The Lake District

A glacial landscape in England

[Welcome to the small spate of new subscribers from the USA – due to Geoff Gibson’s Substack I suspect – and from Halifax, UK. To the former – commiserations on the ‘American act of intentional self-harm’ (Noah Smith) overnight. The latter allows me to include a photo of the famous Piece Hall at the end of the post – I suspect one subscriber in New South Wales will like that. But first…

The latest in the series on Glaciation in the British Isles. New subscribers can find previous posts on this topic, and many others, in the archive. This post is a re-working of an article published in 2018 by my colleague in the geography world, Phil Banks. My thanks to him for his permission to post this.]

Figure 1. Cumbria, including the Lake District

About two million years ago, the Lake District was a mountainous area broken by river valleys radiating outwards from the centre - approximately where High Raise is now (Figure 1). A period of climatic oscillations led to a series of glaciations during which ice built up and then flowed out from this central core.

Today, the centre is occupied by a glaciated massif of mountains and ridges intersected by wide, steep-sided glacial troughs, often occupied by ribbon lakes. Moving to the fringe of the Lake District, the fells give way to more muted scenery – low hills and valleys often filled by drumlin suites. Still further out, there are outwash deposits.

Erosional landforms

The central fells of the Lake District are underlain by the 'Borrowdale Volcanics' – very hard lavas and ashes formed in large eruptions about 450 million years ago. It was this rock that formed the pre-glacial uplands, and they make up the highest and craggiest mountains today: Scafell, Helvellyn and the Langdale Pikes. Further north, older rocks make up the rounded massif of Skiddaw; to the south, less resistant sedimentary rocks form the gentler scenery of southern Lakeland.

Prior to the ice advances, the pattern of valleys would have been as they are today but would have been narrow, V-shaped valleys rather than the present-day troughs. The highest peaks would have been only a few metres higher than today since they were above the ice and suffered little erosion, but the valley floors would have been 100 m to 200 m higher.

The role of weathering

Weathering, in the form of frost action and associated nivation processes, occurs in areas that fluctuate in temperature above and below 0°C. Frost action weakens rocks (particularly those which are jointed or have pronounced bedding), facilitating later glacial processes. The rocks on protruding mountain peaks have been broken up by this process before, during and after glaciation to form boulder fields such as can be found on the top of Scafell Pike. Frost action on high valley sides caused unsorted angular boulders to fall on the ice below, with some forming moraines and some finding their way to the base of the glacier and acting as abrasive tools. The frost action process continues today, forming such features as the Wasdale screes.

Erosion

During each glacial period, snow built up on the mountaintops. As in nivation hollows, this accumulation of snow added pressure to its lower layers and caused it to turn to ice; eventually the Lake District was buried under about 900 m of ice. The weight of this ice resulted in a downward flow from this zone of accumulation. The ice flowed down pre-existing river valleys.

Where there are a series of corries that are back-to-back around a mountain, a pyramidal peak with associated aretes can be formed. Fairfield, one of the eastern fells, is an example. It is surrounded by six corries (Figure 2). Some corries have a lip and have filled with water to form a corrie lake, such as Blea Tarn (Figure 3).

Figure 2. Sketch map of the Fairfield massif

Figure 3. Blea Tarn

Glaciers flowing off the mountaintops merged with corrie glaciers in the former river valleys to create valley glaciers, with some being 400 m in depth. These huge masses of moving ice are much more erosive and inflexible than rivers, resulting in the valleys being straightened and deepened. The resultant landforms are glacial troughs - upper Borrowdale in the central fells is a good example (Figure 4). The inflexible moving ice plucked at the valley sides and removed the interlocking spurs of the former river valley to create regularly spaced, steep rock outcrops called truncated spurs. In places where the underlying rock is less resistant than elsewhere, deep hollows were scoured out by abrasion and then filled with postglacial water to form ribbon lakes (e.g. Ullswater). Tributary glaciers with less erosive force produced shallower troughs that did not reach the depth of the main valley. These have been left as hanging valleys.

Figure 4. Upper Borrowdale

The long profile of a glaciated valley shows a series of steps as it progresses from the peaks to the valley floor (Figure 5).

Figure 5. The long section of the Grizedale Valley

Smaller erosive landforms appear in the valleys. Striations are simply grooves left by particularly erosive fragments at the base of the glacier that have scraped against the bedrock. Roche moutonnees are protruding lumps of bedrock found on the valley floor that have been polished by abrasion on the upstream side with a steep, craggy downstream side where they have been plucked.

Deposition

Material dropped directly by glaciers is known as till, which is an angular, unsorted mixture of clay, sand, gravel and boulders. Till gathers to form two main types of landform – moraines and drumlins.

Moraines generally appear as:

· linear features, where till accumulated on the sides (lateral moraine) or snout (end moraine) of a glacier. In the Lake District, the advances and retreats of the ice were particularly dynamic and so these types of moraine are very difficult to identify. One example of an end moraine can be found in St John’s in the Vale, where the moraine at one time trapped meltwater to form a proglacial lake

· hummocky features, such as around Easedale Tarn, or as a blanket deposit over some of the valley floors, formed by ground moraine.

Drumlins are low, streamlined hills whose origins are not well understood. It is thought that they are the result of thick ground moraine deposited by stagnant melting ice being reshaped either by subglacial meltwater or by a reinvigoration of the ice that caused it to begin moving again. There are fine examples of drumlins in South Cumbria, near Kendal.

Fluvioglaciation

Fluvioglacial features can mainly be found in West Cumbria where the West Cumbrian glaciers flowed towards the Irish Sea. There are sand and gravel outwash deposits with some long low eskers, producing a ridge and valley landscape. These deposits have been disturbed by the nearby Irish Sea ice stream or have been overlain by modern sediments. In some places, kettle holes occupy hollows in the surface of the glacial deposits, such as the Sizergh kettle hole, though many, like Sizergh, are filled with more recent peat bogs.

Finally, as promised…. The Piece Hall, Halifax (UK).

Photo: David Redfern