EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 15, 15-24 (1990)

A DISTINCTION BETWEEN ICE-PUSHED AND ICE-LIFTED LANDFORMS O N LACUSTRINE AND MARINE COASTS

ROBERT GILBERT Department of Geography, Queen's University. Kingston, Ontario K7L 3N6. Canada

Received 3 August 1987 Revised 21 March 1989

ABSTRACT

Study of a small lake, Second Roach Pond, in Maine, U.S.A. clarifies the distinction between shoreline features created by ice push which occurs on both lacustrine and marine coasts, and those created by ice lift related to tidal action in the marine environment. Ice lifting occurred as littoral sediments were frozen into the lake ice, followed by a rise in water level due to damming of the lake before the nival melt each spring. In the period 1905 to 1969 prominent barricades were constructed by this means. A classification based on this distinction is proposed. Ice-pushed landforms include ramparts, ridges in coarse and fine sediments, stone pavements, ice keel grooves, pushed boulder grooves, and tails off promontories. Ice-lift features include boulder barricades, perched stones, stone garlands, and ice keel depressions.

KEY WORDS Coasts Lake ice Sea ice Ice push Littoral sediments

INTRODUCTION

The action of floating ice on shores may be divided into two categories: (1) processes of ice push, and (2) processes of lifting and rafting caused by changing water levels, especially related to tides in the sea. Ice push is driven by thermal expansion (Wilson et al., 1954), or by winds (Tsang, 1974) or, in rivers, by currents (Mackay and MacKay, 1977; Dionne, 1985). Wind-driven ice push occurs both in lakes (especially larger lakes and those in the Arctic and Subarctic-Washburn, 1947; Weber, 1958; Dionne, 1979) and in northern seas (Hume and Schalk, 1964; Kovacs and Sodhi, 1980). Thermal ice push is reported to occur only in lakes, and is best developed in small lakes of mid-latitude (Zumberge and Wilson, 1953; Pessl, 1969; Wagner, 1970; McLellan, 1971).

Ice lifting occurs during the rise and fall of tides as sediment is frozen into the ice during formation and later released by melting, in some cases after movement in ice rafts formed during breakup (Dionne, 1981; Gilbert and Aitken, 1981; Drake and McCann, 1982; Gilbert, 1983). In some circumstances both processes of ice lifting/rafting and ice push act in concert. But because changing water levels are so much more evident on the sea coast, ice lifting is known only from this environment (Wagner, 1970), although the movement offshore of some marked stones in New England lakes during Goldthwait's (1957) experiments suggests that at least some ice rafting may have occurred here.

Although the action of ice and the resulting ice-pushed ridges on the shores of temperate and northern lakes are well documented, the boulder barricades which are such an important aspect of the northern marine littoral environment have not been reported previously from lakes. The distinction between the processes of ice push, and ice lifting and rafting is made in this work from a study of the occurrence of boulder barricades in a small lake in northern Maine. A classification of shoreline features is proposed on the basis of the difference in processes illustrated in the study.

01 97-9337/90/0 1 0 0 1 5-10$05.00 0 1990 by John Wiley & Sons, Ltd.

16 R. GILBERT

DESCRIPTION

The boulder barricades have formed in Second Roach Pond, a small lake in the headwaters of the Kennebec River Basin in northern Maine (Figure 1). The terrain immediately around the shore slopes gently up from the water at 2 to lo". The drainage basin is thickly mantled with glacial sediment, and an esker extends through the lake from Hinckley Cove on the northwest to the southeast of the lake (Figure 1). The esker consists of poorly-sorted sands and gravels, and appears to be a significant source of the materials on the shore of the lake. Bathymetry was determined with a conventional echo sounder and four shore types were mapped: boulder barricades, ice-pushed ridges, beaches, and marshy shores (Figure 1).

