Glacial Deposition & a Glaciated Upland: The Lake District

Why Glaciers Deposit Material

Throughout its journey, a glacier carries enormous quantities of rock debris — fragments shattered by freeze-thaw, blocks ripped out by plucking, material scraped from valley floors and walls by abrasion, and rock falls from valley sides onto the glacier's surface. All this material is transported within the ice and at its base. When the glacier begins to melt — either at its snout where it reaches warmer temperatures, or at the end of an ice age when the climate warms — it loses the energy to carry this load and deposits it. The speed of deposition and the way water is involved determines whether the resulting sediment is till (unsorted, dumped directly from the ice) or outwash (sorted and layered, deposited by meltwater streams beyond the ice front). Till is the most diagnostic deposit — its total lack of sorting, with clay and boulders mixed indiscriminately, tells geologists immediately that ice, not water, put it there.

Erratics

An erratic is a boulder or block of rock that is completely different from the bedrock on which it rests — because the glacier carried it from a distant source and dropped it when the ice melted. Erratics are dramatic evidence of ice movement. The Norber erratics in the Yorkshire Dales are Silurian greywacke boulders sitting on much younger Carboniferous limestone pavements — the ice carried them uphill from the south and deposited them on the limestone pavement. The limestone pavement has been dissolved away around the protected bases of the boulders, leaving them perched on limestone pedestals up to 50 cm high. By matching erratic rock types to their source quarries, geologists can map the exact direction and distance of ice flow during the last ice age.

Drumlins

Drumlins are smooth, oval-shaped hills of glacial till with a distinctive asymmetric profile: the stoss (upstream) end is blunt, steep and high; the lee (downstream) end tapers gently in the direction of ice flow. They range from a few metres high and 100 m long to 50 m high and over 1 km long. Their exact formation is still debated, but the accepted mechanism is that the moving glacier moulded pre-existing till into streamlined shapes aligned with the ice flow, possibly around a harder core of rock or lodged boulder. Drumlins are rarely found alone — they occur in swarms called basket-of-eggs topography. The Eden Valley in Cumbria has one of England's best examples of a drumlin swarm: dozens of smooth elongated hills rising from the valley floor, all oriented in the same direction showing the southward flow of ice from the Lake District fells.

Types of Moraine

Moraine is the collective term for till deposited in specific positions by a glacier. There are four main types. Lateral moraine forms along the sides of a valley glacier as freeze-thaw debris from the valley walls falls onto the glacier's edges and is carried along. When the glacier retreats, lateral moraine is left as a ridge running parallel to the old glacier margins, high on the valley sides. Medial moraine forms when two valley glaciers merge: their lateral moraines meet and form a dark streak of debris running down the centre of the combined glacier. When the ice melts, medial moraine is left as a low ridge in the middle of the valley. Terminal moraine (or end moraine) marks the furthest point reached by a glacier — material bulldozed ahead and deposited at the snout forms a curved ridge across the valley floor. It marks the limit of glacial advance like a physical bookmark in the landscape. Recessional moraines are smaller ridges deposited at successive positions as the glacier retreats in stages, each marking a pause in the retreat. A glacier's ground moraine is the sheet of till deposited across the whole valley floor as the glacier melts.

Case Study: The Lake District

The Lake District in Cumbria, England's largest national park (2,362 km²), provides a complete glacial landscape — both erosional and depositional landforms are visible at every scale. The central dome of ancient Borrowdale Volcanic rocks provided the source area for multiple corrie glaciers that later merged into a major valley glacier system radiating outwards in all directions.

Erosional landforms: Corries are found on the north and northeast faces of the main fells — Red Tarn corrie sits at 720 m below Helvellyn's eastern face, its backwall rising more than 200 m. Striding Edge and Swirral Edge are the arêtes bordering Red Tarn on either side. The major valleys — Borrowdale, Langdale, Wasdale and Ennerdale — are classic glacial troughs with flat floors, near-vertical sides and truncated spurs. The ribbon lakes that fill their floors are some of England's largest: Windermere (18 km, 64 m deep), Ullswater (12 km), Coniston Water and Wastwater (the deepest at 79 m). Aira Force waterfall near Ullswater drops from a hanging valley left by a smaller tributary glacier.

Depositional landforms: The Eden Valley to the east of the Lake District contains an extensive drumlin swarm — tens of smooth oval till mounds aligned to show southward ice flow. Terminal and recessional moraines cross the lowland valleys at the edge of the national park, marking successive positions of the retreating glacier snouts after the last ice age maximum. Ground moraine covers the valley floors throughout the park — the rich farmland of the Vale of Keswick sits on glacial till. The Bowder Stone in Borrowdale, a large boulder of andesite lava from the Borrowdale Volcanic Group, is traditionally cited as an erratic but is now considered by geologists to be a rockfall from Bowder Crag directly above, dislodged after the glacier retreated.