Introduction

Medial moraines will be used here to study the structural peculiarities which affect subglacial and englacial water flow. In temperate glaciers, water accumulates at the base, where its subglacial action is strongly influenced not only by the morphology of the glacier bed but also by the glacial structures imprinted into the ice during its flow over undulating ground.

The connection between glacier flow conditions, the morphology of the glacier bed, and subglacial water in recent glaciers also helps us to understand flow conditions in the Pleistocene ice sheets.

Breidamerkurjökull, Iceland

This large outlet glacier on the coastal side of the southern slope of Vatnajökull flows from a mountainous area near Öraefajökull down to the sandur plain at the coast of the Atlantic Ocean. It consists of the different ice bodies of several feeder glaciers separated from each other in the mountains by nunataks; they ultimately become united to form one large glacier. The different ice bodies of this glacier can be distinguished very easily by their individual medial moraines.

For the present study it is important that Breidamerkurjökull is not affected nearly as much by volcanic activity as the glaciers to the west of it. The extent of that glacier is sufficiently large to develop different subglacial features side by side. Its terminus at the sandur plain has a front of about 16 km. Between 1700 and 1890 the glacier was mainly advancing, though slowly, with halts and short recessions. After 1890 a great retreat began, which accelerated after the first tens of years and continues until the present time. During the time of this recession large areas of formerly ice-covered land became ice-free and now show the imprint of the activity of both glacier ice and subglacial water.

Geomorphological features due to the glacier in the recession area, combined with reports of occasional visitors (Reference ThorarinssonThorarinsson, 1943; Reference EythórssonEythórsson, 1949; Reference TodtmannTodtmann, 1960; Reference PricePrice, 1969), enable the reconstruction of subglacial waterways and their connection with the structural properties of the glacier, and the situation of the medial moraines on its surface (Fig. 1).

Fig. 1.

Breidamerkurjökull, Iceland. 1. mountains, elevation in meters; 2. sandur plain with river beds (1. in action; 2. abandoned); 3. ice edge about the year ….. medial moraine A–G, ice bodies of feeder glaciers I-VI; 4. lake; 5. melt – water outlets (some of them with numbers); 6. undirvarp; 7. esker; 8. drainage system at the contact with the Fjallsjökull succession I-III; 9. supposed situation of subglacial and englacial melt-water channels.

The dating of the 1850–90 stage is from Reference ThorarinssonThorarinsson (1943), of the 1936 stage from Flosi Björnsson, of Kvisker in Iceland, but the later stages have been determined from air photographs taken in specific years.

The outermost moraine ridges are incised by many more melt-water outlets than the inner later stages in the recession area. The disappearance of many of the melt-water outlets was accompanied by an enlargement of the remaining melt-water channels. This is a consequence of the concentration of melt water during the ice recession, a feature which will be found at almost all receding glaciers which had more than one melt-water outlet at their greatest extent.

At Breidamerkurjökull, it is conspicuous that during its recession most of the melt-water outlets in the eastern part (in ice body No. VI, Fig. 1) disappeared at an early stage, whereas in the western part of the glacier where the ice bodies I–V are situated some of the large melt-water outlets have persisted within or near the medial moraines (melt-water outlets Nos. 2–8) until the present time.

But melt-water outlets Nos. 1 and 10 (Fig. 1), which are situated near the western and eastern glacier flanks, have also persisted. Their permanence can easily be understood because they flow through the thinner, more crevassed parts of the glacier, which receive more heat from the adjacent rock slopes. Therefore, glacier flanks are places for the preferential development of subglacial rivers. Almost all large glaciers with their termini on plains have water outlets along their flanks.

Persisting melt-water outlets at the terminus, but far away from the glacier flanks, arise in or near medial moraines. Their positions in relation to these moraines seem to have been constant during the recession since 1890.

A good example of this is medial moraine D−E (Fig. 1), where eskers and abandoned melt-water channels in the recession area can be related to the outflow of melt water at the glacier terminus. Reference PricePrice (1969) has given a detailed description of this area. There are similar configurations of medial moraines and melt-water outlets near medial moraines A–B–C. There is only one large melt-water outlet (No. 5 in Fig. 1) which is not situated within a clearly visible medial moraine, but it is always at a constant distance. Also, the largest outflow from Breidamerkurjökull, the river Jökulsá (No. 8in Fig. 1), is situated at a constant distance east of a medial moraine (F–G in Fig. 1). Reference JónssonJónsson (1955) described the situation during the years 1927–28. At that time, the glacier was still in contact with its outermost moraine ridges near the river Jökulsà, which could not be crossed because it was too turbulent; the glacier was therefore used for crossing. Around the Jökulsá outflow the glacier surface was considerably lowered. This lowering around melt-water outlets is also well known from other glaciers. In Iceland it is called “undirvarp” (Reference AhlmannAhlmann, 1938; Jónsson, 1955). Jónsson explained it as due to subglacial erosion by the melt water, which causes the glacier to sink into the erosion hole.

