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Solifluction and related processes, eastern Banks Island, N.W.T.
P. A. EGGINTON Terrain Sciences Division, Geological Survey of Canada, Ottawa, Ont., Canada K I A 0E8
AND
H. M. FRENCH Departments of Geography and Geology, The University of Ottawa, Ottawa, Ont., Canada KIN 6N.5
Received January 24, 1985 Revision accepted June 6, 1985
Mean surface soil movement measured over the period 1972- 1983 on eight relatively low angle (< 10°), well drained slopes on eastern Banks Island averaged 0.6 cm/year. Substantial variation in mean movement was recorded amongst the slopes. The variation relates to differences in soil moisture conditions, soil grain size, and the position of the measurement transect relative to the crest or base of the slope. The former two variables and therefore the rate of movement can change rapidly across any given slope, particularly when stripes are present.
Three types of movement are identified in the study area: (I) mud burst and mud flow, (2) classic solifluction, and (3) pluglike flow. The latter occurs along discrete shear planes located both within and at the base of the active layer. The existence of episodic and relatively rapid mass movement on these slopes suggests that long-term monitoring programs are necessary to adequately assess and measure slope movement.
Un mouvement moyen de 0.6 cm/an a CtC mesurC, a la surface du sol, sur huit pentes faibles (<lo0) et bien drainCes de I'est de I'ile de Banks, de 1972 a 1983. L'ampleur de ce mouvement varie considCrablement selon les pentes CtudiCes. On attribue ces variations a des diffkrences de teneur en eau du sol, a des diffkrences du granulomktrie et 2 I'orientation du transect ou les mesures ont CtC prClevCes, par rapport au sommet ou 2 la base de la pente. De plus la granulomCtrie et la teneur en eau peuvent changer brusquement le long d'une pente, surtout si les sols sont striCs. Ces changements affectent directement le taux de mouvement.
Trois types de mouvement furent identifiks dans la rkgion CtudiCe: (1) eruption et coulke de boue, (2) solifluction classique et (3) coulCe-en-bloc. On retrouve ce dernier mouvement le long de plans de cisaillement a la base et a I'interieur du mollisol. La prksence de mouvements de masse intermittents et relativement rapides sur ces pentes requiert des ttudes de longue durCe afin de les Cvaluer et de les mesurer de faqon satisfaisante.
Can. J. Earth Sci. 22, 1671 - 1678 (1985)
Introduction Solifluction is commonly regarded as one of the most wide-
spread processes of soil movement in periglacial regions. Andersson (1906, p. 75) first used the term to describe the " . . . slow flowing from higher to lower ground of masses of waste saturated with water." In regions underlain by perma- frost, the slow movement of soil downslope involves both gelifluction (flow) and frost creep (a rachetlike movement, Washburn 1980, pp. 192-201). Since gelifluction and creep can occur simultaneously and since creep is difficult to measure directly, the authors prefer to use the general term solifluction to embrance both processes.
In periglacial environments, solifluction may be the domi- nant mass wasting process on a year to year basis on a given slope, but episodic and more rapid processes may be of con- siderable importance in terms of total mass movement. As a consequence, long-term studies are essential if the nature and significance of mass wasting processes are to be determined. This paper presents observations and measurements of solifluc- tion and related processes on slopes on eastern Banks Island over the 11 year period between 1972 and 1983.
The study area A field camp was maintained on the hummocky morainal
terrain of eastern Banks Island (72O5 1 'N and 1 19O3 1 'W) during the summers of 1972 and 1973, at which time instrumentation was installed (Fig. 1). Brief visits to the sites were made in 1977 and 1980, followed by a detailed survey in 1983. FIG. 1. Location of the study area on eastern Banks Island.
The Quaternary history of this part of Banks Island is com-
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CAN. J . EARTH SCI. VOL. 22. 1985
RG. 2. Typical slopes in the study area: ( a ) slope F, dominated by vegetated stripes 1.5-2.0 m wide (GSC-204117-C); (b) slope D, dominated by flat-topped to slightly rounded hummocks 0 .5-0 .8 rn in diameter (GSC-203502-V); (c) slope G, a fairly well vegetated slope with poorly developed hummocks (GSC-203 165-V).
