- Email: [email protected]
Preservation of landforms under ice sheets and ice caps
Geomorphology,9 (1994) 19-32
Preservation of landforms under ice sheets and ice caps Johan Kleman Department of Physical Geography, Stockholm Universit3,, 106 91 Stockholm, Sweden
(Received May 22, 1992; revised August 30, 1993; accepted September 7, 1993)
Abstract
This article addresses the question of whether or not distinct glacial and non-glacial landforms can survive beneath ice sheets and ice caps with little or no morphological alteration. A review of recent work documents the existence of pre-last stadial landforms and landscapes in areas covered by the Fennoscandian and Laurentide ice sheets. A substantial number of independent works indicate that landforms such as eskers, drainage channels and boulder fields have escaped destruction despite complete ice overriding during several tens of millenia. Full preservation of former ground surfaces or delicate landforms probably is linked to areas where the ice-sheet base was continuously frozen to its bed. Larger "robust" landforms, such as large drumlins, appear to have been preserved even under wet-based conditions. In glaciated areas, patches preserved under dry(cold)-based conditions provide important windows towards the past, showing landscapes that were destroyed in surrounding areas affected by wet-based and eroding ice. Some consequences for the research fields of non-glacial geomorphology, archaeology and botany include the possibility of subglacial museums and refugia. A time/space model describes geomorphological access to information from older events in glaciated areas.
1. Introduction
Although global climatic signals and ice volume estimates can be extracted from deep-sea and ice-sheet cores, the evolution of individual paleo-ice sheets cannot be traced in such data. Understanding of the glacial geology and morphology in formerly glaciated areas is necessary to reconstruct the changing configurations and masses of the waxing and waning mid-latitude ice sheets. Much of our knowledge about paleo-ice sheets is based on spatial analysis of glacial landform patterns. A question of fundamental importance for the reconstruction of glacial history and ice-sheet dynamics is whether small-scale landforms can survive prolonged ice-sheet coverage with no or little morphological alteration, and if so, under what conditions. If landforms that predate ice-sheet burial can indeed 0169-555X/94/$07.00 © 1994Elsevier Science B.V. All rights reserved S S D I O 1 6 9 - 5 5 5 X ( 93 ) E 0 0 3 8 - E
survive subglacial conditions and we fail to recognize this possibility, we will likely draw erroneous conclusions about ice-sheet dynamics and landscape development. Potential information about older events will then remain buried. From field observations (Goldthwait, 1960; Holdsworth and Bull, 1970) as well as theoretical work about the bond strength at the ice/substratum interface, glaciologists have concluded that basal sliding cannot occur under cold-based conditions (Boulton, 1972; Hughes, 1973; Paterson, 1981; Drewry, 1986), or is exceedingly slow (Shreve, 1984). Early hints that block fields may have survived icesheet overriding were given by Ives ( 1957, 1966). The first who specifically attributed the preservation of landforms such as tors to frozen bed preservation was probably Sugden (1968). At about the same time, evidence for almost negligible erosion of a repeatedly
20
J. Kleman/ Geomorphology 9 (1994) 19-32
glaciated area in northern Finland was presented by Kaitanen (1969). Korpela (1969), in a classic work, showed that shallowly buried interstadial organic deposits are numerous in northern Finland, but it was not until the work of Kujansuu (1975) that it became clear that not only interstadial deposits but also landforms (eskers) had survived through the entire Late Weichselian glaciation in that area. On the basis of comparisons with recent ice sheets, Schytt (1974) postulated that the Scandinavian ice sheet may have been cold-based and non-eroding over large central areas. More recent work (Sugden and Watts, 1977; Dyke, 1983; Mangerud et al., 1987; Dyke and Morris, 1988; Lagerb~ick, 1988a, b; Lagerb~ick and Robertsson, 1988: Rodhe, 1988; Borgstr6m, 1989; Boulton and Clark, 1990; Follestad, 1990; Kleman and Borgstr6m, 1990; Dyke et al., 1992; Kleman, 1992; Kleman et al., 1992) has identified landforms and former ground surfaces that have survived ice-sheet overriding with little or no morphological alteration, thus challenging the oldstanding assumption of glacial "erasure". Several different lines of evidence, morphologic as well as stratigraphic, were followed by these workers. In most cases preservation has been explained by continously cold(dry)-based conditions, although Boulton and Clark (1990) linked the preservation to former icedivide areas, and Mangerud et al. (1987) and Kleman et al. (1992) demonstrated that the landforms in question were overrun at least for some time by wet-based ice. The large amount of age control distinguishes Lagerb~ick's work from that of the others, who relied mainly on morphologic relationships. The purpose of this article is to review and discuss the evidence for landform preservation beneath ice sheets and temporal superposition of landforms in formerly glaciated areas. The emphasis is on the areas covered by the Scandinavian and Laurentide ice sheets.
2. Evidence for preservation 2.1. Direct observation at glacier and ice cap margins
Goldthwait (1960) found undisturbed patterned ground as well as lichens and mosses in a tunnel dug into the margin of a cold-based Greenland glacier. The ice thickness was around 40 m and the ice was actively
Fig. 1. The northeastern glacier margin of Stor6yj6kulen, Stortiya, Svalbard. Noticehow the beach ridges disappearunder the ice margin. The ice cap has undergone some recession during the last decades. Air Photo: Norsk Polarinstitutt ($48-602). From Jonsson (1983). flowing over the site, but without any basal sliding. Radiocarbon ages of the organic material indicated that the site had been ice-covered less than 400 years. This reference is, together with Holdsworth and Bull (1970), cited in textbooks (Sugden and John, 1976; Paterson, 1981 ) as the prime evidence for the absence of basal sliding of cold-based ice masses. Jonsson (1983) observed the emergence of morphologically unchanged cobble beach ridges (Fig. 1 ) and unstriated outcrops from under the margin of the small cold-based ice cap Stor6yj~kulen, Svalbard, which has advanced and retreated across the landforms. Although the sequence of events is clear, little is known about the duration of ice flow over the ridges and outcrops.
J. Kleman / Geomorphology 9 (1994) 19-32
2.2. Morphostratigraphic evidence The most convincing evidence for preservation is offered by landforms or former ground surfaces composed of or stratigraphically overlain by deposits that predate the last period of ice-sheet coverage at the site. Mangerud et al. (1987) described a morphologically distinct high-elevation marine terrace on western Svalbard overlain by a basal till containing striated clasts and displaying a strong preferred orientation. Two shell samples in the terrace sands yielded ages of around 36 ka. Clearly, the terrace was overrun by an ice sheet, and for at least part of the time, that ice sheet was wetbased. Perhaps the weightiest evidence for complete preservation of pre-last stadial landscapes is reported from northeastern Sweden (Lagerb~ick, 1988a, b; Lagerb~ick and Robertssson, 1988). Lagerb~ick and Robertsson
21
(1988) describe two overlapping regional landform systems, the younger one discontinuous and characterized by patches of S-N to SSW-NNE trending drumlins and fluting, and the older more continuous with NW-SE trending drumlins. The older landform system contains directionally integrated eskers (Fig. 2) and other deglaciation landforms, notably Veiki moraines. In situ organic deposits (peat, gyttja, lacustrine silts) from kettle holes overlying esker material and on Veiki moraine plateaus yielded radiocarbon dates in the range 30 ka-infinite (Lagerb~ick, 1988a; Lagerb~ick and Robertsson, 1988). The scores of radiocarbon ages are supported by palynological evidence that also shows that the organic deposits represent two separate Early Weichselian interstadials. At some sites a thin till bed separates the deposits from the two interstadials. The older (first Early Weichselian stadial) landscape was interpreted by Lagerb~ick and Robertsson (1988) to have
Fig. 2. The Vuottarauto esker 30 km SSE of Nattavaara, NE Sweden. Interstadial organic deposits are present at A and B. The esker derives from the deglaciation of the first Weichselian ice sheet and has survived two later glaciations. Photo: R. Lagerb~ick. From Lagerb~ick and Robertsson (1988).
22
J. Kleman / Geomorphology 9 (/994) 1 ~ 3 2
Fig. 3. The WNW-sloping lateral drainage channels on Mt. Hundfj~illet, Transtrand Mountains, western Sweden. The channels are cut in a lower till unit and covered by a Late Weichselian basal till. They are interpreted to have formed during a pre-Late Weichselian deglaciation. From Kleman et al. (1992).
been preserved beneath two later, continuously coldbased, ice sheets. Lagerb~ick (1988b) interpreted periglacial phenomena in northern Sweden, such as ventifacted boulders, cryoturbated sediments, and lowlying block fields to have formed during interstadials with harsh climatic conditions, rather than during the Holocene. Using a combined morphologic and stratigraphic approach, Kleman et al. (1992) tried to resolve the complex glaciofluvial morphology in the western Transtrand mountains, western Sweden. In this low mountain group crosscutting lateral channel systems are abundant, and several lateral channel systems show direct signs (e.g. fluting, partial erosion) of later ice overriding. On the basis of interpreted ice-surface slope direction, crosscutting relationships, and morphologic sharpness, the various channel systems were attributed to four different deglaciation events. Drainage channels
from three of the deglaciations events were interpreted to be present on one of the summits in the area, Mt. Hundfj~illet. In deep sections cut across channels in each of the three channel systems it was found that they were cut at three different stratigraphic levels. The channels from the last deglaciation were cut in the uppermost Late Weichselian (L.W.) basal till, while lateral channels formed during earlier events were cut in lower till units and covered by the L.W. basal till (Fig. 3). A paleosol in an adjacent section was stratigraphically correlated with the third latest deglaciation event and yielded an infinite (more than 45 ka) radiocarbon age and palynological evidence of a cold flora. The study shows that pre-Late Weichselian landforms are preserved, although the number of events is still open to debate. Of importance is the fact that some of the older channels have at least for some time been overrun by wet-based ice.