After 1930, when the ice edge near Jökulsá receded from the outermost moraine ridge, a lake (Jökulsarlon) formed and the subglacial outflow of Jökulsá at the ice edge was hidden below the surface of the lake. However during the recession the undirvarp always remained at the same position in relation to the adjacent medial moraines F–G on the receding glacier terminus. Even until today it marks the position of subglacial erosion below the glacier.

The existence of constant melt-water outlets near the flanks of large glaciers helps to explain the melt-water conditions near medial moraines because they are contact zones between the lateral parts of feeder glaciers within them. There must be a connection between medial moraines and persisting melt-water outlets. It can be understood by a study of their development.

In the junction area between feeder glaciers (Fig 2), behind nunataks or valley slopes, where lateral moraines become medial moraines, the melt water cannot usually flow over the glacier surface because it is inclined to the nunatak or the rock slope. In some cases a lake will form. Crevassing of the ice near rock slopes enables melt water to penetrate into the ice, where most of it finds its way to the glacier bed. The water cannot freeze in a temperate glacier and normally it cannot flow in any direction other than that of the ice flow. If the quantity of subglacial water is sufficiently large, it will form a subglacial river or, if not, it will become trapped in a chamber which moves down-stream with the ice to the glacier terminus. Because new water chambers are always developing in the junction area, they form a row of water chambers in the ice-now direction. When these chambers approach the terminus, increased melting connects them to subglacial or englacial channels.

Fig. 2.

Medial moraines, and subglacial and englacial waterways in a composite glacier. 1. glacier base; 2. ice bodies, separated by nunataks and medial moraines; 3. lateral and medial moraines M1, M2; 4. subglacial and englacial water in chambers and channels; 5. water flow into and out of the glacier.

Subglacial channels also occur at a constant distance from medial moraines. In such cases, the melt water can flow beneath the base of the glacier away from the contact zone of the feeder glaciers, if the slope of the valley floor is sufficiently steep. The water is then trapped at a constant distance from the junction area and flows from there in the ice-flow direction.

Typical Features of Composite Glaciers in Pleistoncene Ice Sheets

One of the main reasons for studying recent glaciers is to seek a better understanding of the fossil traces left by the vast ice sheets of the Pleistocene. However, most of the glaciers studied today are valley glaciers, which correspond primarily to fossil valley glaciers and not to large ice sheets covering lowlands.

Of all the recent glaciers in Europe, Breidamerkurjökull seems to be one of the best examples of a large composite glacier, which flows from the mountains to a lowland area. Its structural properties, acquired mainly in the junction areas between different feeder glaciers, still govern the behaviour of subglacial water in the lowland areas. The ice sheets of the Pleistocene, which moved over hilly ground, acquired a similar structural formation to composite glaciers, and the subglacial water was also directed by these structures.

It makes no great difference whether the hills penetrated to the glacier surface as nunataks or whether they remained covered by ice. Of importance for its influence on subglacial waterways was the structural formation near the base of the glacier.

In this sense, the structural development of composite glaciers can have some similarities to the structures of the advancing ice sheets of the Pleistocene.

In two examples, some features of Breidamerkurjökull are used as models for an explanation of fossil forms.

Münsterländer Kiessandzug - an Esker of the Saalian Glaciation

During the Saalian glaciation the North European ice sheet entered the Münsterland and left drift, ground moraine, and melt-water deposits in many places, but today they are mostly covered by younger sediments.

No long terminal moraine ridges have been left to assist the reconstruction of the ice-flow development. The distribution of erratics led Reference HesemannHesemann (1975) to the conclusion that the ice came from the north-east, crossing the ridge of the Teutoburger Wald. Reference SeraphimSeraphim (1972, Reference Seraphim1973) found several drumlins near Gütersloh (a town in the north-eastern part of the Münsterland) which seemed to have been formed by ice flow from the north-west.

One large esker, the Münsterländer Kiessandzug, remains. This consists of a channel, cut subglacially to a depth of 10–30 m into the Upper Cretaceous marls and limestones, and with a length of about 80 km and a width of 100–700 m. In this channel, a ridge of melt-water sand with some pebbles had accumulated and today at a number of places it penetrates through younger cover sands to the surface (Fig. 4).

