3.1.4.7, 3.1.4.4 - Local glacial processes and landscape evidence

A relict glacial landscape in Eryri

Cwm Idwal lies in the northern Eryri mountains, south of the Nant Ffrancon valley, in a cool, wet upland setting. Bedrock structure and resistance influenced the relief inherited by former glaciers. The reserve partnership identifies a bowl-shaped corrie, sharp ridges and morainic deposits near Llyn Idwal; Nant Ffrancon provides the broader U-shaped trough with a comparatively small modern river.

The BGS Snowdon geological memoir, published in 1997 for 1:50,000 sheet 119, combines rock and Quaternary field evidence to interpret landscape development. Former ice during repeated cold phases enlarged hollows and valleys. These are relict glacial forms: no glacier excavates the corrie today, and current wet or freezing weather does not establish modern permafrost there.

The setting matters to the enquiry. It connects a small hollow and its deposits to a larger valley system, allowing local-scale observations to test a wider glacial interpretation. The course's enquiry is: how strongly do topography and sediment evidence support different processes in this relict landscape? It has a defined area and evidence types, rather than assuming that every visible feature has the same origin or age.

Connecting processes, landforms and time

Snow accumulating in favourable hollows enabled glacier formation. Rotational flow, basal abrasion and plucking contributed to the corrie floor and headwall, while neighbouring erosion left sharp ridges. A larger valley glacier widened and deepened Nant Ffrancon, truncating former spurs and leaving tributary valleys higher than the main floor. These features form a spatially connected erosional assemblage.

Transport carried debris from slope weathering and basal erosion. Deposition left morainic accumulations, including prominent deposits along the eastern side of Llyn Idwal described by the reserve partnership. Their position links former ice margins to the corrie. However, neither a ridge nor a trough dates itself, and sediment and chronological evidence are needed to distinguish successive cold phases.

After retreat, streams, rockfalls, weathering, soil formation and vegetation altered the inherited landscape. Present frost action can add scree; slope movement can rework deposits. A broad landform may therefore reflect long glacial development, while a fresh scar records a recent event. The correct explanation connects process, timescale, material and form, and distinguishes glacial, periglacial and later adjustment rather than assigning every feature to moving ice.

Published terrain and field evidence

Use the actual terrain resource to examine broad relief, then test its interpretation against geological and field observations. This map is deterministically derived from OS Terrain 50; its contour interval is 100 m, deliberately coarser than the underlying 50 m height-grid spacing.