Alignment of the drumlins and ice flaw

The ice flow indicated by the drumlin alignment is somewhat complex. Two converging flows are indicated, consisting of a dominant flow from the north-west which meets and diverts a second flow from the north. This flow pattern is substantiated by the striae and roches moutonnées marked on the Geological Survey of Ireland mapsFootnote * of the area (Fig. 2b). Because of the differing flow directions, it is possible to outline zones of drumlins of similar alignment by inspection. These zones are arbitrary but they serve to aid discussion of different parts of the area (Fig. 2). The two zones on the south and west of the lough (zones I and II) are distinguished by geographic isolation rather than by differences in drumlin alignment.

The flow within the zones is quite distinct; in zones I, II and III the flow is similar. Ice moved from the north-west and most of the drumlins trend between 130° and 150°. In zone IV, flow was from between 100° and 130°. In zone V ice moved directly from west to east, and in VII from north to south. Zone VI is transitional between zones V and VII (Fig. 2).

Size and shape of drumlins

Of the total of 873 drumlins recognized on air photographs, 786 were measured; the remainder were not measured as they are either drowned in Strangford Lough or have been so modified by wave action at the present or earlier sea-levels that their true shape is not discernible.

On each drumlin the long and the short axes were measured, and the ratio of the length (long axis) to the breadth (short axis) was calculated. The means and the standard deviations (a) of the axial lengths and the ratios were determined on the Deuce Calculator of the Department of Applied Mathematics, The Queen’s University of Belfast.

• The mean length is 308 m.; σ=95m.

• The mean breadth is 155 m.; σ=41m.

• The mean axial ratio is 2 03; σ=0.56

Table 1 gives the means for the area and for the zones.

Table 1

Reference CharlesworthCharlesworth (1939, p. 262–63) and Reference EbersEbers (1926; quoted by Reference CharlesworthCharlesworth (1939)) collected drumlin dimensions from the literature, and reported that the average drumlin was less than 1 km. long, although the length varied from 30 m. to 15 km. The breadth ranged from 50 m. to 800 m. but was usually less than 600 m.; the axial ratio was from 1:1 to 13:1, the most common being 2:1 or 3:1.

Reference CharlesworthCharlesworth (1939) also measured the drumlins of Ards Peninsula and found a mean length of 420 yd. (384 m.), a breadth of 245 yd. (224 m.) and an axial ratio of 1.7:1. While these measurements differ from those of the present study, they are within the limits of standard deviation of the data.

Drumlin distribution

There are two types of drumlin distribution to be considered. The first is the density of drumlins per unit area on the ground; the second is the spacing of drumlins, that is, the relation of one drumlin to its neighbours.

The density of the drumlins was studied by dividing the 1 km. squares of the Irish grid into four squares of 0 5 km. by 0 5 km. and counting the number of drumlins in each square. The resulting density pattern was then contoured and is shown in Figure 3.

The density pattern on Figure 3 shows that the drumlins are concentrated in bands both perpendicular and parallel to ice flow. In zones I, II and III the ice flow may be considered to have been uninterrupted and the strongest concentration of linear groups is perpendicular to the ice flow, with less developed bands parallel to the flow. The distance between lines is about 3 km.

Fig. 3.

Drumlin density

The flow is disturbed in the northern zones and the density pattern reflects this disturbance. In zone IV the strongest concentration is parallel to both the ice flow and to the zone boundaries, probably as a result of pressure from the Scottish ice moving in from the north and being diverted eastwards (Fig. 3). The pattern in the northern zones (V, VI and VII) is dominantly radial, thus remaining perpendicular to the swinging ice flow.

The drumlins often tend to occur together, usually in en échelon groups. Most, however, (114, or 19 per cent in the present study) occur singly with no obvious relationships to others. Groups of two make up 13 per cent (= 84 drumlins) of the total; the remainder are disposed in groups of up to 40. Only the 733 drumlins readily identified on land were used.