Boulder barricades extend along 6-71 km (47 per cent) of the shoreline. The barricade is a ridge of boulders and cobbles 3 to 15 m wide (Figure 2), standing 1 to 2 m above the surrounding lake bottom, from 5 to more than 100 m from the shoreline at the highest water level (Figure 3). The shape of the barricades and their position with respect to the shore are related to the slope of the shore above about 1.5m depth, and apparently are unrelated to the bathymetry below that depth (in Figure 3 compare profile 5 with the others). Along gently sloping shores (about 2-4") the barricades are large and well developed. Shoreward are lagoons, some open to the lake, others completely enclosed (Figure 2c and d; Figure 3 profiles 2, 5, 10, and 12). At the east end of the lake the barricades are lower and smaller, in some places just emergent above the present water level. On moderately sloping shores (about 4-6") the barricades, while still well developed, are closer to shore. Behind are smaller enclosed ponds (Figure 2b; Figure 3 profiles 1,4, and 7) or marshy areas above the present lake level. On steeper shores (about 6-8") barricades begin to merge with the shore and are transitional between true boulder barricades and ice-pushed ramparts (Figure 2a; Figure 3 profile 6). On the steepest shores (greater than about 8") are found ice-pushed ramparts which cover a further 1-75 km (12 per cent) of the shore of the lake.

The plan form of the shore has little relation to the occurrence of barricades or ramparts. Barricades are as well developed around promontories as in bays, while elsewhere ramparts occur adjacent to barricades in bays and around promontories. In some locations the barricades are steeper on the lakeward side and there is a sharp break in slope with the lake floor (Figure 3, profiles 1, 2, 4, 9, and 12). At several locations large boulders have been pushed shoreward into the barricades. The boulder shown in Figure 4 has a trail 2 m wide and 0.5 m deep in the sandy bottom extending offshore at an angle of 70" to the line of the barricade. The

.-A Boulder barricade 0 Exposed shore below I 5 5 m - Isobaths, interval 2m - Profiles 1-12

Figure 1. Second Roach Pond showing the location of boulder barricades. Profiles 1 to 12 are shown in Figure 3

SHORELINE FEATURES CREATED BY ICE-PUSH 17

18 R. GILBERT

Figure 3. Selected profiles of the shore of Second Roach Pond from echo sounding and Abney level and chain surveys. Locations are shown in Figure 1

Figure 4. Ice-pushed boulder and deformation of the boulder barricade. View is looking east over profile 4 at boulder. Scale extends 1.3 m above the surface

smaller stones of the barricade have been pushed aside and the boulder ploughed into the barricade. These observations suggest the modification of the barricades by ice push.

The barricades are composed mainly of cobbles and small boulders. Offshore and onshore of the barricade the lake bottom is muddy sand with scattered cobbles and boulders on the surface in no recognizable pattern and no concentration along either the present shore or the highest water line. At the east end of the lake where

SHORELINE FEATURES CREATED BY ICE-PUSH 19

there are no barricades, isolated boulders occur in the shallow water, resting on the nearly flat surface of fine sediment in a manner much like the perched boulders of arctic intertidal flats (Rosen, 1979; Gilbert and Aitken, 1981).

The remaining part of the shore (41 per cent excluding Hinckley Cove) is made up of sand beaches (especially along the esker where it emerges from the lake in the northwest and southeast) and marshy shore (especially in the bays at the east and southwest ends of the lake), neither showing evidence of significant ice action.

FORMATION OF THE BARRICADES

The formation of boulder barricades in the sea and in Second Roach Pond is related through the action of changing water level. In the sea the water level change occurs in daily tides, and in Second Roach Pond it is associated with artificial control to aid logging activity and hydroelectric generation downstream. In 1905 C.E., a dam was built at the outlet of Second Roach Pond. There are no records of the natural water level before this date, but features having the appearance of ice-pushed shores (Figure 3 profiles 2,3 , and 1 l), some with a ridge (profiles 8 and lo), occur about 2.7m below the present water level. The present level is maintained 0-66 m above the bottom of the old dam by a second dam recently constructed 300 m farther into the lake (Figure 1). The maximum lake level marked by vegetation change is 1.55 m above the present lake level.

Normal operation of the first dam involved closing of the sluice gate in winter as logs were piled on the lake ice from the land around. During nival melt in spring, the lake level rose as much as 2 m or more. A moat of ice-free water formed around the shore, probably in part from this water level rise and in part from melting of the nearshore ice in shallow water and under the influence of slightly warmer runoff from the nearby land (Figure 6). After the ice had completely melted, releasing the logs, they were rafted to the outlet, the dam was opened and they were flushed down Roach River to First Roach Pond (Figure 1) and beyond. This was repeated during the summer, depending on the availability of water and logs. Normally by October the lake level was near the bottom of the sluice as a result of the practice, where it remained for the winter until the inflow of melt water in the spring (Figure 5). The log drives ended in 1969, and the sluice gate was removed.