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EGGINTON AND FRENCH 1673
Method
MOVEMENT (em) MovEMemr (em)
FIG. 3. Histograms showing the percentage of stone targets moving various distances over the period 1972 - 1983.
plex and is discussed in detail elsewhere (Vincent 1982, 1983). The area was last glaciated in pre-late Wisconsinan times. Con- sequently, the primary landscape has been subjected to mass wasting for tens of thousands of years at least. The near-surface sediments are ice rich, and both active and inactive thermokarst forms are common in the area (French and Egginton 1973; French 19746).
Eight low- to moderate-angle (3-8") slopes located on the edge of two lake basins, which at one time were part of an extension of proglacial lake Ivitaruk, were chosen for detailed study. The instrumented transects occur at roughly the same elevation (85 k 15 m asl). On the upper slopes beach sands up to 15 cm thick overlie Jesse till of the Amundsen Glaciation, but on the middle and lower slopes the sands are locally absent or they have been incorporated into the underlying till. All measurement transects but one are located within 100 m of an inteffluve, and as such the studied slopes are relatively dry. Vegetation cover is variable but is typical of the mid-Arctic and is dominated by Dryas tundra and Dryas barrens complexes (S. A. Edlund, personal communication, 1983). Patterned ground phenomena occur widely in the form of stripes and tundra hummocks (Fig. 2). On these slopes active layers are typically 90 cm deep.
Two marker rods, spaced 15-30 m apart, were driven into frozen ground along a contour on each of the eight slopes in early August 1972. A line was run between the two rods, and painted marker stones 1.5-2.5 cm in diameter were placed at random along these transects to measure surface displace- ments. Aluminum foil strips (2 cm by 10 cm) were buried along each transect and oriented with the long axes parallel to the transect such that the top of the strips were at the ground surface and the bottoms at 2 cm depth. These strips provide a measure of surface or more precisely near-surface movement. In some instances, additional foil strips were buried in pits at various depths to provide data on subsurface movement.
When the aluminum foil stripes were installed, surface and near-surface sediments were-examined and a representa- tive sample for grain-size analysis was collected from each transect. On the basis of the visual inspection, the sediments were generally uniform across each 15-30 m transect. Only when vegetated stripes were present (Fig. 2a) was a significant difference in grain size observed. In these cases, a representa- tive sample was collected from both the vegetated and unvege- tated of the transect.
Movement of both types of target (marker stones and foil) were measured relative to the line run between the two marker posts. The technique is the same as that used and described in some detail by French (1974~) and similar to that described by Mackay (1981), who used wooden pegs as targets. Although the technique is crude, the precision of the measurement is +5 mm. In this case the installation provides a means of ob- taining a measurement of mass movement to depths of approx- imately 70 cm. Because the marker rods were only driven to a maximum of 120 cm depth, and more typically 10- 15 cm into permafrost in 1972, accurate measurement of deeper seated movement if and when it occurs is not possible. On the other hand, at the time of installation it was anticipated that measured solifluction (i.e., creep and gelifluction) would rapidly ap- proach zero at depths of 70 cm or less (Benedict 1970; French 1974a; Price 1973; Rudberg 1962, 1964; Washburn 1980).
Surface movement Stone movement 1972 - 1983
A summary of the measured displacement of marker stones on each of the eight slope transects bver the 11 year period July 1972 to July 1983, along with other pertinent data, is presented in Table 1, and histograms of target movement are present in Fig. 3. There was a significant range in mean target movement amongst the slopes. The greatest movement (14.7 cm) was recorded on transect D, which had the highest percentage of silt and clay, whereas the smallest movement (2.9 cm) was recorded on transect G, which is of similar angle to D but with a relatively low percentage of fines (Table 1). The differ- ence in movement is not due solely to grain-size considerations; soil moisture conditions are quite different at the two sites. Transect D is situated immediately below an area covered by a fairly thick snowbed for much of the early summer. As a result, this transect is consistently wetter than the other tran- sects. Transect G, on the other hand, is more exposed and dries rapidly in early spring, and by late summer the surface is desiccated and cracked.