The spacing of the drumlins was studied by measuring the angle between the ice flow (long axis of drumlins) and a line joining the centres of adjacent drumlins (Fig. 4a). This angle, together with the length of the line was used with a traverse table to determine the spacing perpendicular and parallel to the ice flow. The results are presented in Figure 4. The line joining the centres of adjacent drumlins makes various angles with the line of ice flow (elongation of drumlins); the commonest is 0°, the next commonest is 90°, with all gradations between. Angles in the right-hand quadrant (facing the ice flow) are commoner than those in the left (Fig. 4b).

Fig. 4.

The spacing of drumlins

• a. Explanation of measurements. α₁ is the angle between a line joining drumlin centres and the line of ice flow, measured clockwise. α₂ is the same angle counter-clockwise. x is the distance between the centres of adjacent drumlins. y is the distance between drumlin centres measured parallel to the ice flow. z is the distance between drumlin centres measured perpendicular to the ice flow.

• b. Rose diagram of angles α₁, and α₂.

• c. Histogram showing variation in x.

• d. Histogram showing variation in y.

• e. Histogram showing variation in z.

The spacing of drumlins is also variable; the mean distance between drumlin centres is about 335 m. (Fig. 4c). The mean distance between drumlin centres measured parallel to the ice flow is about 240 m. (Fig. 4d) and the mean distance perpendicular to the ice flow is about 176 m. (Fig. 4d). Reference ReedReed and others (1962) found that in New York these distances were 2,280 ft. (695 m.) parallel to the ice flow and 1,540 ft. (469 m.) perpendicular to the ice flow.

Ice flow

The ice-flow pattern established by this study is that of a dominant flow from the northwest. This can only have been Irish ice flowing from a centre over Lough Neagh. The lesser flow of ice from a northerly direction may have come from a Scottish source. This picture is radically different from that given by Reference DwerryhouseDwerryhouse (1923) and Reference CharlesworthCharlesworth (1939), both of whom postulated a dominance of Scottish ice over north-east Ireland.

Dwerryhouse was the first to postulate a strong flow of Scottish ice over eastern Ireland. He reached this conclusion mainly from a study of indicators (chiefly Scottish). There are, however, some contradictions in his evidence. He cited the case of the Scrabo Hill dolerite (Reference DwerryhouseDwerryhouse, 1923, p. 396) which was carried south-eastwards, but he did not comment on the fact that this implies a movement of ice from the north-west. He also stated that the striae everywhere support a southward movement of ice down Strangford Lough; in fact, the striae recorded on the Geological Survey maps indicate movement from the north-west, except in the area north of Newtownards where they do trend north-south.

Reference CharlesworthCharlesworth (1939) reproduced Dwerryhouse’s ice-flow diagrams without change, and his map of the drumlins of north-east Ireland shows a general south-south-easterly trend over the Ards Peninsula/Strangford Lough area. He did not apparently recognize either the east-west drumlins of zone V or the north-south drumlins of zones VI and VII. This may have been due to the difficulty of determining the true elongation of drumlins without air photographs, especially if, as in zones V and VI, the drumlins are not markedly elongated.

The drumlins, the striae and the carriage of Scrabo Hill dolerite all indicate a strong flow of Irish ice over north-east Ireland at least during the drumlin. forming stage of the late glacial period. Scottish ice, if present at all, was present only on the north of Ards Peninsula. This supports the view of Reference Synge and StevensSynge and Stevens (1960) that the flow of Scottish ice over Ireland at this time was much weaker than had been supposed.

Drumlin origin

The typical smooth elliptical form of drumlins is a streamlined shape similar to that of objects designed to pass through a medium (or to allow a medium to flow past the object) with least disturbance (Charley, 1959). The surfaces of such objects (for example, air foils, birds, fish, etc.) are smooth with no breaks to allow eddy formation or to cause drag. Because of this shape and because of the parallelism of long axes with ice flow, drumlins are accepted as forming beneath flowing ice.