From this description a scenario similar to that proposed for the formation of marine boulder barricades (Rosen, 1979) is reasonable. Figure 7a is a model showing the probable conditions shortly after the dam was put into operation, During the low water levels of winter, sediment, including cobbles and boulders, was frozen into the ice now forming 2 m above the natural level of the lake before the dam was built.

metres 16-

I2-

08-

04-

0 -

-04- - .-/

\ 1 -0

--o

Figure 5. Mean and range of water levels in Second Roach Pond 1929-1950 interpreted from data in U.S. Geological Survey (1954). ( 1 ) refers to the lake level when full (1.55 m), (2) to the present level (0 m), and (3) to the bottom of the sluice (- 0.66 m)

20 R. GILBERT

In the spring as the water level rose, some sediment was lifted and floated by the ice and a band of open water formed around the shore (Figure 6). Calculations by Drake and McCann (1982) indicate that even small ice pans several metres in diameter could float most of the stones now in the barricade. However the few largest (for example Figure 4) could only be moved by ice push. As in the marine environment (Rosen, 1979), the ice containing sediment from the 'rafting zone' (Figure 7) probably also was at least partially broken up due to the presence of the sediment and its effect of lowering the albedo of the surface. Although lake ice is weaker during break-up than sea ice because of the different crystal structures (Kindle, 1924), stones from the rafting zone were transported lakeward to the edge of the still-frozen, clean lake ice and dropped as the pans melted to form the nucleus of the barricade (Figure 7b). I t is not known why net drift does not occur shoreward, piling stones near the high water mark, but none has been found in either marine settings (Gilbert and Aitken, 1981; Rosen, 1979) or around Second Roach Pond. As the barricade formed, thermal ice push during winter steepened the outer edge, raising and straightening the ridge crest and forcing the largest stones shoreward into the barricade.

Over ;I period of 64 years from 1905 to 1969, the barricade grew as rafting lakeward and ice push shoreward continued. (As far as can be determined, the dam was operated as described above almost every year during that period.) An estimate ofthe rate of growth can be made from the volume of the barricade and the size ofthe stones in it. From the measured profiles the mean volume of the barricade is 17.4 m3 m- ' and thus the average growth was 0.272 m 3 a - ' m - during the 64 years of operation. The mean size of particles in the barricades is 0-58 m diameter. Assuming the porosity of the sediments is about 30 per cent (Fraser, 1935), then an average of about 2.0 stones was brought to the barricade each year per metre length.

If this rapid development also occurs in the sea, it helps explain how barricades occur on shores such as the Labrador coast (Figure 8) where rates of isostatic sea level lowering are of the order of 0.3 m 100 a - (Andrews, 1970), and thus where long periods are not available for development of littoral landforms at a given level. Contrast this with the statement by Hult (1971) that the 'formation time of ice[-pushed] ramparts may be rather long' (p. 12).

Figure 6. Vertical air photograph GS-DL 299 (U.S. Department of Agriculture) taken on 10 April, 1947 showing position of ice with respect to the shore

SHORELINE FEATURES CREATED BY ICE-PUSH 21

Clean lake Ice Rafting zone

Spr ing r J

Figure 7. Sketch of the method of formation of boulder barricades in Second Roach Pond: (a) at the initial stage, and (b) after the barricade has formed

Figure 8. Coastline at Ford Harbour near Nain, Labrador (56"27' N, 61" 12' W) showing at low tide the boulder barricade similar to those in Second Roach Pond. Mean tidal range is 1.7 m, and spring tide range is 2.65 m, compared to the range of 2.2 m in Second Roach

Pond during the formation of barricades there

The occurrence of boulder barricades in Second Roach Pond shows the necessity of changes in water level associated with freezing and melting, for the barricades have formed only in this lake. Other lakes in the area similar in other respects, except the changing water levels, do not have boulder barricades.

DISCUSSION AND CONCLUSIONS

From the discussion above it is apparent that two distinct processes, ice push and ice lift, operate to create the ice-related features on shorelines, and that lift-related features are normally restricted to the sea because water level changes in lakes are usually inconsequential. The features associated with each are summarized in a conceptual model shown as Figure 9, in which the additional variables of slope of the shore, texture of littoral sediments, and direction of ice push are also seen to affect the forms produced.

