Hanging Valleys, Truncated Spurs, and Ribbon Lakes

🌊 Hanging Valleys, Truncated Spurs, and Ribbon Lakes

Three further erosional landforms are found in and around U-shaped valleys. All three are products of the same glacial sculpting process, just seen from different angles.

Hanging Valleys — tributaries left behind

A hanging valley is a tributary valley whose floor sits significantly higher than the floor of the main glacial trough below it. To understand why it forms, you need to think about what happens when a smaller glacier joins a larger one.

During the ice age, the main valley glacier was larger and more powerful than its tributary glaciers. The main glacier eroded its valley floor deeply — deepening it by perhaps 200–300 metres. The smaller tributary glacier, with much less ice volume and erosive power, could only erode to a much shallower depth. When both glaciers melted at the end of the ice age, the tributary valley floor was left at a much higher elevation than the main valley floor. The two valleys do not meet at the same level — the tributary hangs above the main valley.

After deglaciation, streams flowing down the tributary valley reach the edge of the hanging valley and fall steeply down to the main valley floor — creating waterfalls. These waterfalls are one of the most dramatic post-glacial features of U-shaped valley landscapes.

UK named example: Watendlath valley above Borrowdale, Lake District. Also: many hanging valleys visible in Lauterbrunnen, Switzerland, where more than 72 waterfalls cascade into the main trough. In Wales: Pistyll Rhaeadr, Wales — at 73 m, the highest single-drop waterfall in England and Wales, where a hanging valley tributary falls into the Afon Disgynfa below.

Truncated Spurs — interlocking spurs cut off by the glacier

Before glaciation, V-shaped river valleys had interlocking spurs — ridges that projected from alternate sides of the valley, forcing the river to wind around them. When a glacier filled the valley, it was too large and powerful to follow these bends. Instead of deflecting around each spur, the glacier eroded straight through the ends of the spurs, cutting them off cleanly. The result is a series of cliff-like rock faces on the sides of the U-shaped valley where the ends of the spurs were removed — these are truncated spurs. They appear as flat-fronted triangular rock faces (triangular facets) set into the valley sides.

UK named examples: visible throughout Nant Ffrancon, Snowdonia, and Borrowdale, Lake District. The triangular facets on the valley sides are the telltale sign.

Ribbon Lakes — over-deepened troughs filled with water

A ribbon lake is a long, narrow lake occupying the floor of a glacial trough. It forms where the glacier eroded the valley floor more deeply in one section — creating a rock basin — which then fills with water after the ice melts. The deeper erosion can occur because of softer or more fractured rock (which erodes more easily), a section where the glacier was thicker and more powerful, or where ice flow was slowed by valley narrowing, causing the ice to press down with greater force.

Many ribbon lakes also have a moraine dam at one end — a ridge of till deposited at the glacier's snout that helps retain the water in the basin. The combination of a rock basin and a moraine dam creates some of the largest and deepest lakes in upland Britain.

UK named example: Windermere, Lake District — 17 km long and up to 67 m deep, the longest lake in England. Also: Ullswater (14.8 km), Coniston Water, and Thirlmere — all ribbon lakes in glacial troughs in the Lake District. In Snowdonia: Llyn Tegid (Bala Lake) — the largest natural lake in Wales, occupying a glacial trough.