J. Kleman / Geomorphology 9 (1994) 1 9 3 2
2.3. Morphological evidence England (1986) documented a locality on Ellesmere Island where two lateral channels cross an otherwise unaltered alluvial fan. The evidence appears unambigious, there can be little doubt about the subaerial nature of the fan and the inconsequent nature of the channels prove the later existence of ice at the locality. Little is known about the duration of the glacial event. England attributes the preservation to patchy coldbased conditions. From arctic Canada, Newfoundland, the Gasp6 peninsula and western Norway there exist numerous accounts (Ives, 1957; Dahl, 1966; Sollid and S6rbel, 1979; Nesje et al., 1988; Grant, 1989) of weathering zones and of "trimlines" separating landscapes with marked differences in morphologic maturity and degree of weathering. The age and history of the different weathering zones has been controversial. Andrews and Miller (1976) argue for a trimline interpretation, mainly on the basis of the lack of direct evidence for ice cover and supposed inability of boulder fields to survive ice-sheet overriding. Sugden and Watts (1977), however, document fresh striated erratics from a tor area in the uppermost weathering zone, thereby giving evidence for preservation despite ice-sheet overriding. Sugden and Watts (1977) attributed the preservation to frozen-bed conditions and specifically pointed out that tors and felsenmeers need not be diagnostic of areas that escaped glaciation. Nesje et al. (1988) mapped the lower limit of blockfields in the southern Scandinavian mountains and, using a trimline interpretation, reconstructed the maximum elevation of the Late Weichselian ice sheet in southern Norway. The resulting reconstruction involved an ice-sheet profile that was exceedingly low in eastern Norway and the border region with Sweden; numerous nunataks and ice thicknesses of only 500 m over plains and valleys in the central parts of the ice sheet were predicted. The reconstruction involved low, almost shelf-like, gradients halfway between ice divide and margin, steepening towards the ice divide as well as the margin. Basal shear stresses in the critical area of the easternmost supposed nunataks would be as low as 0.04 bar. Such conditions are highly unlikely in a region with exposed shield rocks and sandy tills. In the area of the easternmost supposed nunataks there is also direct morphological evidence for a Late Weichselian
23
ice cover (Kleman and Borgstr6m, 1990; see below). The reconstruction in western Norway by Nesje et al. was contested by Follestad (1990), who described autochtonous summit block fields in the fjord landscape and interpreted them to have survived overriding by Late Weichselian ice. The topographically independent flow pattern indicated by striae and till fabric could only have existed with an ice surface much higher than the altitude of the boulder fields. Kleman and Borgstr6m (1990) described two distinct landscape types (Fig. 4) on the low-relief Mt. Fulufj~illet plateau, western Sweden. In discrete zones on this plateau, vast boulder fields without internal topography, boulder depressions and large sorted polygons tbrm a landscape characterized by subaerial processes. Other parts of the plateau are characterized by minor fluting, boulder masses elongated in the last recorded ice flow direction, perched boulders, and a lack of boulder depressions and polygons. At the border between the two landscape types, boulder tails emanating from boulder depressions, fluted patches within subaerially formed boulder fields, and thrust ridges in these boulder fields indicate patchy glacial reshaping of a preexisting landscape. At least 60 km: of the plateau was interpreted to be a relict periglacial landscape, preserved by continuously cold-based conditions. Similar relict surfaces are also described from other localities in northern and western Sweden by Kleman and Borgstr6m (1994). Rodhe (1988) described four localities with crosscutting lateral glaciofluvial drainage channel systems in northwestern Sweden and assigned the older systems to the deglaciation from one of the two Early Wechselian ice sheets documented by Lagerb~ick (1988a). Kleman (1992) made a stratified morphologic map of a large part of northwestern Sweden and reconstructed the basic pattern of the last deglaciation on the basic of the locally youngest and morphologically undegraded forms. He described an older underlying stratum of deglaciation landforms, including a coherent zone of lateral moraines. The lateral moraine zone was interpreted to mark the eastern margin of a west-centered pre-Late Weichselian ice sheet during a halt in its recession. The older, occasionally degraded, landform system was formed by ice flow from the northwest and is probably of Early Weichselian age. Through spatial analysis of regional drumlin systems Dyke and Morris (1988) could delineate areas within
J. Kleman / Geomorphology 9 (1994) 19-32
24
Boulder depression Boulder blanket, glacially t r a n s p o r t e d u ~ -
m
'/
/
Limit of p a t t e r n e d ground Fluting Linear till ridge
-.. E s k e r - l i k e ridge F a u l t - r e l a t e d ridge & depression 0 i
/
,D
t
angsj/3stugan
1 km i
"9
y
I/ I I
Preserved periglacital landscape f
c~
Preserved periglacial landscape
0 o
I
Fig. 4. The distribution of glacially overridden periglacial surfaces and glacially reshaped patches (A, B, C and the R. Tang~n valley) on the Fulufj~llet plateau, western Sweden. The "'tails" (1, H and IH), emanating from boulder depressions at the border zone, show that the glacial e~ent ( Late Weichselian) postdates the formation of the periglacial landscape. Slightly modified after Kleman and Borgstr6m (1990).
a large paleo-ice stream on Prince of Wales Island, Arctic Canada, that were unaffected by basal sliding (Fig. 5). Within the 10-20 km long lenticular "patches" older and directionally divergent drumlin
sets were left morphologically unaltered. The topographical conditions in the area are such that a topographical "trimline" explanation is unreasonable. The older drumlin sets were interpreted to have been pre-
25
J. Kleman / Geomorphology 9 (1994) 19-32
Bedrock B
Bedrock A
"~
side of dispersal train
AXIALPLUk4E I
~ndistinct margins)// COLD BASED (old landforms preserved]
10"rl
mli~% n
DIVERGENT FLOW (striated bedrock)
Dru '1
J"
I
---3--
Fig. 5. Schematicrepresentationof the majorcomponentsof the large "'northernBoothiadispersal train" and probableglaciologicalconditions that lormed them. FromDyke and Morris (1988). served in frozen-bed zones and patches by Dyke and Morris (1988). Dyke et al. (1992) reconstructed frozen and thawed bed zones on the basis of this morphologic evidence. Dyke (1983) also decribes how preLate Wisconsinan colluvium surfaces have survived ice sheet overriding in the central parts of Somerset Island. Boulton and Clark (1990) reconstructed the changing configuration of the Laurentide ice sheet through the last glacial cycle, using satellite imagery as the primary data source for defining sets of glacial lineations. Aerial photography was used for interpreting the relative ages of the features. Boulton and Clark found that in certain regions (eastern Keewatin, central Labrador, James Bay Lowlands) up to 4 lineation sets were present. Using the combined evidence of lineation
patterns, relative age assignments and correlations to key stratigaphic sites they arrived at a reconstruction of changing centres of outflow, flow patterns and marginal configuration for the last glacial cycle. The study convincingly shows that old morphology is preserved in a discernible form over large areas. The preservation is mainly ascribed to low erosion/sedimentation rates in ice divide areas by Boulton and Clark (1990). 2.4. Conditions under which preservation occurs
Most works ascribe preservation to continously coldbased conditions. Boulton and Clark (1990), however, attribute preserved older drumlin sets mainly to "under ice-divide preservation".
J. Kleman / Geomorphology 9 (1994) 19-32
26
alte___ssJ~k k ~
948
• Mt. Tjuolma
/ Preserved landscape
/
J
830.
periglacial
(Boulder fields and large sorted polygons)
Gj
861
/ 850
I
1 km
I
-827 Fig. 6. Border beween a Late Weichselian glacial landscape and a preserved ( interstadial or older) periglacial landscape at Tjuolma. the Ultevis plateau, NW Sweden. The topography is such that the boundary A-B cannot represent a trimline. It is interpreted to mark the boundary between frozen and thawed bed during a late stage of the last deglaciation. Spot heights are given in meters. From Kleman (1992).
J. Kleman / Geomorphology 9(1994) 19-32
I am of the opinion that this explanation is insufficient, and that at least some of that preservation must be linked to frozen-bed conditions. The fact that the flow traces are present in sharply delineated sets ( Prest et al., 1968; Bouiton and Clark, 1990)indicates strong lateral contrasts between flow trace creation and nochange conditions. Under continuously wet-based conditions and the influence of a migrating ice divide or dispersal centre we should expect a much "fuzzier" picture of lineations, with continuous transitions between sets, instead of.the "jumps" indicated by most of the lineation sets. On the other hand such "jumps", indicating gaps in lineation development, are what we could expect if the flow pattern changed under frozenbed conditions, a possibility first pointed out by Vorren (1979). It is not inconceivable that landforms can survive long periods under wet-based ice-divides, but that does not explain how they could also survive the transgression of a wet-bed marginal zone during the ensuing deglaciation. Only in the unusual case of a fully symmetrical retreat will the ice-divide zone remain fixed in position, until the ice sheet is all gone. This difficulty in ascribing preservation mainly to an "under icedivide location" is illustrated by the case of northern Scandinavia, where the westernmost preserved areas (Kleman, 1992) are situated within 170 km of the western ice-sheet margin. With any realistic ice-divide position, these areas were located roughly two thirds of the distance towards the ice margin. The zone of patchy preservation extends for more than 400 km to the east (Lagerb~ick and Robertsson, 1988; Hirvas, 1991), and is cut by at least one Late Weichselian drumlin fan, the Pajala fan (Kleman, 1990). The existence of sharply defined borderlines (Fig. 6) between the last glacial fluting and older landscapes (Dyke and Morris, 1988; Kleman and Borgstr6m, 1990, 1994; Kleman, 1992) are interpreted to indicate the presence of a fundamental process boundary. The most likely candidate appears to be the boundary between thawed and frozen bed. It is difficult to see how such boundaries could form with a spatially uniform basal condition, be it a frozen bed or a thawed bed, To sum up: Full preservation of minor or fragile forms (boulder fields, paleo-surfaces, sharp-crested eskers, etc.) probably requires continuous frozen-bed conditions. Large-scale landforms such as large drum-
27
lins can probably be morphologically discernible ( something very different from full preservation) after a substantial time of wet based ice-overriding.