In lakes and rivers ice push is created by thermal expansion and by wind and current-driven ice pans. On steep slopes, especially in coarse sediments, ice-pushed ramparts occur (Mansikkaniemi, 1970). On moderate slopes ice-pushed ridges develop. They may be composed of boulders and cobbles (Dionne, 1979), of sand (Scott, 1927) or of fine sediments commonly supported by vegetation as a turf (Buckley, 1901). These landforms, although usually distinguishable, form a continuum on shorelines, and rigorous distinction among them (based on height to width ratio, for example) is usually unnecessary. In the case where water-saturated

22

I .- ..

I L e

R. GILBERT

ICE -PUSHED LANDFORMS

Multidirectional I Unidirectional

Ri& in coarse s8dimnt Rim in fine sediment I Taif from promonfwy

I I

mvement fce keef gmaves and 1

pushed bwlckr grooves

ICE - LIFTED LANDFORMS

Lacustrine barricades Marine borricades f h f a 1

Perched stones

Garlands Ice keel depr8sions

Figure 9. Classification of ice-pushed and ice-lifted and rafted coastal landforms. Drawing by John R. Glew

SHORELINE FEATURES CREATED BY ICE-PUSH 23

sand is frozen for some distance up the beach, the ridge forms not at the edge of the lake ice, but at that point up the beach where the solidly frozen sand gives way to dry, easily deformed sand (Hult, 1971). Thus, small structures such as docks on shore may be moved shoreward undisturbed.

On gently sloping shores pavements often form (Scott, 1927; Worsley, 1975; Dionne, 1985); on soft substrates, ice keel grooves (Wilson et al., 1954; Pessl, 1969; Wagner, 1970) or pushed boulder grooves occur (Goldthwait, 1957; Hume and Schalk, 1964; Mansikkaniemi, 1970; McLellan, 1971). Both are common on rivers where the ‘push’ from currents is more nearly parallel to the shore (Dionne, 1985; Mackay and MacKay, 1977; Guimont and Laverdiere, 1980). Pavements have also been reported from the marine environment where currents associated with tides occur (Hansom, 1983, Jahn, 1977; Gilbert et al., 1984).

The features listed above are associated with nearly undirectional ice push. At the end of promontories or around islands, especially, thermally-induced ice push may push from two directions as expansion occurs in basins on either side of the point or island (Goldthwait, 1957). This may result in the production of a tail out from the end of the end of the land fed by ‘longshore’ transport of sediment by ice pushing at a slightly oblique angle to the shore.

The second ice process, that of lifting, occurs mainly in the marine environment because it is here that daily tides provide the mechanism. Ice rafting and ice pushing may be associated with ice lifting, but they are usually of secondary importance, and in fact, ice-pushed features on marine coasts are generally less common than on lacustrine shores except where exposed to the effects of major storms (Kovacs and Sodhi, 1980). The landforms that are created by ice lifting are boulder barricades (by the mechanism described by Rosen, 1979, and Lauriol and Gray, 1980), and perched boulders. The latters are lifted in each tidal cycle, and set down gently on the substrate as the ice melts. They may occur as isolated boulders, in random patterns or in well- defined ‘garlands’ in shallow water near shore (Gilbert and Aitken, 1981). In fine sediments, depressions associated with ice being lowered on the surface are also common, as are disturbances to the primary sedimentary structures of coastal landforms (Dionne, 1985).

The study at Second Roach Pond described in this paper illustrates that where large ranges in water level occur in a lake, ice-lifted features very similar to the marine forms also occur. However, it is important that water level rise occurs after freeze up, when sediments have been frozen into the ice at the shoreline and therefore may be lifted by the rising ice. The speed with which these features have formed in Second Roach Pond where there was only one cycle per winter of rising water, suggests that their formation on sea coasts with rapidly changing sea level is not precluded.

ACKNOWLEDGEMENTS

Field work was supported with a grant from the natural Sciences and Engineering Research Council of Canada. Mr. A. J. Corson of the Kennebec Water Power Company provided useful information on past logging practice and water levels at Second Roach Pond. Comments by reviewers improved the paper significantly.

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