The range of marker movement on each of these slopes is also quite different (Fig. 3). The majority of the recorded movements on slope G clusters about a single class interval (2-4 cm), whereas on slope D the distribution is distinctly
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CAN. J. EARTH SCI. VOL. 22, 1985
TABLE 1. Summary of stone movement 1972- 1983, eastern Banks Island
Mean total Slope movement Standard Percent angle 1972- 1983 deviation Percent silt and
Transect Orientation (deg) (cm) (cm) Observations clay clay
A North 7 9.9 3.3 8 22.4 58.6 B North 6 3.1 2.8 26 24.9 61.5 D Northeast 7 14.7 4.3 36 11.7 73.0 E Northeast 2 3.9 2.0 33 10.7 63.6 F South 6 7.6 2.3 19 20.2 53.4 G Southwest 8 2.9 2.0 30 22.1 58.4 H West 5 3.8 2.6 59 27.3 68.4 I East 6 5.5 4.3 48 12.7 59.3 B , North 6 3.8 3.6 18 24.9 61.5 B, North 6 1.8 1.6 8 7.0 36.9 Fs South 6 8.0 1.7 14 20.2 53.4 F, South 6 5.4 2.6 5 - A
NOTES: The subscript s denotes bare mud surface; v denotes vegetated surface.
bimodal. Unimodal histograms covering few class intervals are indicative of fairly uniform conditions across the slope. Bimodal and (or) broad-based distributions (e.g., D and F, Fig. 3) suggest more than one style of movement and (or) variable conditions across a slope. Some of the possible reasons for the variability in movement are examined below.
In the study area, the postion of the transect with respect to the crest and the base of the slope is of some importance. For example, transects A and B are located on the same slope, but A is located about 50 m from the hill crest, whereas B is located 30 m farther downslope. The average total surface movements for A and B over the 11 year period were 9.9 and 3.1 cm, respectively (Table 1). Transects D and E are similarly situated: D is located 50 m from the hill crest, and E is located near the base. Surface movements for D and E were 14.7 and 3.9 cm, respectively. It is clear that on these two slopes movement is greater in the upper to midslope area than near the base of the slope.
Not only are rates of movement variable from slope to slope and at different positions on a slope, as discussed above, but also they vary over short distances along individual transects. The grain-size distribution, vegetation cover, and patterned ground form are relatively uniform across transect I (Table 1). Despite this, the displacement of marker stones from their original 1972 position to their 1983 position is markedly differ- ent from one side of the transect to the other (Fig. 4). The north side of the transect (0- 15 m) moved substantially farther than the south side (15-25 m): mean values are 8.7 and 2.7 cm, respectively. This variability is attributable to local soil mois- ture conditions. A small ridge running across the slope about 10 m upslope from the transect collects drifting snow and funnels surface runoff to the north side of the transect.
In this area, the variation in surface movement across a slope is more pronounced and regular where stripes are well devel- oped. The stripe pattern is given by vegetation; however, the pattern also reflects a change in sediment across the slope. Transects B and F are covered by stripes (Table I; Fig. 2). The bare surfaces comprise colluvium derived primarily from till, whereas the vegetated parts of the stripe are underlain by col- luvium derived from lacustrine sands. Surface movement in the sandy, well vegetated parts of the transects was only 50-70% of that recorded in the adjacent unvegetated parts (cf. B,, V,,
and F,, F,, Table 1). However, there is sufficient overlap between the recorded movements on this transect that the histo- grams of marker movement are unimodal (B and F, Fig. 3).
Although considerable variation in soil movement exists between slopes, reflecting differences in a variety of slope conditions, the data presented are not suitable for multiple regression analysis. This is primarily because some of the crit- ical variables are difficult to quantify and (or) because of the difficulty in collecting continuous data over an extended time period (i.e., I 1 years).
Short- versus long-term movement Many workers have estimated rates of soil movement by
repeated surveys of marker stones or other targets. Although the method is simple, the question of whether the rate of tracer movement can be equated with soil movement often arises. Particles of different size move at different rates, and marker stones of any one size may or may not be equated with mean surface movement (Carson and Kirkby 1972, p. 203). An addi- tional problem is that tracers often sit proud on the ground surface and are not initially incorporated into the soil itself. Thus, if marker stones are used, in the short term they may move downslope more rapidly than local stones that form part of the surface matrix. In the short term (i.e., 1 -2 years) this may introduce a positive bias as high as 300% in rates of surface movement (Caine 198 1).