3. The basal and marginal geomorphic systems Hitherto there has been a consensus that subglacial landforms such as flutings and drumlins are indicators of basal sliding; i.e. wet-based conditions. The landforms commonly used in reconstructions of glacial dynamics can be described as belonging either to the "wet-bed system", or to the meltwater system, which includes landforms such as eskers, channels and icedammed lake traces. In contrast to the accepted link between the "classic" glacial landforms and wet-based conditions, there has not been a consensus around criteria for discerning frozen-bed conditions. The reviewed work documents the existence of relict surfaces and landforms in formerly glaciated areas, and strongly indicates that the preservation of small or fragile landforms is caused by frozen-bed conditions. It thus appears justified to define a '~dry-bed system", complementing the wet-bed system and marginal meltwater system. In the following, three morphologic systems are defined, and the time-space relationships of landforms in formerly glaciated areas are discussed with the aid of a model based on the inherent properties of the three systems. The dr)., bed system: No new landforms are created, with the exception of occasional glaciotectonic features. The frozen bed condition leads to a hiatus in landform development. The wet bed system: Basal sliding occurs. Flow oriented lineations are continuously produced, destroyed and reoriented. Flow-aligned landforms are fossilized at the cessation of wet-based conditions, i.e. by deglaciation or refreezing of the bed. The marginal melm,ater system: A coherent system of linear features (eskers and/or ice-directed drainage channels) is produced. Despite its small areal coverage the meltwater system carries important information. Forming a spatially coherent system it reflects the decay pattern, and may occasionally be the only system doing SO.
Fig. 7 describes three geomorphic systems present at the base and margin of the ice sheet. An additional
J. Kleman / Geomorphology 9(1994) 19-32
28
Ice sheet
,4 C
Geomorphic systems
Meltwater system ele~ end's Production ot linear In coherent pattern
Fig 7. The figure shows the geomorphic systems that result from processes at the ice sheet base and margin. A full explanation is given in the texl.
system, the supraglacial drift depositional system, is not included in the model. Although this system is important in deglaciation landscapes close to the maximum ice-sheet extent, it appears to be of little importance in the former core area zones with palimpsest lineation landscapes, and has therfore not been included in the model. Where present, it mainly masks older landforms. Following the description of the three geomorphic systems resulting from processes at the ice-sheet base and margin, a set of "viewing rules" can be deduced.
We are interested in the age, location and type of information transmitted upwards through time, parts of it eventually observable to a present day observer (geomorphologist). The easiest way to illustrate these relationships is, however, to invert the system, i.e. to describe instead what happens to the geomorphologist's "line of sight" as it penetrates further and further back in time at specific points in space. What we seek is the intersection between line of sight and the time/ space domains of landform formation. The dry bed system is transparent in the sense that
J. Kleman / Geomorpholo~y 9 (1994) 19-32
we can see through it back in time until we meet the time of landform development, be it subaerial or glacial landforms. When the line of sight meets the wet-bed system, it does not penetrate, if subglacial landform development was intense. Intensity of landform creation, and thereby destruction of older forms is largely a function of flow velocity. Occasionally, as in ice-divide zones, the line of sight can penetrate deeper back in time. The system is opaque to semi-transparent. When the line of sight meets the meltwater system it mostly penetrates. Meltwater traces are typically narrow linear features that only to a minor areal extent destroy or mask the landscape on which they are superposed. For the purpose of tracing regional landform systems, the meltwater system can be regarded as
nearly transparent.
b e d a ne d m etl t w a t e r a g/e o mbo'er dp ,h i/cd r ys yw stems through time
®1 E
bj
Observer system
29
3.1. Time-space relationships of ~eomorphic systems To graphically illustrate the consequences of "preservation" on the interpretation of glacial landform systems, a simple model based on the evolution of a hypothetical ice sheet will be presented. The basic thermal zonation of the hypothetical ice sheet was chosen on the basis of the thermal ice-sheet model of Hooke (1977), the view on temperature conditions of the Scandinavian ice sheet of Sollid and S6rbel (1988), and the works of Lagerbfick (1988a) and Kleman ( 1990, 1992), as well as recent modeling of the Scandinavian ice sheet by G.S. Boulton and co-workers (G. Boulton, pets. commun., 1993 ). Fig. 8a is constructed by plotting an assumed evolution along the transect I-II in Fig. 7 through time. In the real world, with changing flow patterns, ice flow would not always be parallel to the plane of the paper.
C J L i n e of sight penetration back in time d J
Time - s p a c e t o p o l o g y of features
Present interglacial
I-
o
"3 o • c
Q. ~
o
o
~.~_
Q,o
o'~
S p a c e (proflleacroas Ice-sheet base)
f
Marginal me#water system, p~oduces coherent set of linear features
Line of sight penetration: The system is nearly transparent
Wet bed system, continuous production, destruction and reorientation of forms
The systemis semi-tran~parent ~o opaque
Dry bed system, preservation of landforms
The system is transparent
Interstadlal subaerlal system, productlonof periglacial landforms
The system is nearly transparent 7 " ~l°,o,
Fig. 8. The figure describes the time/I D-space relationships of the geomorphic systems that are visible to a present-day observer of a glaciated area. A full explanation is given in the text.
30
J. Kleman / Geomotphology 9 (1994) 19-32
The following is assumed for the hypothetical ice sheet in Fig. 8: During the early build-up phase the last ice sheet is mainly cold-based, in line with conclusions drawn by Hughes ( 1973 ) about ice-sheet inception in permafrost areas. It ends its life partly wet-based. During the early build-up phase of the last ice sheet (stadial 2 in Fig. 8) insignificant or only minor meltwater traces are produced, as ice-sheet growth locks water into the ice sheet. "['he last ice sheet was foregone by (an) earlier ice sheet(s). During the intervening interstadial minor subaerial landforms were created. Fig. 8b illustrates the line of sight of the scientist, a morphologist trying to interpret the glacial traces in such terms as flow direction and time. The 2D case presented here can only illustrate time, a 3D graph would be necessary to show flow direction. Fig. 8c shows the intersection of the geomorphologist's "line of ~ight" with the domains where significant landforms are" produced. The line of sight stops at some (time) depth in the wet-bed system but penetrates the other systems. Fig. 8d shows the landform systems that are visible to lhe observer (us), and topology of that information in lhis t i m e / ( I D ) space diagram. The most important features in Fig. 8d are the continuous and time-transgressive nature of meltwater traces, the age discontinutties of flow traces, and the potential of information from older glacial, as well as non-glacial events. Points (A) and (D) in space represent "classical" glacial landscapes where all the landforms, flow traces as well as meltwater forms, represent a short time span and rettect the same flow regime. In such areas mapping of glacial morphology without time-stratification is meaningful. At point (B), the flow traces and meltwater traces were created during the same stadial but there is a large time gap between them, and they reflect different flow regimes. At point (C) the situation is even more complex. One complete morphological system, the meltwater system from the last deglaciation, as well as three older, only to a lesser extent obscured, landform systems are present. A subaerially formed, probably periglacial, "'varnish" overlies the flow trace~meltwater couple from the preceeding stadial. The four systems represent three different events. Maps over areas with palimpsest such as (B) and (C) should preferably
include representation of the relative age of crosscutting features, or be made in a time-stratified fashion. Rarely can such time stratification be done afterwards by other scientists, even on good single-layer mediumscale maps of glacial morphology (such as Sollid and Kristiansen, 1982; Borgstr6m, 1989).
4.
Discussion
The scheme in Fig. 8 focuses on the temporal relationships between morphological features, but is spatially highly simplified. Real ice sheets have nontextbook flow configurations, ice streams, and complex and patchy basal thermal configurations. This makes the reconstruction of flow pattern evolution even more difficult, but does not affect the basic arguments regarding "line of sight penetration" and the geomorphologist's access to older morphologic strata in areas with continuous frozen-bed conditions. Inversion procedures for extracting the evolution of flow configuration from flow trace palimpsests have been suggested by Kleman (1990) and Boulton and Clark (1990). The evidence for preservation comes from a large variety of regions, glaciers and glaciated areas in a wide variety of settings and environments, and also on a variety of spatial scales. Of the works cited above, Mangerud et al. (1987), England, 1986, Sugden and Watts (1977) and Rodhe (1988) give what can be described as "point" evidence for preservation. More information about basal thermal patterns are provided by Dyke and Morris (1988), Dyke et al. (1992), Kleman and Borgstr6m ( 1990, 1994) and Kleman (1992), who describe intermediate-scale patches of sharply delineated relict surfaces, marking areas of frozen-bed conditions. These observations verify Hughes' (Denton and Hughes, 1981 ) postulate that ice sheet bases have wide transition zones with a patchwork of frozen and thawed areas. Sugden and John's (1976) concept of "selective linear erosion" emphasises topographic, and thereby basal thermal, control over patterns of erosion and preservation. The relationship between topography, basal thermal regime and erosion seems to be widely accepted for fjord landscapes, and there are also indications that some topographic control of basal thermal regime exists also in more subdued topography (Kleman and Borgstr6m, 1993). The "preservation case" has implications outside
Z Kleman / Geomorphology 9 (1994) 1 9 3 2
ice-sheet research. In non-glacial geomorphology it is often assumed (Oilier, 1979; Fairbridge and Finkl, 1980) that study of pre-Quaternary landform development is best performed on non-glaciated cratonic surfaces such as Australia and parts of Africa. It is thought that the Quaternary glaciations have so extensively changed the preglacial morphology in the areas covered by them as to make study of long-term landform development overly difficult. The described evidence for preservation indicates that upland areas and plateaus in arctic Canada and northern Scandinavia have been covered by fully dry (cold)-based non-erosive ice caps or ice sheets during most of the Quaternary. Parts of these areas may therefore well be "Tertiary museums" with a periglacial "varnish". In archaeology the "assumption of glacial destruction" has previously given archaeologists no hope of finding traces of ancient (pre-Holocene) man in the areas covered by the last ice sheets. If he ( and she) were indeed present in those northerly areas before the onset of the last stadial, the possibility of finding traces of humans or their activities cannot any longer be neglected in areas interpreted to have been covered by dry-based ice sheets. In botany the problem of bicentric species and possible nunatak "refugia" (for a comprehensive bibliography, see Nesje et al., 1988) still awaits a satisfactory answer. In the light of the evidence o f ' 'recycled" ground surfaces it may be speculated whether viable material can survive beneath an ice sheet, i.e. whether subglacial refugia may have existed. This possibility was also indicated by Rapp (1992). Indications that seeds can be viable for very long periods in a dry, cold environment were given by Porsild et al. ( 1967 ) who described a find of viable seeds in a muck deposit in Alaska, interpreted to be at least 10,000 years old. A structure such as a boulder field beneath a cold(dry)based ice sheet provides a cold and cosmic-radiation protected environment where seeds could possibly remain viable for long periods of time.
