two corries.
  • 17. Arêtes – are formed as two corries are enlarged back to back. As this happens the ridge between them narrows.
  • 18. Pyramidal Peak - formed where three or more corries intersect back to back, forming a sharp, pointed hilltop between them Mount Snowdon
  • 19.
    • 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
  • 20. - 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)
  • 21. Diffluent Col - a low pass at a valley side cut by ice spreading out over a col from its valley into an adjoining one.