Key Terms

📖 Key Terms

corrie (cirque/cwm) — An armchair-shaped hollow in a mountainside formed by glacial erosion, specifically the combination of rotational flow, abrasion (floor), and plucking (back wall). The hollow is over-deepened relative to the lip at its front. Contains a tarn (small lake) after deglaciation. Called a cwm in Welsh, a cirque in French.

tarn — A small mountain lake occupying the over-deepened floor of a corrie after the glacier has melted. Retained by the rock lip and/or moraine ridge at the corrie's front.

rotational flow — The characteristic movement of ice within a corrie, where the glacier mass rotates as it moves — pivoting forward and downward in a curved arc. This concentrates erosion on the floor of the hollow and drives the development of the over-deepened bowl shape.

arête — A narrow, knife-edge ridge between two adjacent corries or glacial valleys, formed when glaciers erode from both sides of a ridge simultaneously, progressively thinning and sharpening it. The classic example is Striding Edge, Helvellyn.

pyramidal peak — A sharp, pointed mountain summit formed when three or more corries erode inward from different aspects of the same mountain, leaving the remaining rock as a pointed peak between them. Examples: Snowdon (Wales), the Matterhorn (Switzerland).

U-shaped valley (glacial trough) — A wide, flat-floored valley with steep, straight sides, carved by a glacier moving through and greatly enlarging a pre-existing river valley. The U-shaped cross-profile (steep sides + flat floor) contrasts with the V-shaped profile of a river valley (sloping sides meeting at a point).

hanging valley — A tributary valley whose floor is significantly higher than the floor of the main glacial valley below. Formed because the smaller tributary glacier lacked the erosive power to erode as deeply as the main glacier. Characteristically marked by a waterfall where streams fall from the hanging valley to the main trough below.

truncated spur — A cliff-like, triangular rock face on the side of a U-shaped valley, formed where a glacier cut through the end of a former interlocking spur. The glacier was too large to deflect around the spur and simply eroded through it, leaving a flat-fronted rock face.

ribbon lake — A long, narrow lake occupying the floor of a glacial trough. Forms where the glacier over-deepened the valley floor by eroding into weaker rock, creating a rock basin that fills with water after deglaciation. Often also retained by a terminal moraine dam. Example: Windermere (17 km, the longest lake in England).

misfit stream — A river flowing through a U-shaped valley that is far too small to have carved the valley. The valley was carved by a glacier; the small stream is all that remains after deglaciation. The oversized valley relative to the tiny stream is a clear indicator of former glacial erosion.

drumlin — A smooth, oval-shaped hill of glacial till, elongated in the direction of ice flow. The blunt, steep end (stoss face) faces the direction from which the ice came; the tapered end (lee face) points in the direction the ice was travelling. Drumlins appear in swarms (drumlin fields) and are used to reconstruct past ice flow directions. Example: Eden Valley, Cumbria.

stoss face — The steep, blunt upstream end of a drumlin, facing into the direction from which the glacier came. The ice hit the stoss face first.

lee face — The gently tapering downstream end of a drumlin, pointing in the direction the glacier was travelling.

erratic — A rock fragment or boulder transported by a glacier from its source and deposited far away, on bedrock of a different rock type. Erratics are used to trace former ice flow paths. Example: Norber Erratics, Yorkshire Dales — Silurian greywacke boulders on Carboniferous limestone, transported approximately 2.5 km by ice.

terminal moraine — A curved ridge of unsorted till deposited at the snout of a glacier at the maximum extent of glaciation. Marks the furthest point the ice reached. Often acts as a dam, retaining a ribbon lake in the glacial trough behind it.

till — Glacial sediment deposited directly by the glacier itself. It is characteristically unsorted (particles of all sizes mixed together) and unstratified (no layers). The unsorted nature of till distinguishes it from water-sorted sediment.

outwash plain (sandar) — A flat, gently sloping plain of sorted, stratified fluvioglacial sediment deposited by meltwater rivers flowing away from the glacier snout. Sediment grades from coarser (near the glacier) to finer (further away). Braided rivers are characteristic. Also called a sandar (Icelandic).

kettle hole — A bowl-shaped depression in an outwash plain formed when a buried block of dead ice finally melts, causing the overlying sediment to collapse. If the hole intersects the water table, it fills with water to become a kettle lake.

esker — A long, sinuous ridge of sorted gravel and sand deposited by a meltwater river flowing within or beneath the glacier (a subglacial river). When the glacier melts, the ice walls of the subglacial tunnel disappear, leaving the river's sediment as a winding ridge. Common in Ireland, Scotland, Scandinavia, and Canada.