5. Conclusions
- Small- and medium-scale landforms predating the last glacial event occur scattered in the areas covered by the Fennoscandian and the Laurentide ice sheets. - Preserved landforms mark areas where the last ice
31
sheets were continuously dry (cold)-based. - The morphological impact of an ice sheet is related to migration over time of the wet-bed system, dry-bed system and the marginal meltwater system.
Acknowledgements This study was funded through a grant from the Swedish Natural Science Research Council. I want to thank Bo Str6mberg, Stig Jonsson, Ingmar Borgstr6m, Clas H~ittestrand and two unknown referees for constmctive comments on the manuscript.
References Andrews. J.D. and Miller, G.H., 1976. Quaternary glacial chronology of the eastern Canadian arctic; A review and a contribution on amino acid dating of Quaternary molluscs from the Clyde Cliffs. In: W. Mahaney (Editor), Quaternary Stratigraphy of North America. Dowden, Hutchinson and Ross, Stroudsburg, Pa., pp. 1-32. Borgstrtim, I., 1989. Terr~ingformerna och den glaciala utvecklingen i de s6dra fj~illen. Department of Physical Geography, Stockholm University, Meddelande 234, 133 pp. Boulton, G.S., 1972. The role of thermal regime in glacial sedimentation. Inst. Br. Geogr. Spec. Publ., 4: 1-19, Bouhon, G,S. and Clark, C.D., 1990. A highly mobile Laurentide ice sheet revealed by satellite images of glacial lineations. Nature, 346: 813-817. Dahl, R., 1966. Block fields, weathering pits and tor-like forms in the Narvik mountains, Nordland, Norge. Geogr. Ann., 48A: 5585. Denton, G.H. and Hughes, T.J., 1981. The Last Great Ice Sheets. Wiley, New York, 484 pp. Drewry, D., 1986. Glacial Geologic Processes. Edward Arnold, London, 276 pp. Dyke, A.S., 1983. Quaternary geology of Somerset Island, District of Franklin. Geol. Surv. Can. Mem., 404:32 pp. Dyke, A.S. and Morris, T.F., 1988. Drumlin fields, dispersal trains, and ice streams in Arctic Canada. Can. Geogr., 32: 86-90. Dyke, A.S., Morris, T.F., Green, D.E.C. and England, J., 1992. Quaternary geology of Prince of Wales Island Arctic Canada. Geol. Surv. Can. Mem., 433:142 pp. England, J., 1986. Glacial erosion of a high Arctic Valley. J. Glaciol., 32: 60-64. Fairbridge, R.W. and Finkl, C.W., 1980. Cratonic erosional uncomfortuities and peneplains. J. Geol., 88: 69-82. Follestad, B,A., 1990. Block fields, ice-flow directions and the Pleistocene ice sheet in Nordm0re and Romsdal, West Norway. Norsk Geol. Tidsskr., 70: 27-33. Goldthwait. R.P., 1960. Study of ice cliff in Nunatarssuaq, Greenland. Tech. Rep. Snow Ice Permafrost Res. Establ., 39: 1-103,
32
J. Kleman / Geomotphology 9 (1994) 19-32
Grant, D.R., 1989: Quaternary geology of the Atlantic Appalachian region of Canada. In: R.J. Fulton (Editor), Quaternary Geology of Canada and Greenland. Geological Survey of Canada, Geology of Canada, No. 1, pp. 393~-~40. Hirvas, H., 1991. Pleistocene stratigraphy of Finnish Lapland. Geol. Surv. Finland Bull., 354 pp. Hntdsworth, G. and Bull, C., 1970. The flow law of cold ice; investigations on Meserve glacier, Antarctica. In: A.J. Gow et al. (Editors), Int. Syrup. on Antarctic Glaciological Exploration (ISAGE). Int. Assoc. Sci. Hydrol. Publ., 86: 204-216. Hooke, R. LeB., 1977. Basal temperatures in polar ice sheets: A qualitative review. Quat. Res., 7: 1-13. Hughes, T., 1973. Glacial permafrost and Pleistocene ice ages. In: Permafrost: The North American Contribution to the Second International Conference. National Academy of Sciences, Washington, DC. Ires. J.D.. 1957. Glaciation of the Torngat mountains, northern Labrador. Arctic, 10( 2): 67-88. lw:s, J.D., 1966. Associated weathering forms on mountain tops and the nunatak hypothesis. Geogr. Ann., 48A(4): 220-223. Jonsson. S., 1983. On the geomorphology and past glaciation of StorOya, S valbard. Geogr. Ann., 65A( 1/2): 1-17. Kaitanen, V., 1969. A geographical study of the morphogenesis of Northern Lapland. Fennia, 99( 5 ): 1-85. Kk~man, J., 1990. On the use of glacial striae for reconstruction of paleo-ice sheet flow patterns - - With application to the Scandinavian ice sheet. Geogr. Ann., 72A ( 3/4) : 217-236. Klcman, J., 1992. The palimpsest glacial landscape in northern S w e d e n - Late Weichselian deglaciation landforms and traces of older west-centered ice sheets. Geogr. Ann., 74A(4): 305-325, Kleman, J. and BorgstrOm, 1., 1990. The boulder fields of Mr. Fulufjfillet, west-central Sweden - - Late Weichselian boulder blankets and interstadial periglacial phenomena. Geogr. Ann., 72A( I ): 63-78. Kleman, J. and Borgstr6m, 1., 1994. Glacial landforms indicative of a partly frozen bed. J. Glaciol., in press. Kleman, J., Borgstr6m, I., Robertsson, A.-M. and Lilliesk01d. M.. 1992. Morphology and stratigraphy from several deglaciations in the Transtrand mountains, western Sweden. J. Quate. Sci., 7(1): 1-16. Korpela, K.. 1969. Die Weichsel-eiszeit und ihr interstadial in Per~.pohjola ( N6rdlisches NordFinnland) im licht yon submor~.nen sedimenten. Ann. Acad. Sci. Fenn., Ser. A III, 99:108 pp. Kujansuu, R., 1975. Interstadial esker at Marrasjfirvi, Finnish Lapland. Geologi, 27(4):45-50. Lagerbfick, R.. 1988a. The Veiki moraines in northern Sweden - widespread evidence of an early Weichselian deglaciation. Boreas, 17,469-486. Lagerbfick, R., 1988b. Periglacia[ phenomena in the wooded areas
of northern Sweden - - relicts from the T~irend6 interstadial. Boreas. 17: 487-500. Lagerbfick, R. and Robertsson, A.-M,, 1988. Kettle holes I stratigraphical archives for Weichselian geology and paleoenvironment in northernmost Sweden. Boreas, 17: 439-468. Mangerud, J., Bolstad, M., Elgersma, A., Helliksen, D., Landvik, J.Y., Lycke, A.K., L6nne, I., Salvigsen, O.. Sandahl, T. and Sejrup, H.P., 1987. The Late Weichselian glacial maximum in Svalbard. Polar Res., 5 (n.s.) : 275-278. Nesje, A., Dahl, S.O., Anda, E. and Rye, N., 1988. Block fields in southern Norway: Significance for the Late Weichselian ice sheet. Norsk Geol. Tidskr., 68: 149-170. Oilier, C.D., 1979. Evolutionary geomorphology of Australia and Papua-New Guinea. Inst. Br. Geogr. Trans. New Ser., 4: 516539. Paterson, W.S.B., 1981. The Physics of Glaciers. Pergamon, Oxford, 380 pp. Porsild, A.E.. Harington, C.R. and Mulligan, G.A., 1967. Lupinus arcticus Wats. grown from seeds of Pleistocene age. Science, 158: 113-114, Prest, V.K., Grant, D.R. and Rampton, V.N., 1968. Glacial Map of Canada. Geol. Surv. of Canada, Map 1253A. Rapp, A., 1992. K~irkevagge revisited. Field excursions on geomorphology and environmental history in the Abisko mountains, Sweden. Sver. Geol. Unders. Ser. Ca, 81: 269-276. Rodhe, L., 1988. Glaciofluvial channels formed prior to the last deglaciation: examples from Swedish Lapland. Boreas, 17:511516. Schytt, V., 1974. Inland ice sheets - - recent and Pleistocene. Geol. F6ren. Stockholm F6r., 96: 298-309. Shreve. R.L., 1984. Glacier sliding at subfreezing temperatures. J. Glacol., 30: 341-347. Sollid, J.L. and Kristiansen, K., 1982. Hedmark Fylke, KvartaergeoIogi og geomorfologi 1:250,000. Geografisk Institutt, Oslo Universitet. Sollid, J.L. and S6rbel, L., 1979. Deglaciation of western Central Norway. Boreas, 8: 233-239. Sollid, J.L. and SOrbel, L., 1988. Influence of temperature conditions in formation of end moraines in Fennoscandia and Svalbard. Boreas, 17: 553-558. Sugden, D.E., 1968. The selectivity of glacial erosion in the Cairngorm mountains, Scotland. Trans. Inst. Br. Geogr., 45: 75-92. Sugden, D.E. and John, B., 1976. Glaciers and Landscape. Edward Arnold, London, 376 pp. Sugden, D.E. and Watts, S.H., 1977. Tors, felsenmeer and glaciation in northern Cumberland peninsula, Baffin Island. Can. J. Earth Sci., 14: 2817-2823. Vorren, T., 1979. Weichselian ice movements, sediments and stratigraphy on Hardangervidda, South Norway. Norges Geol. Unders., 350:1-117.