To test for this bias, surface stone movement was measured on three slope transects E, F, and I (previously described in Table 1) 1 year after the initial installation. The average dis- placements were 0.2, 2.1, and 2.5 cm, respectively. If these values are compared with the mean annual surface displace- ments over the 1 1 year period 1972- 1983, derived by dividing the mean total movement given in Table 1 by 1 I, the values are quite similar for transect E but the short-term values are 310 and 500% greater than the longer term values for F and I, respectively. Whether or not the higher short-term values result from a bias in marker movement, as suggested by Caine (1981), is difficult to assess. Summer 1973 was significantly warmer and wetter than other years during the measurement period, and the higher rates of surface marker displacement in that year may well be consistent with the prevalent climatic and soil conditions. In either case, the implication is the same:
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EGGINTON AND FRENCH
1
0 ' 1 1 N I 1 6 10 16 20 26
Horizontal distance along the contour (m)
FIG. 4. Stone target movement recorded along transect I between 1972 and 1983.
Movement of foil targets (cm)
I FIG. 5. Graph showing the relationship between the movement of stone and foil targets located within 0.25 m of each other on the same slope.
short-term measurements of marker movement may not pro- vide an adequate measure of average annual slope movement.
Foil strips buried at 0-2 cm depth used in this study cannot be displaced downslope more rapidly than the soil mass itself. Foil target displacement, at a depth 0-2 cm, over the period 1972- 1982 is compared with the displacement of the nearest surface marker stone, typically located less than 0.25 m away (Fig. 5). The readings are taken from all eight study slopes and cover the range of displacements measured. There is quite good agreement between both types of marker movement. This im- plies that if the movement of the marker stones is biased rela- tive to the foil markers this effect is diminished in the long run. It is concluded, therefore, that the 1972- 1983 data on sur- face stone movement are representative of the relative surface (0-2 cm depth) soil movement on the study slopes.
Styles of movement On the basis of surface and subsurface measurements and
pits dug across the study slopes, three types of movement are recognized in this area: (I) mud burst and mud flow, (2) classic
solifluction, and (3) pluglike movement. Movement is classed into these three styles for convenience only. Mass movement phenomena form a continuum from the very slow to the very fast, and evidence of all of the above types of movement may be found on any one slope.
Mud burst and mud pow On some slopes, surface materials appear to liquefy and
flow for some distance downslope before losing moisture and consolidating. The snouts of these mud flows have a character- istic transversely ribbed surface (Fig. 6). The mud flows are episodic in nature, as the mud surfaces are found in various stages of revegetation. New (i.e., more recent than 1972) mud flows were not identified on the study slopes in 1973, 1977, or 1983, and the process did not contribute to the downslope displacement of marker stones on the measurement transects. Nevertheless, the process does contribute periodically to mass movement on these slopes.
In this area, the mud flows can often be traced 5- 10 m upslope to where they originate as a mud burst from under 15-30 cm of lacustrine sand. Although the longer mud flows are more spectacular, smaller flows 1-2 m in length are more typical. The mud surface may or may not revegetate before burial by another mud flow. In cross section, this produces a series of parallel to subparallel sheets or lobes up to 30 cm thick (Fig. 6). Mud bursts and (or) injection structures are not un- common and have been reported elsewhere on Banks Island (French 1976) and in other areas (i.e., Shilts 1978; Egginton and Dyke 1982). In this area, mud bursts occur only on the upper or mid parts of the slope, near the base of snowbanks. The liquefaction of the mud and its movement to the surface could result from positive differential pore-water pressures alone; however, differential soil movement on these slopes may also determine the location of these failures. In an earlier section, attention was drawn to the fact that the upper slopes moved three or more times farther than the lower slopes over the period 1972- 1983 (cf. A and B or D and E, Table 1). This situation requires that the upper slope shear and ovemde the slower moving lower slope. The zone of shearing may favour periodic mud burst and mud flow. Whatever the precise mech- anism, this type of movement is rapid and it is distinct from,
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1676 CAN. 3. EARTH SCI. VOL. 22, 1985
FIG. 6. Snout of a mud flow in the study area (GSC-203165-Y). The shovel locates the upper surface; the buried organic layer (middle arrow) is at the same elevation as the vegetation in front of the feature (arrow at left). On the basis of the continuous organic layer, the mud flow is about 30 cm thick at the snout.
although possibly related to, solifluction. As the muds origi- nate at depth, the movement must be a mid- to late-summer phenomenon.