Drumlins are one of a group of streamlined forms described as “ice-moulded”. Also in this group are subglacial ridges (Reference DysonDyson, 1952), which are relatively long compared to their height and breadth; subglacial ridges are usually 200 m. long by 0 5 m. high. These forms are produced by an extrusion process, in which plastic, waterlogged till beneath a glacier is squeezed up into grooves in the base of the ice. The grooves are probably formed by a rock knob or a boulder projecting upwards into the ice from the floor. The flowing ice moulds itself to the obstacle, due to the pressure on the stoss side, but it does not flow back into the space behind the obstacle; thus a cavity is left which is filled with till and is gradually elongated as the ice flows on. This process has been observed in the case of subglacial ridges which often have boulders at their up-stream ends. Ice-moulding around an obstacle, leaving a cavity behind, has been observed by Reference CarolCarol (1947) who managed to descend a crevasse and reach a roche moutonnée on the floor of an active glacier. He observed that ice flow in a zone just above the rock surface was much faster than either the main ice flow or the flow in the ice adjacent to the rock. Carol reasoned that this fast-moving ice was made more mobile by pressure-melting and moved from a high pressure in front of the roche moutonnée to a zone of low pressure behind it.

Reference HoppeHoppe (1951) suggested an extrusive origin for drumlins; till is assumed to have been squeezed up into hollows or caves in the base of a decaying glacier, which then became reactivated and overrode the mass of till.

Reference GravenorGravenor (1953), Reference KupschKupsche (1955) and others have found stratified fluvio-glacial materials within drumlins. Gravenor suggested that drumlins formed by a combination of erosion and deposition following a re-advance over moraine and outwash.

Some drumlins consist entirely of rock and are similar to whaleback ridges and roches moutonnées; others have partial or complete rock cores. Reference CrosbyCrosby (1934) found that drumlins near Boston, Massachusetts, were of this type. Such forms must be wholly erosional if entirely formed of rock. When a rock core is present it is usually at or near the stoss end (front) of the drumlin (Flint, [1957], p. 68) and is thus analogous to the boulder at the head of sub-glacial ridges, or to crag and tail.

Reference AldenAlden (1911) and Reference TaylorTaylor (1931) pointed out that drumlins tended to form in places where the ice flow was radial or dispersive. Alden suggested that such flow would contain areas of low pressure that might be favourable to drumlin development.

One feature that might indicate ice conditions is the size and shape of drumlins. Reference WrightWright (1912) observed that in Donegal the largest and most elongated drumlins occurred in areas of strong unidirectional ice flow. This is true of the Ards Peninsula/Strangford Lough area, the smallest and least elongated drumlins being located in zones V, VI and VII (Table 1), where the southward-moving Scottish ice met and was diverted by the stronger Irish ice flowing from the north-west, while the largest and most elongated drumlins are found in the zones where the flow is undisturbed.

One factor that seems necessary to drumlin development is the presence of some obstacle to ice flow (such as a hummock) on the floor of the glacier. This hummock may be a rock knob, a large boulder or a group of boulders, till squeezed up into holes in the basal ice, or outwash fillings in crevasses or moulins. In the case of till or outwash, this material must have accumulated during a period of ice stagnation and then been frozen and overridden by a re-advance of the ice. Rock knobs or frozen drift, when overridden create a pressure difference between the front and the back of the obstacle, the high pressure on the front encourages pressure-melting of the ice, thus making it more mobile. The mobile ice moves from high to low pressure more quickly than its load of debris; thus a zone of debris accumulates near the obstacle and is deposited as lodgment till. Drumlins therefore grow by deposition on the stoss and top sides and continue to grow as long as the pressure due to flow is great enough to cause pressure-melting on the front of the drumlin. This process would be encouraged by pressure differences produced by other phenomena such as those suggested by Alden or Taylor.

The drumlins are densest in bands perpendicular to the ice flow in zones I, II, III and IV (Fig. 3); it is possible that this represents waves in the ice forming belts of low pressure favourable to drumlin formation.