Preservation of landforms under ice sheets and ice caps Johan Kleman Department of Physical Geography, Stockholm Universit3,, 106 91 Stockholm, Sweden
(Received May 22, 1992; revised August 30, 1993; accepted September 7, 1993)
Abstract
This article addresses the question of whether or not distinct glacial and non-glacial landforms can survive beneath ice sheets and ice caps with little or no morphological alteration. A review of recent work documents the existence of pre-last stadial landforms and landscapes in areas covered by the Fennoscandian and Laurentide ice sheets. A substantial number of independent works indicate that landforms such as eskers, drainage channels and boulder fields have escaped destruction despite complete ice overriding during several tens of millenia. Full preservation of former ground surfaces or delicate landforms probably is linked to areas where the ice-sheet base was continuously frozen to its bed. Larger "robust" landforms, such as large drumlins, appear to have been preserved even under wet-based conditions. In glaciated areas, patches preserved under dry(cold)-based conditions provide important windows towards the past, showing landscapes that were destroyed in surrounding areas affected by wet-based and eroding ice. Some consequences for the research fields of non-glacial geomorphology, archaeology and botany include the possibility of subglacial museums and refugia. A time/space model describes geomorphological access to information from older events in glaciated areas.
1. Introduction
Although global climatic signals and ice volume estimates can be extracted from deep-sea and ice-sheet cores, the evolution of individual paleo-ice sheets cannot be traced in such data. Understanding of the glacial geology and morphology in formerly glaciated areas is necessary to reconstruct the changing configurations and masses of the waxing and waning mid-latitude ice sheets. Much of our knowledge about paleo-ice sheets is based on spatial analysis of glacial landform patterns. A question of fundamental importance for the reconstruction of glacial history and ice-sheet dynamics is whether small-scale landforms can survive prolonged ice-sheet coverage with no or little morphological alteration, and if so, under what conditions. If landforms that predate ice-sheet burial can indeed 0169-555X/94/$07.00 © 1994Elsevier Science B.V. All rights reserved S S D I O 1 6 9 - 5 5 5 X ( 93 ) E 0 0 3 8 - E
survive subglacial conditions and we fail to recognize this possibility, we will likely draw erroneous conclusions about ice-sheet dynamics and landscape development. Potential information about older events will then remain buried. From field observations (Goldthwait, 1960; Holdsworth and Bull, 1970) as well as theoretical work about the bond strength at the ice/substratum interface, glaciologists have concluded that basal sliding cannot occur under cold-based conditions (Boulton, 1972; Hughes, 1973; Paterson, 1981; Drewry, 1986), or is exceedingly slow (Shreve, 1984). Early hints that block fields may have survived icesheet overriding were given by Ives ( 1957, 1966). The first who specifically attributed the preservation of landforms such as tors to frozen bed preservation was probably Sugden (1968). At about the same time, evidence for almost negligible erosion of a repeatedly
20
J. Kleman/ Geomorphology 9 (1994) 19-32
glaciated area in northern Finland was presented by Kaitanen (1969). Korpela (1969), in a classic work, showed that shallowly buried interstadial organic deposits are numerous in northern Finland, but it was not until the work of Kujansuu (1975) that it became clear that not only interstadial deposits but also landforms (eskers) had survived through the entire Late Weichselian glaciation in that area. On the basis of comparisons with recent ice sheets, Schytt (1974) postulated that the Scandinavian ice sheet may have been cold-based and non-eroding over large central areas. More recent work (Sugden and Watts, 1977; Dyke, 1983; Mangerud et al., 1987; Dyke and Morris, 1988; Lagerb~ick, 1988a, b; Lagerb~ick and Robertsson, 1988: Rodhe, 1988; Borgstr6m, 1989; Boulton and Clark, 1990; Follestad, 1990; Kleman and Borgstr6m, 1990; Dyke et al., 1992; Kleman, 1992; Kleman et al., 1992) has identified landforms and former ground surfaces that have survived ice-sheet overriding with little or no morphological alteration, thus challenging the oldstanding assumption of glacial "erasure". Several different lines of evidence, morphologic as well as stratigraphic, were followed by these workers. In most cases preservation has been explained by continously cold(dry)-based conditions, although Boulton and Clark (1990) linked the preservation to former icedivide areas, and Mangerud et al. (1987) and Kleman et al. (1992) demonstrated that the landforms in question were overrun at least for some time by wet-based ice. The large amount of age control distinguishes Lagerb~ick's work from that of the others, who relied mainly on morphologic relationships. The purpose of this article is to review and discuss the evidence for landform preservation beneath ice sheets and temporal superposition of landforms in formerly glaciated areas. The emphasis is on the areas covered by the Scandinavian and Laurentide ice sheets.
2. Evidence for preservation 2.1. Direct observation at glacier and ice cap margins
Goldthwait (1960) found undisturbed patterned ground as well as lichens and mosses in a tunnel dug into the margin of a cold-based Greenland glacier. The ice thickness was around 40 m and the ice was actively
Fig. 1. The northeastern glacier margin of Stor6yj6kulen, Stortiya, Svalbard. Noticehow the beach ridges disappearunder the ice margin. The ice cap has undergone some recession during the last decades. Air Photo: Norsk Polarinstitutt ($48-602). From Jonsson (1983). flowing over the site, but without any basal sliding. Radiocarbon ages of the organic material indicated that the site had been ice-covered less than 400 years. This reference is, together with Holdsworth and Bull (1970), cited in textbooks (Sugden and John, 1976; Paterson, 1981 ) as the prime evidence for the absence of basal sliding of cold-based ice masses. Jonsson (1983) observed the emergence of morphologically unchanged cobble beach ridges (Fig. 1 ) and unstriated outcrops from under the margin of the small cold-based ice cap Stor6yj~kulen, Svalbard, which has advanced and retreated across the landforms. Although the sequence of events is clear, little is known about the duration of ice flow over the ridges and outcrops.
J. Kleman / Geomorphology 9 (1994) 19-32
2.2. Morphostratigraphic evidence The most convincing evidence for preservation is offered by landforms or former ground surfaces composed of or stratigraphically overlain by deposits that predate the last period of ice-sheet coverage at the site. Mangerud et al. (1987) described a morphologically distinct high-elevation marine terrace on western Svalbard overlain by a basal till containing striated clasts and displaying a strong preferred orientation. Two shell samples in the terrace sands yielded ages of around 36 ka. Clearly, the terrace was overrun by an ice sheet, and for at least part of the time, that ice sheet was wetbased. Perhaps the weightiest evidence for complete preservation of pre-last stadial landscapes is reported from northeastern Sweden (Lagerb~ick, 1988a, b; Lagerb~ick and Robertssson, 1988). Lagerb~ick and Robertsson
21
(1988) describe two overlapping regional landform systems, the younger one discontinuous and characterized by patches of S-N to SSW-NNE trending drumlins and fluting, and the older more continuous with NW-SE trending drumlins. The older landform system contains directionally integrated eskers (Fig. 2) and other deglaciation landforms, notably Veiki moraines. In situ organic deposits (peat, gyttja, lacustrine silts) from kettle holes overlying esker material and on Veiki moraine plateaus yielded radiocarbon dates in the range 30 ka-infinite (Lagerb~ick, 1988a; Lagerb~ick and Robertsson, 1988). The scores of radiocarbon ages are supported by palynological evidence that also shows that the organic deposits represent two separate Early Weichselian interstadials. At some sites a thin till bed separates the deposits from the two interstadials. The older (first Early Weichselian stadial) landscape was interpreted by Lagerb~ick and Robertsson (1988) to have
Fig. 2. The Vuottarauto esker 30 km SSE of Nattavaara, NE Sweden. Interstadial organic deposits are present at A and B. The esker derives from the deglaciation of the first Weichselian ice sheet and has survived two later glaciations. Photo: R. Lagerb~ick. From Lagerb~ick and Robertsson (1988).
22
J. Kleman / Geomorphology 9 (/994) 1 ~ 3 2
Fig. 3. The WNW-sloping lateral drainage channels on Mt. Hundfj~illet, Transtrand Mountains, western Sweden. The channels are cut in a lower till unit and covered by a Late Weichselian basal till. They are interpreted to have formed during a pre-Late Weichselian deglaciation. From Kleman et al. (1992).
been preserved beneath two later, continuously coldbased, ice sheets. Lagerb~ick (1988b) interpreted periglacial phenomena in northern Sweden, such as ventifacted boulders, cryoturbated sediments, and lowlying block fields to have formed during interstadials with harsh climatic conditions, rather than during the Holocene. Using a combined morphologic and stratigraphic approach, Kleman et al. (1992) tried to resolve the complex glaciofluvial morphology in the western Transtrand mountains, western Sweden. In this low mountain group crosscutting lateral channel systems are abundant, and several lateral channel systems show direct signs (e.g. fluting, partial erosion) of later ice overriding. On the basis of interpreted ice-surface slope direction, crosscutting relationships, and morphologic sharpness, the various channel systems were attributed to four different deglaciation events. Drainage channels
from three of the deglaciations events were interpreted to be present on one of the summits in the area, Mt. Hundfj~illet. In deep sections cut across channels in each of the three channel systems it was found that they were cut at three different stratigraphic levels. The channels from the last deglaciation were cut in the uppermost Late Weichselian (L.W.) basal till, while lateral channels formed during earlier events were cut in lower till units and covered by the L.W. basal till (Fig. 3). A paleosol in an adjacent section was stratigraphically correlated with the third latest deglaciation event and yielded an infinite (more than 45 ka) radiocarbon age and palynological evidence of a cold flora. The study shows that pre-Late Weichselian landforms are preserved, although the number of events is still open to debate. Of importance is the fact that some of the older channels have at least for some time been overrun by wet-based ice.