Fig. 4.

Münsterl and glacier of the Saalian glaciation, north-west Germany. 1. outermost ice margin; 2. contours of the surface beneath the ice, (a) 100 m intervals, (b) less than 100 m; 3. flow direction of the ice.

The large size of this esker corresponds very well to the large amounts of subglacial water available for (Figure.4) drainage. The water was supplied by an ice-dammed lake east of the Teutoburger Wald ridge. This lake, nourished not only by melting ice but even more so by the discharge of the Weser and Elbe Rivers catchment areas, had been dammed since the time the ice sheet, approaching from the north, met the north-western end of the Teutoburger Wald ridge.

Where the ice surmounted the low hills on the northern border of the Münsterland it was water-free but where it entered the valleys, it was accompanied by large amounts of water at its base. It formed large subglacial channels below the Ems Plain; these channels are now covered by younger sediments and are not visible today. But, subglacial water also entered the valley of the Steinfurther Aa, and on its way south from there developed the channel of the Münsterländer Kiessandzug. At first, the course of the channel, and of the ice flow, was directed towards the south, indicating spreading of the ice body to the east of it. This ice body east of the Münsterländer Kiessandzug entered the Münsterland through the Ems Valley plain. It was more extensive than the ice body entering from the west of the esker.

Fig. 5.

Junction between the Norwegian glacier and the Baltic Sea glacier during the Weichselian glaciation. Jutland (Denmark). The angle at the outermost glacier terminus is well preserved by moraines and the Karup Sandur; the angle of the recession stage cannot be reconstructed, because the terminal moraines of the retreat are far too destroyed. The melt-water river, arising at the head of the Karup Sandur, drained radially over that sandur (away from the ice); during recession it changed to a tangential flow along the retreating ice front of the Norwegian glacier. 1. ice limit; 2. sandur; 3. melt-water outlets and flow direction.

On its way south this esker has some small bends, which may have been caused by the influence of the morphology of the terrain on the ice flow. In the area of the town of Münster, the esker turns to the east, indicating a marked diminution of the eastern ice body. The left flank of this ice body moved along the slope of the Teutoburger Wald ridge, where several incisions in the ridge acted as overflows from the ice-dammed lake behind that ridge. The overflow water caused intense melting of the left flank of the Münsterland glacier.

Yet, another point, a comparison between Breidamerkurjökull and the Münsterland glacier is of interest.

At Breidamerkurjökull, the eskers are not situated in the channels occupied by the main subglacial rivers; the latter are marked by deep erosion to peripheral lakes along the glacier terminus. The eskers are usually situated near the medial moraine D–E (Fig. 1) without large lakes at the terminus, and in a more “peripheral” position to the subglacial drainage and on a subglacial water divide.

The site of the Münsterlander Kiessandzug esker was not the main subglacial water flow below the Münsterland glacier, for even larger channels are situated east of that esker below the plain near the town of Rheine in the Ems Valley. Also, the Münsterländer Kiessandzug is situated in a peripheral position in relation to the subglacial drainage of the Münsterland glacier.

Junction Between the Norwegian and the Baltic Sea Glaciers in Jutland (Denmark) During the Last Glaciation

Similar sequences in geomorphological development, as at the junction between Breidamerkurjökull and Fjallsjökull, took place at the junction between other Icelandic glaciers: Skaftafellsjökull and Svinafellsjökull, and the now separated branches of Heinabergsjökull (Reference ThomeThome, 1972).

On a much larger scale, it also occurred at the junction between the Norwegian glacier and the Baltic Sea glacier of the North European ice sheet during the (last) Weichselian glaciation (Fig. 5).

During its maximum extent, the terminus of the Norwegian glacier and the Baltic Sea glacier formed an angle at their contact in northern Jutland, where one of the largest melt-water rivers from that ice sheet was responsible for the accumulation of the well-known Karup Sandur. When the recession began, the smaller Norwegian glacier left its outermost moraine ridges first. The melt water, escaping from the contact zone, followed and formed deeper river beds between the higher surface of the Karup Sandur and the receding front of the Norwegian ice. Also, the receding glacier front is not well preserved here by moraine ridges, but only melt-water channels show a geomorphological development similar to that at Breidamerkurjökull and other Icelandic glaciers. During the recession, there was wandering of the melt-water channels from a radial drainage pattern in the contact area to the front of the glacier that receded first. A spreading of the Baltic Sea glacier into the flank of the Norwegian glacier can also be assumed. The mixing of boulders carried by. both glaciers (Reference MilthersMilthers, 1942) in that part of the area may also be due to these movements.