Classic solifluction Frost creep and gelifluction are dependent upon ice segre-
gation in the active layer, its distribution, and the resultant heave. Ice distribution within the active layer determines both the depth and timing of movement. On some transects (e.g., F, Table I), foil markers at or near the surface were displaced farther than those buried progressively deeper in the active layer. This type of velocity profile, concave downslope, has been commonly reported in the literature (e. g . , Benedict 1970; Price 1973; Washburn 1980). It is associated with shearing and may result in material at depth being moved progressively upward as it moves downslope. A section in the vicinity of transect F illustrates this phenomenon (Fig. 7). Here, organics and beach sand are carried upward as a sheet of colluvium 60 cm thick, derived primarily from till, moves over the sands. The path taken by the sands and organics indicates that the upward component to movement can be equal to the downslope component. At this site, clean sand occurs at depths of 60 cm and extends beyond the bottom of the active layer, effectively prohibiting seasonal ice segregation below 60 cm.
(1981) found that this type of movement occurs along the base of the active layer on slopes on Garry Island. On eastern Banks Island, field evidence indicates that pluglike flow occurs not only at the base of the active layer - permafrost boundary but also along discrete zones within the active layer.
The surface of transect D is dominated by hummocks 50- 80 cm in diameter (Figs. 2 b; Table 1). Excavations on this slope in 1983 indicate that cracks extend from the borders to depths of 25 cm and more, penetrating organic layers that underlie the hummocks (Fig. 8). The upper organic layer, at a depth of 15 to 20 cm, is discontinuous beneath the hummocks. The organics trail off in an upslope direction, giving the im- pression that the hummocks slide downslope as discrete plugs 15 cm thick while surface organics are subducted and over- ridden. Support for this interpretation comes from two addi- tional pieces of field evidence. (1 ) Matches placed in a vertical position in 1972 around the periphery of one hummock in the middle of transect D were still in place in 1983 despite a mean displacement of surface stones of 14.7 cm over this period (Table 1). For the form to be preserved so exactly, flow must be pluglike along a discrete plane or planes at some depth within the active layer. (2) Several straight-sided pits were dug into the slope in the vicinity of transect D in 1972. By 1983, hummocks were moving along an organic layer and into the pit as discrete units 15-20 cm thick (Fig. 9). The pluglike move- ment was particularly pronounced at the back or upslope edge of the pit. Detailed data are lacking, but ice lensing is common in the upper active layer; this type of movement is probably related to the preferential development of ice lenses at a depth of 15-20 cm.
Casual observation of other slopes in the area indicates that pluglike movement can occur late in the summer along the base of the active layer. On one slope of 6" underlain by fine sands and silts, midsummer borings in 1973 revealed excess ice con- tents of 45% by volume at or near the base of the active layer and in the upper 50 cm or more of permafrost. In mid-August 1973, surface displacements of 20 cm were recorded on this slope in one 24 h period. Because the zone of failure was along the base of the active layer, the hummock surface form was retained. In fact, tagged hummocks were still intact when the site was revisited in 1983.
An examination of the records for Sachs Harbour on south- em Banks Island, the nearest permanent climatological station to the study area, indicates that on the basis of the length of thaw season and the number of thawing degree-days summer 1972 was an average or typical year, whereas summer 1973 was atypical in that it was one of the warmest on record. In the study area, measured active layer depths were up to 30% greater in 1973 than those recorded at the same sites in 1972. The pluglike movement recorded in 1973 was associated with permafrost degradation.