J. Kleman / Geomorphology 9 (1994) 1 9 3 2
2.3. Morphological evidence England (1986) documented a locality on Ellesmere Island where two lateral channels cross an otherwise unaltered alluvial fan. The evidence appears unambigious, there can be little doubt about the subaerial nature of the fan and the inconsequent nature of the channels prove the later existence of ice at the locality. Little is known about the duration of the glacial event. England attributes the preservation to patchy coldbased conditions. From arctic Canada, Newfoundland, the Gasp6 peninsula and western Norway there exist numerous accounts (Ives, 1957; Dahl, 1966; Sollid and S6rbel, 1979; Nesje et al., 1988; Grant, 1989) of weathering zones and of "trimlines" separating landscapes with marked differences in morphologic maturity and degree of weathering. The age and history of the different weathering zones has been controversial. Andrews and Miller (1976) argue for a trimline interpretation, mainly on the basis of the lack of direct evidence for ice cover and supposed inability of boulder fields to survive ice-sheet overriding. Sugden and Watts (1977), however, document fresh striated erratics from a tor area in the uppermost weathering zone, thereby giving evidence for preservation despite ice-sheet overriding. Sugden and Watts (1977) attributed the preservation to frozen-bed conditions and specifically pointed out that tors and felsenmeers need not be diagnostic of areas that escaped glaciation. Nesje et al. (1988) mapped the lower limit of blockfields in the southern Scandinavian mountains and, using a trimline interpretation, reconstructed the maximum elevation of the Late Weichselian ice sheet in southern Norway. The resulting reconstruction involved an ice-sheet profile that was exceedingly low in eastern Norway and the border region with Sweden; numerous nunataks and ice thicknesses of only 500 m over plains and valleys in the central parts of the ice sheet were predicted. The reconstruction involved low, almost shelf-like, gradients halfway between ice divide and margin, steepening towards the ice divide as well as the margin. Basal shear stresses in the critical area of the easternmost supposed nunataks would be as low as 0.04 bar. Such conditions are highly unlikely in a region with exposed shield rocks and sandy tills. In the area of the easternmost supposed nunataks there is also direct morphological evidence for a Late Weichselian
23
ice cover (Kleman and Borgstr6m, 1990; see below). The reconstruction in western Norway by Nesje et al. was contested by Follestad (1990), who described autochtonous summit block fields in the fjord landscape and interpreted them to have survived overriding by Late Weichselian ice. The topographically independent flow pattern indicated by striae and till fabric could only have existed with an ice surface much higher than the altitude of the boulder fields. Kleman and Borgstr6m (1990) described two distinct landscape types (Fig. 4) on the low-relief Mt. Fulufj~illet plateau, western Sweden. In discrete zones on this plateau, vast boulder fields without internal topography, boulder depressions and large sorted polygons tbrm a landscape characterized by subaerial processes. Other parts of the plateau are characterized by minor fluting, boulder masses elongated in the last recorded ice flow direction, perched boulders, and a lack of boulder depressions and polygons. At the border between the two landscape types, boulder tails emanating from boulder depressions, fluted patches within subaerially formed boulder fields, and thrust ridges in these boulder fields indicate patchy glacial reshaping of a preexisting landscape. At least 60 km: of the plateau was interpreted to be a relict periglacial landscape, preserved by continuously cold-based conditions. Similar relict surfaces are also described from other localities in northern and western Sweden by Kleman and Borgstr6m (1994). Rodhe (1988) described four localities with crosscutting lateral glaciofluvial drainage channel systems in northwestern Sweden and assigned the older systems to the deglaciation from one of the two Early Wechselian ice sheets documented by Lagerb~ick (1988a). Kleman (1992) made a stratified morphologic map of a large part of northwestern Sweden and reconstructed the basic pattern of the last deglaciation on the basic of the locally youngest and morphologically undegraded forms. He described an older underlying stratum of deglaciation landforms, including a coherent zone of lateral moraines. The lateral moraine zone was interpreted to mark the eastern margin of a west-centered pre-Late Weichselian ice sheet during a halt in its recession. The older, occasionally degraded, landform system was formed by ice flow from the northwest and is probably of Early Weichselian age. Through spatial analysis of regional drumlin systems Dyke and Morris (1988) could delineate areas within
J. Kleman / Geomorphology 9 (1994) 19-32
24
Boulder depression Boulder blanket, glacially t r a n s p o r t e d u ~ -
m
'/
/
Limit of p a t t e r n e d ground Fluting Linear till ridge
-.. E s k e r - l i k e ridge F a u l t - r e l a t e d ridge & depression 0 i
/
,D
t
angsj/3stugan
1 km i
"9
y
I/ I I
Preserved periglacital landscape f
c~
Preserved periglacial landscape
0 o
I
Fig. 4. The distribution of glacially overridden periglacial surfaces and glacially reshaped patches (A, B, C and the R. Tang~n valley) on the Fulufj~llet plateau, western Sweden. The "'tails" (1, H and IH), emanating from boulder depressions at the border zone, show that the glacial e~ent ( Late Weichselian) postdates the formation of the periglacial landscape. Slightly modified after Kleman and Borgstr6m (1990).
a large paleo-ice stream on Prince of Wales Island, Arctic Canada, that were unaffected by basal sliding (Fig. 5). Within the 10-20 km long lenticular "patches" older and directionally divergent drumlin
sets were left morphologically unaltered. The topographical conditions in the area are such that a topographical "trimline" explanation is unreasonable. The older drumlin sets were interpreted to have been pre-
25
J. Kleman / Geomorphology 9 (1994) 19-32
Bedrock B
Bedrock A
"~
side of dispersal train
AXIALPLUk4E I
~ndistinct margins)// COLD BASED (old landforms preserved]
10"rl
mli~% n
DIVERGENT FLOW (striated bedrock)
Dru '1
J"
I
---3--
Fig. 5. Schematicrepresentationof the majorcomponentsof the large "'northernBoothiadispersal train" and probableglaciologicalconditions that lormed them. FromDyke and Morris (1988). served in frozen-bed zones and patches by Dyke and Morris (1988). Dyke et al. (1992) reconstructed frozen and thawed bed zones on the basis of this morphologic evidence. Dyke (1983) also decribes how preLate Wisconsinan colluvium surfaces have survived ice sheet overriding in the central parts of Somerset Island. Boulton and Clark (1990) reconstructed the changing configuration of the Laurentide ice sheet through the last glacial cycle, using satellite imagery as the primary data source for defining sets of glacial lineations. Aerial photography was used for interpreting the relative ages of the features. Boulton and Clark found that in certain regions (eastern Keewatin, central Labrador, James Bay Lowlands) up to 4 lineation sets were present. Using the combined evidence of lineation
patterns, relative age assignments and correlations to key stratigaphic sites they arrived at a reconstruction of changing centres of outflow, flow patterns and marginal configuration for the last glacial cycle. The study convincingly shows that old morphology is preserved in a discernible form over large areas. The preservation is mainly ascribed to low erosion/sedimentation rates in ice divide areas by Boulton and Clark (1990). 2.4. Conditions under which preservation occurs
Most works ascribe preservation to continously coldbased conditions. Boulton and Clark (1990), however, attribute preserved older drumlin sets mainly to "under ice-divide preservation".
J. Kleman / Geomorphology 9 (1994) 19-32
26
alte___ssJ~k k ~
948
• Mt. Tjuolma
/ Preserved landscape
/
J
830.
periglacial
(Boulder fields and large sorted polygons)
Gj
861
/ 850
I
1 km
I
-827 Fig. 6. Border beween a Late Weichselian glacial landscape and a preserved ( interstadial or older) periglacial landscape at Tjuolma. the Ultevis plateau, NW Sweden. The topography is such that the boundary A-B cannot represent a trimline. It is interpreted to mark the boundary between frozen and thawed bed during a late stage of the last deglaciation. Spot heights are given in meters. From Kleman (1992).
