Glacial systems, the processes of erosion, transport and deposition, and the landforms they produce in glaciated uplands.

What this dot point is asking

WJEC wants you to treat a glacier as a system with a mass balance, explain the processes of glacial erosion, transport and deposition, and account for the erosional and depositional landforms of glaciated uplands, with located examples such as Snowdonia.

The answer

The glacial system and mass balance

A glacier is an open system: inputs are snow and avalanched debris; the store is the ice itself; transfers are ice flow and meltwater; and outputs are meltwater and deposited debris. The upper accumulation zone gains mass and the lower ablation zone loses it, with the equilibrium line between them. During the Last Glacial Maximum around $20{,}000$ years ago, ice sheets up to several hundred metres thick covered most of upland Wales, and it is this ice that sculpted the landforms still visible in Snowdonia (Eryri) today. Glaciers move by basal sliding (lubricated by meltwater) and internal deformation, fastest in the centre and slowest at the bed and margins.

Glacial processes

Erosion works by abrasion (rock debris embedded in the ice scours the bedrock, leaving striations) and plucking (meltwater freezes ice to jointed rock and tears blocks away as the glacier moves). Freeze-thaw weathering above the ice supplies angular debris through repeated freezing and expansion of water in joints. Transport carries material on the surface (supraglacial), within (englacial) and at the base (subglacial). Deposition drops unsorted till when the ice melts, while sorted fluvioglacial material is laid down by meltwater streams.

Erosional landforms

A corrie (cwm in Wales) is an armchair hollow with a steep back wall and a rock lip; two eroding back to back form an arete, such as Crib Goch on Snowdon, and three or more leave a pyramidal peak. A valley glacier carves a straight, steep-sided glacial trough (U-shaped valley) such as Nant Ffrancon, truncating spurs and leaving tributary hanging valleys above it; over-deepened sections fill with ribbon lakes such as Llyn Ogwen. Roches moutonnees show a smooth abraded up-glacier (stoss) side and a plucked, steep down-glacier (lee) side, recording the direction of ice flow.

Depositional landforms

When ice melts it dumps moraine: lateral (along the sides), medial (where two glaciers merge), terminal (marking the maximum advance) and ground moraine. Drumlins are streamlined till mounds aligned with ice flow, with a steep stoss end and a tapering lee end. Erratics are boulders carried far from their source rock and stranded on different geology, for example fragments of distinctive Welsh rock transported by ice. Till sheets blanket lowland areas beyond the uplands.

Examples in context

Example 1. The Snowdon (Yr Wyddfa) massif, Snowdonia. The summit area of Snowdon shows the full erosional sequence. Several corries (Cwm Glas, Cwm Dyli, Cwm Llan) were carved into the flanks by small glaciers during the last glaciation. Back-to-back erosion of neighbouring corries left the knife-edge arete of Crib Goch, one of the sharpest ridges in Britain, and the convergence of three or more corries gives the summit its pyramidal character. Tarns such as Glaslyn and Llyn Llydaw fill the corrie floors behind rock lips. The massif is the classic Welsh case study because every major upland glacial landform can be located within a few kilometres.

Example 2. Nant Ffrancon and Cwm Idwal, Ogwen Valley. The Ogwen Valley contains Nant Ffrancon, a textbook U-shaped glacial trough with a flat floor, steep walls and truncated spurs, carved by a valley glacier flowing north from the central Snowdonia ice. Hanging valleys with waterfalls enter above the main valley floor, and the ribbon lake Llyn Ogwen occupies an over-deepened section. Above it, Cwm Idwal is a hanging corrie holding the tarn Llyn Idwal, with moraines and erratics on its floor. The site, a National Nature Reserve, lets students see erosion (trough, corrie, arete) and deposition (moraine, erratics) in one landscape.

Try this

Q1. Define glacial mass balance. [2 marks]

• Cue. The net difference between accumulation and ablation over a year.

Q2. Explain how a glacial trough is formed. [3 marks]

• Cue. A valley glacier erodes by abrasion and plucking, straightening and deepening the valley into a steep-sided U-shape and truncating spurs.