crag and tail

till

Drumlin in Alberta (Royal Alberta Museum)

Simple model of drumlin form

Drumlin interior exposed in Galway Bay, Ireland. Note the bedded sand and gravel below the cap of till

Partly drowned drumlin swarm in Clew Bay, Ireland. Google earth image.

Drumlin

Drumlins occur widely within the moulded and streamlined scenery of the central lowlands of Scotland. Each drumlin is a small hill, tending towards an egg shape, with its steepest slopes and summit at the up-ice end. Drumlins rarely occur singly, however, and are found in groups or swarms, with the tapered end of each hill pointing in the direction of glacier flow. Together the drumlins give a streamlined, undulating terrain, with intervening boggy depressions which have been termed basket of eggs topography. Drumlin swarms occur within major end moraine systems and so the drumlin is a subglacial feature.

Drumlins vary widely in shape. In the Lothians the drumlins are typically 5-25 m high and 0.1 to 5 km in length. The most elongate are more akin to ribs than eggs. The elongation may relate to the speed and duration of glacier flow and the resistance to deformation of the sediment at the core of the drumlin.

Drumlins have been traditionally regarded as landform formed entirely in till which has been shaped by moving ice. Although large exposures are rare – there is little mineral wealth in the excavation of sticky boulder clay – some drumlins do seem to be composed entirely of till. Many drumlins contain a core of rock and it is clear that drumlins and crag and tails represent a continuum of forms. Whilst the classic drumlin is entirely a depositional form and the classic crag and tail is entirely an erosional feature, most drumlins and crag and tails show evidence of both deposition and erosion. A further complication is that there is growing evidence that drumlins may be formed of materials from more than one period of ice flow. Some drumlins contain a core of sand and gravel; others are superimposed on older glacier bedforms. In the central lowlands, the drumlins align with the direction of flow of the last ice sheet but this is an area where topographic control may have guided successive ice sheets along the same path.

The variety of form and the sedimentology of drumlins suggests that these streamlined bedforms may be convergent landforms, developed by a range of glacial processes. Most drumlins have a capping layer of lodgement till. Those with a core of bedded sediment may show deformation and shearing in these basal layers, with more thoroughly mixed material above. Others, however, show fills of water-lain sediment on the lee side of the drumlins, suggesting the former presence of a cavity at the base of the glacier.

There is probably no unifying theory of drumlin formation. Drumlins with a rock core probably form in a similar way to crag and tails, with the addition of the till tail in the final stages of flow of the last ice sheet. Boulton (1987) has suggested that drumlins cored by rock or stratified sediment have formed by differential erosion and deposition acting across a heterogeneous but generally deformable bed. Deformable sediment, under great pressure from the overlying ice and probably with consequent high pore water pressures, flows around cores of less-deformable rock or sediment. Deposition occurs where water can drain from the bed, such as into permeable sediment. The addition of till takes place incrementally, adding to the height and length of the drumlin.