J. Kleman / Geomorphology 9(1994) 19-32
I am of the opinion that this explanation is insufficient, and that at least some of that preservation must be linked to frozen-bed conditions. The fact that the flow traces are present in sharply delineated sets ( Prest et al., 1968; Bouiton and Clark, 1990)indicates strong lateral contrasts between flow trace creation and nochange conditions. Under continuously wet-based conditions and the influence of a migrating ice divide or dispersal centre we should expect a much "fuzzier" picture of lineations, with continuous transitions between sets, instead of.the "jumps" indicated by most of the lineation sets. On the other hand such "jumps", indicating gaps in lineation development, are what we could expect if the flow pattern changed under frozenbed conditions, a possibility first pointed out by Vorren (1979). It is not inconceivable that landforms can survive long periods under wet-based ice-divides, but that does not explain how they could also survive the transgression of a wet-bed marginal zone during the ensuing deglaciation. Only in the unusual case of a fully symmetrical retreat will the ice-divide zone remain fixed in position, until the ice sheet is all gone. This difficulty in ascribing preservation mainly to an "under icedivide location" is illustrated by the case of northern Scandinavia, where the westernmost preserved areas (Kleman, 1992) are situated within 170 km of the western ice-sheet margin. With any realistic ice-divide position, these areas were located roughly two thirds of the distance towards the ice margin. The zone of patchy preservation extends for more than 400 km to the east (Lagerb~ick and Robertsson, 1988; Hirvas, 1991), and is cut by at least one Late Weichselian drumlin fan, the Pajala fan (Kleman, 1990). The existence of sharply defined borderlines (Fig. 6) between the last glacial fluting and older landscapes (Dyke and Morris, 1988; Kleman and Borgstr6m, 1990, 1994; Kleman, 1992) are interpreted to indicate the presence of a fundamental process boundary. The most likely candidate appears to be the boundary between thawed and frozen bed. It is difficult to see how such boundaries could form with a spatially uniform basal condition, be it a frozen bed or a thawed bed, To sum up: Full preservation of minor or fragile forms (boulder fields, paleo-surfaces, sharp-crested eskers, etc.) probably requires continuous frozen-bed conditions. Large-scale landforms such as large drum-
27
lins can probably be morphologically discernible ( something very different from full preservation) after a substantial time of wet based ice-overriding.
3. The basal and marginal geomorphic systems Hitherto there has been a consensus that subglacial landforms such as flutings and drumlins are indicators of basal sliding; i.e. wet-based conditions. The landforms commonly used in reconstructions of glacial dynamics can be described as belonging either to the "wet-bed system", or to the meltwater system, which includes landforms such as eskers, channels and icedammed lake traces. In contrast to the accepted link between the "classic" glacial landforms and wet-based conditions, there has not been a consensus around criteria for discerning frozen-bed conditions. The reviewed work documents the existence of relict surfaces and landforms in formerly glaciated areas, and strongly indicates that the preservation of small or fragile landforms is caused by frozen-bed conditions. It thus appears justified to define a '~dry-bed system", complementing the wet-bed system and marginal meltwater system. In the following, three morphologic systems are defined, and the time-space relationships of landforms in formerly glaciated areas are discussed with the aid of a model based on the inherent properties of the three systems. The dr)., bed system: No new landforms are created, with the exception of occasional glaciotectonic features. The frozen bed condition leads to a hiatus in landform development. The wet bed system: Basal sliding occurs. Flow oriented lineations are continuously produced, destroyed and reoriented. Flow-aligned landforms are fossilized at the cessation of wet-based conditions, i.e. by deglaciation or refreezing of the bed. The marginal melm,ater system: A coherent system of linear features (eskers and/or ice-directed drainage channels) is produced. Despite its small areal coverage the meltwater system carries important information. Forming a spatially coherent system it reflects the decay pattern, and may occasionally be the only system doing SO.
Fig. 7 describes three geomorphic systems present at the base and margin of the ice sheet. An additional
J. Kleman / Geomorphology 9(1994) 19-32
28
Ice sheet
,4 C
Geomorphic systems
Meltwater system ele~ end's Production ot linear In coherent pattern
Fig 7. The figure shows the geomorphic systems that result from processes at the ice sheet base and margin. A full explanation is given in the texl.
system, the supraglacial drift depositional system, is not included in the model. Although this system is important in deglaciation landscapes close to the maximum ice-sheet extent, it appears to be of little importance in the former core area zones with palimpsest lineation landscapes, and has therfore not been included in the model. Where present, it mainly masks older landforms. Following the description of the three geomorphic systems resulting from processes at the ice-sheet base and margin, a set of "viewing rules" can be deduced.
We are interested in the age, location and type of information transmitted upwards through time, parts of it eventually observable to a present day observer (geomorphologist). The easiest way to illustrate these relationships is, however, to invert the system, i.e. to describe instead what happens to the geomorphologist's "line of sight" as it penetrates further and further back in time at specific points in space. What we seek is the intersection between line of sight and the time/ space domains of landform formation. The dry bed system is transparent in the sense that
J. Kleman / Geomorpholo~y 9 (1994) 19-32
we can see through it back in time until we meet the time of landform development, be it subaerial or glacial landforms. When the line of sight meets the wet-bed system, it does not penetrate, if subglacial landform development was intense. Intensity of landform creation, and thereby destruction of older forms is largely a function of flow velocity. Occasionally, as in ice-divide zones, the line of sight can penetrate deeper back in time. The system is opaque to semi-transparent. When the line of sight meets the meltwater system it mostly penetrates. Meltwater traces are typically narrow linear features that only to a minor areal extent destroy or mask the landscape on which they are superposed. For the purpose of tracing regional landform systems, the meltwater system can be regarded as
nearly transparent.
b e d a ne d m etl t w a t e r a g/e o mbo'er dp ,h i/cd r ys yw stems through time
®1 E
bj
Observer system
29
3.1. Time-space relationships of ~eomorphic systems To graphically illustrate the consequences of "preservation" on the interpretation of glacial landform systems, a simple model based on the evolution of a hypothetical ice sheet will be presented. The basic thermal zonation of the hypothetical ice sheet was chosen on the basis of the thermal ice-sheet model of Hooke (1977), the view on temperature conditions of the Scandinavian ice sheet of Sollid and S6rbel (1988), and the works of Lagerbfick (1988a) and Kleman ( 1990, 1992), as well as recent modeling of the Scandinavian ice sheet by G.S. Boulton and co-workers (G. Boulton, pets. commun., 1993 ). Fig. 8a is constructed by plotting an assumed evolution along the transect I-II in Fig. 7 through time. In the real world, with changing flow patterns, ice flow would not always be parallel to the plane of the paper.
C J L i n e of sight penetration back in time d J
Time - s p a c e t o p o l o g y of features
Present interglacial
I-
o
"3 o • c
Q. ~
o
o
~.~_
Q,o
o'~
S p a c e (proflleacroas Ice-sheet base)
f
Marginal me#water system, p~oduces coherent set of linear features
Line of sight penetration: The system is nearly transparent
Wet bed system, continuous production, destruction and reorientation of forms
The systemis semi-tran~parent ~o opaque
Dry bed system, preservation of landforms
The system is transparent
Interstadlal subaerlal system, productlonof periglacial landforms
The system is nearly transparent 7 " ~l°,o,
Fig. 8. The figure describes the time/I D-space relationships of the geomorphic systems that are visible to a present-day observer of a glaciated area. A full explanation is given in the text.
30
J. Kleman / Geomotphology 9 (1994) 19-32
The following is assumed for the hypothetical ice sheet in Fig. 8: During the early build-up phase the last ice sheet is mainly cold-based, in line with conclusions drawn by Hughes ( 1973 ) about ice-sheet inception in permafrost areas. It ends its life partly wet-based. During the early build-up phase of the last ice sheet (stadial 2 in Fig. 8) insignificant or only minor meltwater traces are produced, as ice-sheet growth locks water into the ice sheet. "['he last ice sheet was foregone by (an) earlier ice sheet(s). During the intervening interstadial minor subaerial landforms were created. Fig. 8b illustrates the line of sight of the scientist, a morphologist trying to interpret the glacial traces in such terms as flow direction and time. The 2D case presented here can only illustrate time, a 3D graph would be necessary to show flow direction. Fig. 8c shows the intersection of the geomorphologist's "line of ~ight" with the domains where significant landforms are" produced. The line of sight stops at some (time) depth in the wet-bed system but penetrates the other systems. Fig. 8d shows the landform systems that are visible to lhe observer (us), and topology of that information in lhis t i m e / ( I D ) space diagram. The most important features in Fig. 8d are the continuous and time-transgressive nature of meltwater traces, the age discontinutties of flow traces, and the potential of information from older glacial, as well as non-glacial events. Points (A) and (D) in space represent "classical" glacial landscapes where all the landforms, flow traces as well as meltwater forms, represent a short time span and rettect the same flow regime. In such areas mapping of glacial morphology without time-stratification is meaningful. At point (B), the flow traces and meltwater traces were created during the same stadial but there is a large time gap between them, and they reflect different flow regimes. At point (C) the situation is even more complex. One complete morphological system, the meltwater system from the last deglaciation, as well as three older, only to a lesser extent obscured, landform systems are present. A subaerially formed, probably periglacial, "'varnish" overlies the flow trace~meltwater couple from the preceeding stadial. The four systems represent three different events. Maps over areas with palimpsest such as (B) and (C) should preferably
include representation of the relative age of crosscutting features, or be made in a time-stratified fashion. Rarely can such time stratification be done afterwards by other scientists, even on good single-layer mediumscale maps of glacial morphology (such as Sollid and Kristiansen, 1982; Borgstr6m, 1989).
4.
