Landforms of glacial deposition: till, drumlins, moraines and erratics

Overview

Till sheet, drumlin swarm and moraine, in general terms.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is a till sheet, and what kind of relief does it produce?

   Model answer

   A till sheet, also called ground moraine or a boulder clay plain, is an extensive blanket of till plastered over the bedrock beneath a former ice sheet, ranging from a thin veneer to tens of metres thick. Because it is deposited beneath moving ice rather than piled at a margin, it smooths and infills the underlying relief rather than forming ridges, giving a gently undulating, subdued lowland surface with hollows and no strong grain. It is the most widespread of all glacial deposits and forms the ground surface over much of lowland Ireland and eastern England. Being clay-rich and compacted it is poorly permeable, so the hollows within it are frequently waterlogged and occupied by peat, marsh or small lakes.

2. 2

   What is a drumlin? Describe its shape and size.

   Model answer

   A drumlin is a smooth, streamlined, egg-shaped or whale-backed hill made of till, sometimes with a core of bedrock or older sediment. It is elongated, with a length to width ratio of roughly two and a half to one, and asymmetrical in long profile: one end is blunt, high and comparatively steep, and the other tapers gradually to a low point. Typical dimensions are 5 to 50 metres high and 100 metres to a couple of kilometres long, with the width following from the ratio, so a 500 metre drumlin is roughly 200 metres across and a 2 kilometre one is roughly 800 metres across. The surface is smooth and unbroken, and the internal material is normally lodgement till, dense and compacted with clasts aligned along the long axis.

3. 3

   Describe what a drumlin looks like in long profile and in cross-section, and what an observer standing beside one would see.

   Model answer

   In long profile it is asymmetrical, like a spoon lying face down: the up-ice or stoss end rises steeply to the highest point within the first third of its length, and from there the surface falls away in a long, gently tapering lee slope to merge with the surrounding ground. In cross-section, taken at right angles to the long axis, it is a smooth symmetrical dome, steepest on the flanks and rounded at the crest. An observer would see a low, rounded, grassy hill with no rock outcrop and no sharp break of slope, one end noticeably steeper than the other, usually with a hedged field pattern running across it and, on looking around, a series of near-identical hills all lying the same way.

4. 4

   How is a drumlin thought to form?

   Model answer

   The explanation generally taught is that heavily debris-laden basal ice becomes overloaded and loses the competence to carry all of its material, so it plasters till onto the bed; the still-moving ice then moulds and streamlines that till into a form offering least resistance to flow, often around a pre-existing obstruction such as a bedrock knoll or a patch of older sediment. A related and widely supported mechanism is the deformation and reshaping of a saturated, soft subglacial till layer beneath fast-flowing ice. The formation of drumlins is not fully settled, and other explanations, including erosion of an existing till sheet and deposition by catastrophic subglacial meltwater floods, have been proposed; what is agreed is that they form subglacially beneath actively moving ice.

5. 5

   Explain how a drumlin shows the direction in which the ice was moving.

   Model answer

   By its asymmetry and its alignment. The long axis of a drumlin is parallel to the direction of ice flow, because the ice moulds it into a streamlined shape offering least resistance. The blunt, steep end faced up-ice, into the oncoming ice, and the gently tapering end points down-ice, in the direction the ice was travelling. So an observer walking from the steep end to the pointed end is walking in the direction the ice went. The alignment of clasts within the till confirms it. Mapping the orientation of a whole drumlin field therefore reconstructs the flow lines of a vanished ice sheet and locates the dome it flowed out from.

6. 6

   What is a drumlin swarm, and what is meant by basket of eggs topography?

   Model answer

   Drumlins almost never occur singly; they form in swarms or fields of tens, hundreds or thousands of individuals, all sharing much the same alignment because they were shaped by the same ice flow. Seen from a height or on a map, the repeated pattern of oval hills of similar size and shape lying parallel to one another, separated by low wet hollows, resembles eggs in a basket, which is where the term basket of eggs topography comes from. The drumlin belt across the north of Ireland, and County Down in particular, is a classic example, and such fields typically lie some distance in from the former ice margin where the ice was flowing fast.

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