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Features of Glacial Erosion

The Nant Ffrancon Valley, Snowdonia

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Hanging Valley – a small tributary valley high above the floor of the Nant Ffrancon valley trough.

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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.

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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.

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Roche Moutonnée – asymmetrical outcrop of resistant rock – smoothed and steepened by glacial erosion.

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The up-valley side has been smoothed by

abrasion as the increased

pressure causes melting and

basal sliding.

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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.

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Ribbon Lake - Llyn Ogwen, Snowdonia

This occupies an area of the trough

floor that was over deepened by

extending and compressing flow.

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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.

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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.

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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

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Steep, frost shattered back walls, subjected to plucking

during glaciation

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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.

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Arête - a knife shaped ridge between two corries.

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Arêtes – are formed as two corries are enlarged back to back. As this happens the ridge between them narrows.

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Mount Snowdon

Pyramidal Peak - formed where three or more corries intersect back to back, forming a sharp, pointed hilltop

between them

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- 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

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- 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)

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Diffluent Col- a low pass at a valley side cut by ice spreading out over a col from its valley into an adjoining one.