In central Keewatin, Egginton (1981) has described a plug- like movement of thin sand and gravel pads that overlie till. Depending upon the pad thickness, the movement is associated with either annual or epsiodic thaw of the ice-rich till. Mackay (1981) attributed pluglike movement at or near the base of the active layer on Garry Island to the thawing of an ice-rich basal zone formed primarily by winter upfreezing and augmented by downward migration during the thaw period (Mackay 1983). In
Pluglike flow other words, the movement is interpreted as a more or less Laboratory experiments have demonstrated that pluglike annual phenomenon. Total movement is dependent upon the
flow results along discrete planes when bands of segregated ice ice content of the soil and its distribution. If the upper perma- in the soil column thaw (Rein and Burrous 1980). Mackay frost is ice rich, it seems likely that the most extensive displace-
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EGGINTON AND FRENCH 1677
FIG. 7. A cross section in the vicinity of transect F. Note the upward movement of sands and organics towards the surface.
FIG. 8. A cross section in the vicinity of transect D. Note the pluglike movement of the hummocks along an organic layer located in the upper active layer.
ments will occur during unseasonably warm years when thaw extends beyond last year's active layer (i.e., the ice-rich basal zone) into the ice-rich upper permafrost. On this basis, pluglike movement may be quite variable on a year to year basis and (or) episodic in nature. Any attempt to separate pluglike movement at the base of the active layer from that occurring at the top of the permafrost table is not practicable.
Summary and conclusions On the hummocky morainal topography of eastern Banks
Island, mean annual surface displacements on eight slopes ranged from 0.3 to 1.3 cm/year over the period 1972 - 1983; these mean values averaged 0.6 cm/year. These values are similar to, although lower than, those reported earlier by French (1974a, 1976) on similar slopes on southern Banks
Island. The study slopes are representative of those found in the study area: the values reported reflect the location of the mea- surement transects on relatively well exposed and drained slopes and the nature of the materials.
Considerable variation was found amongst measurement transects, reflecting amongst other things differences in mois- ture availability, soil grain size, and the relative position of the transect on the slope. On individual transects, relatively subtle differences in soil moisture are associated with substantial dif- ferences in the rates of movement. Striping, where it occurs, is related to differential movement: the finer grained, unvegetated parts move twice as rapidly as the coarser, well vegetated parts. Segregated ice formation and its distribution within the active layer are critical to some styles of mass movement.
Three styles of movement were identified on the study slopes; for convenience we term them mud flow, "classic soli- fluction," and pluglike flow. On the basis of form, the study slopes are covered by solifluction sheets (Harris 1981) or gelifluction sheets (Washburn 1980) and in one instance by solifluction or gelifluction lobes. This terminology is not nec- essarily consistent with the processes involved. All three types of movement identified may occur on any one slope in any one year. Slopes dominated by classic solifluction for a number of years are not necessarily subject to a slow, regular, uniform displacement of material downslope over a longer time frame. Such movement may be punctuated by relatively rapid episodic movement including mud flow and (or) pluglike movement along the base of the active layer.
Pluglike movement on slopes may be more common than previously recognized. Zones within the upper active layer that favour the development of ice lenses can form the horizon along which pluglike movement occurs, as proposed by Rein and Burrous (1980). The ice-rich active layer - permafrost interface may provide another such horizon (Mackay 1981). If permafrost degrades in response to climatic fluctuations, as it did in 1973 on eastern Banks Island, movement may be partic- ular rapid. In the long run, episodic movement may be quite significant to overall slope development. On the basis of our
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1678 CAN. J . EARTH SCI. VOL. 22, 1985
FIG. 9. A straight-sided pit dug in 1972 and photographed in 1983 (GSC-204117-B). Hummocks are moving as discrete units 15-20 cm thick into the pit.
work and other recently published work (e.g., Mackay 1981), we conclude that short-term monitoring studies d o not neces- sarily provide an adequate measure of average slope movement o r a comprehensive description of the processes involved.
Acknowledgments Field studies have been supported by the Geological Survey
of Canada; Polar Continental Shelf Project (Energy, Mines and Resources Canada); the Natural Sciences and Engineering Research Council of Canada; and the University of Ottawa Northern Research Group. Helpful comments on an earlier draft of this paper were provided by D. A. Hodgson, J. A. Heginbottom, and D. G. Harry. Assistance in the field was provided at various times by G. Gruson, J . Lai, and S. A. Edlund.
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