Glaciations

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6. What materials and landforms are deposited underneath the ice?

Glacial ice scratches and erodes rock over which is passes by abrading and plucking this rock. This can be likened to bulldozing. Bedrock is crushed into either boulders, cobbles or pebbles and sand, silt and clay. A mixture of all of these materials is left behind after the glacier melts, and this material is called till or boulder clay. Boulder clay thus blankets much of the bedrock in Ireland, and makes up the soil and subsoil we all encounter when digging.

Boulder clay deposited in this fashion underneath the ice may take the form of

Drumlins
Crag and tails
Rogen moraine
Ground moraine.

Drumlins are oval shaped hills which are often blunt at the up-ice end and elongated at the down-ice end, with a thinning tail. They are thus streamlined with their long axes in line with ice flow direction. This characteristic shape gave the feature its name which is derived from the Irish droimnin (small, round-backed hill). They occur mostly in clusters, or swarms, and are usually tens of metres wide and a few hundred metres long. Their long axes parallel ice flow direction. Sometimes they have rock cores. In many counties (e.g. Cavan and Monaghan) lakes often occupy poorly drained, interdrumlin areas.

Crag and tails are types of drumlins which are formed when a mass of rock obstructs oncoming ice and boulder clay material collects in the lee of the obstruction, always being smoothed by the above ice. A tapering ridge of boulder clay therefore extends from the crag when the ice retreats.

Rogen moraines are crescentic ridges up to 40 m high and 1 km wide which lie transverse to former ice flow. These arcuate ridges may be up to 15 km long and have their outer limbs bent downglacier. They are therefore much larger than drumlins generally. Drumlins are often superimposed on these huge ridges. Rogen moraines also occur in fields and there may be several hundred ridges in one field. They are common in Ireland and in fact the drumlin belt of Cavan/ Monaghan/ Leitrim is mostly superimposed on a field of Rogen moraine.

Rogen moraines displayed on a Digital Elevation
Model of County Monaghan. The image is approximately
40 km across; ice flow was left to right. Note that
many of the ridges have streamlined drumlins
superimposed on them.
Ground moraine is boulder clay which has little or no surface relief form. The landscape is rolling to gently undulating. In area where much bounder clay has been deposited but drumlins and rogen moraines are absent, chances are that the area is underlain by ground moraine. Examples of areas composed of ground moraine in Ireland are in south County Meath, County Kildare, north County Dublin and County Wexford.

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