Discussion
The scheme in Fig. 8 focuses on the temporal relationships between morphological features, but is spatially highly simplified. Real ice sheets have nontextbook flow configurations, ice streams, and complex and patchy basal thermal configurations. This makes the reconstruction of flow pattern evolution even more difficult, but does not affect the basic arguments regarding "line of sight penetration" and the geomorphologist's access to older morphologic strata in areas with continuous frozen-bed conditions. Inversion procedures for extracting the evolution of flow configuration from flow trace palimpsests have been suggested by Kleman (1990) and Boulton and Clark (1990). The evidence for preservation comes from a large variety of regions, glaciers and glaciated areas in a wide variety of settings and environments, and also on a variety of spatial scales. Of the works cited above, Mangerud et al. (1987), England, 1986, Sugden and Watts (1977) and Rodhe (1988) give what can be described as "point" evidence for preservation. More information about basal thermal patterns are provided by Dyke and Morris (1988), Dyke et al. (1992), Kleman and Borgstr6m ( 1990, 1994) and Kleman (1992), who describe intermediate-scale patches of sharply delineated relict surfaces, marking areas of frozen-bed conditions. These observations verify Hughes' (Denton and Hughes, 1981 ) postulate that ice sheet bases have wide transition zones with a patchwork of frozen and thawed areas. Sugden and John's (1976) concept of "selective linear erosion" emphasises topographic, and thereby basal thermal, control over patterns of erosion and preservation. The relationship between topography, basal thermal regime and erosion seems to be widely accepted for fjord landscapes, and there are also indications that some topographic control of basal thermal regime exists also in more subdued topography (Kleman and Borgstr6m, 1993). The "preservation case" has implications outside
Z Kleman / Geomorphology 9 (1994) 1 9 3 2
ice-sheet research. In non-glacial geomorphology it is often assumed (Oilier, 1979; Fairbridge and Finkl, 1980) that study of pre-Quaternary landform development is best performed on non-glaciated cratonic surfaces such as Australia and parts of Africa. It is thought that the Quaternary glaciations have so extensively changed the preglacial morphology in the areas covered by them as to make study of long-term landform development overly difficult. The described evidence for preservation indicates that upland areas and plateaus in arctic Canada and northern Scandinavia have been covered by fully dry (cold)-based non-erosive ice caps or ice sheets during most of the Quaternary. Parts of these areas may therefore well be "Tertiary museums" with a periglacial "varnish". In archaeology the "assumption of glacial destruction" has previously given archaeologists no hope of finding traces of ancient (pre-Holocene) man in the areas covered by the last ice sheets. If he ( and she) were indeed present in those northerly areas before the onset of the last stadial, the possibility of finding traces of humans or their activities cannot any longer be neglected in areas interpreted to have been covered by dry-based ice sheets. In botany the problem of bicentric species and possible nunatak "refugia" (for a comprehensive bibliography, see Nesje et al., 1988) still awaits a satisfactory answer. In the light of the evidence o f ' 'recycled" ground surfaces it may be speculated whether viable material can survive beneath an ice sheet, i.e. whether subglacial refugia may have existed. This possibility was also indicated by Rapp (1992). Indications that seeds can be viable for very long periods in a dry, cold environment were given by Porsild et al. ( 1967 ) who described a find of viable seeds in a muck deposit in Alaska, interpreted to be at least 10,000 years old. A structure such as a boulder field beneath a cold(dry)based ice sheet provides a cold and cosmic-radiation protected environment where seeds could possibly remain viable for long periods of time.
5. Conclusions
- Small- and medium-scale landforms predating the last glacial event occur scattered in the areas covered by the Fennoscandian and the Laurentide ice sheets. - Preserved landforms mark areas where the last ice
31
sheets were continuously dry (cold)-based. - The morphological impact of an ice sheet is related to migration over time of the wet-bed system, dry-bed system and the marginal meltwater system.
Acknowledgements This study was funded through a grant from the Swedish Natural Science Research Council. I want to thank Bo Str6mberg, Stig Jonsson, Ingmar Borgstr6m, Clas H~ittestrand and two unknown referees for constmctive comments on the manuscript.
References Andrews. J.D. and Miller, G.H., 1976. Quaternary glacial chronology of the eastern Canadian arctic; A review and a contribution on amino acid dating of Quaternary molluscs from the Clyde Cliffs. In: W. Mahaney (Editor), Quaternary Stratigraphy of North America. Dowden, Hutchinson and Ross, Stroudsburg, Pa., pp. 1-32. Borgstrtim, I., 1989. Terr~ingformerna och den glaciala utvecklingen i de s6dra fj~illen. Department of Physical Geography, Stockholm University, Meddelande 234, 133 pp. Boulton, G.S., 1972. The role of thermal regime in glacial sedimentation. Inst. Br. Geogr. Spec. Publ., 4: 1-19, Bouhon, G,S. and Clark, C.D., 1990. A highly mobile Laurentide ice sheet revealed by satellite images of glacial lineations. Nature, 346: 813-817. Dahl, R., 1966. Block fields, weathering pits and tor-like forms in the Narvik mountains, Nordland, Norge. Geogr. Ann., 48A: 5585. Denton, G.H. and Hughes, T.J., 1981. The Last Great Ice Sheets. Wiley, New York, 484 pp. Drewry, D., 1986. Glacial Geologic Processes. Edward Arnold, London, 276 pp. Dyke, A.S., 1983. Quaternary geology of Somerset Island, District of Franklin. Geol. Surv. Can. Mem., 404:32 pp. Dyke, A.S. and Morris, T.F., 1988. Drumlin fields, dispersal trains, and ice streams in Arctic Canada. Can. Geogr., 32: 86-90. Dyke, A.S., Morris, T.F., Green, D.E.C. and England, J., 1992. Quaternary geology of Prince of Wales Island Arctic Canada. Geol. Surv. Can. Mem., 433:142 pp. England, J., 1986. Glacial erosion of a high Arctic Valley. J. Glaciol., 32: 60-64. Fairbridge, R.W. and Finkl, C.W., 1980. Cratonic erosional uncomfortuities and peneplains. J. Geol., 88: 69-82. Follestad, B,A., 1990. Block fields, ice-flow directions and the Pleistocene ice sheet in Nordm0re and Romsdal, West Norway. Norsk Geol. Tidsskr., 70: 27-33. Goldthwait. R.P., 1960. Study of ice cliff in Nunatarssuaq, Greenland. Tech. Rep. Snow Ice Permafrost Res. Establ., 39: 1-103,
32
J. Kleman / Geomotphology 9 (1994) 19-32
Grant, D.R., 1989: Quaternary geology of the Atlantic Appalachian region of Canada. In: R.J. Fulton (Editor), Quaternary Geology of Canada and Greenland. Geological Survey of Canada, Geology of Canada, No. 1, pp. 393~-~40. Hirvas, H., 1991. Pleistocene stratigraphy of Finnish Lapland. Geol. Surv. Finland Bull., 354 pp. Hntdsworth, G. and Bull, C., 1970. The flow law of cold ice; investigations on Meserve glacier, Antarctica. In: A.J. Gow et al. (Editors), Int. Syrup. on Antarctic Glaciological Exploration (ISAGE). Int. Assoc. Sci. Hydrol. Publ., 86: 204-216. Hooke, R. LeB., 1977. Basal temperatures in polar ice sheets: A qualitative review. Quat. Res., 7: 1-13. Hughes, T., 1973. Glacial permafrost and Pleistocene ice ages. In: Permafrost: The North American Contribution to the Second International Conference. National Academy of Sciences, Washington, DC. Ires. J.D.. 1957. Glaciation of the Torngat mountains, northern Labrador. Arctic, 10( 2): 67-88. lw:s, J.D., 1966. Associated weathering forms on mountain tops and the nunatak hypothesis. Geogr. Ann., 48A(4): 220-223. Jonsson. S., 1983. On the geomorphology and past glaciation of StorOya, S valbard. Geogr. Ann., 65A( 1/2): 1-17. Kaitanen, V., 1969. A geographical study of the morphogenesis of Northern Lapland. Fennia, 99( 5 ): 1-85. Kk~man, J., 1990. On the use of glacial striae for reconstruction of paleo-ice sheet flow patterns - - With application to the Scandinavian ice sheet. Geogr. Ann., 72A ( 3/4) : 217-236. Klcman, J., 1992. The palimpsest glacial landscape in northern S w e d e n - Late Weichselian deglaciation landforms and traces of older west-centered ice sheets. Geogr. Ann., 74A(4): 305-325, Kleman, J. and BorgstrOm, 1., 1990. The boulder fields of Mr. Fulufjfillet, west-central Sweden - - Late Weichselian boulder blankets and interstadial periglacial phenomena. Geogr. Ann., 72A( I ): 63-78. Kleman, J. and Borgstr6m, 1., 1994. Glacial landforms indicative of a partly frozen bed. J. Glaciol., in press. Kleman, J., Borgstr6m, I., Robertsson, A.-M. and Lilliesk01d. M.. 1992. Morphology and stratigraphy from several deglaciations in the Transtrand mountains, western Sweden. J. Quate. Sci., 7(1): 1-16. Korpela, K.. 1969. Die Weichsel-eiszeit und ihr interstadial in Per~.pohjola ( N6rdlisches NordFinnland) im licht yon submor~.nen sedimenten. Ann. Acad. Sci. Fenn., Ser. A III, 99:108 pp. Kujansuu, R., 1975. Interstadial esker at Marrasjfirvi, Finnish Lapland. Geologi, 27(4):45-50. Lagerbfick, R.. 1988a. The Veiki moraines in northern Sweden - widespread evidence of an early Weichselian deglaciation. Boreas, 17,469-486. Lagerbfick, R., 1988b. Periglacia[ phenomena in the wooded areas
of northern Sweden - - relicts from the T~irend6 interstadial. Boreas. 17: 487-500. Lagerbfick, R. and Robertsson, A.-M,, 1988. Kettle holes I stratigraphical archives for Weichselian geology and paleoenvironment in northernmost Sweden. Boreas, 17: 439-468. Mangerud, J., Bolstad, M., Elgersma, A., Helliksen, D., Landvik, J.Y., Lycke, A.K., L6nne, I., Salvigsen, O.. Sandahl, T. and Sejrup, H.P., 1987. The Late Weichselian glacial maximum in Svalbard. Polar Res., 5 (n.s.) : 275-278. Nesje, A., Dahl, S.O., Anda, E. and Rye, N., 1988. Block fields in southern Norway: Significance for the Late Weichselian ice sheet. Norsk Geol. Tidskr., 68: 149-170. Oilier, C.D., 1979. Evolutionary geomorphology of Australia and Papua-New Guinea. Inst. Br. Geogr. Trans. New Ser., 4: 516539. Paterson, W.S.B., 1981. The Physics of Glaciers. Pergamon, Oxford, 380 pp. Porsild, A.E.. Harington, C.R. and Mulligan, G.A., 1967. Lupinus arcticus Wats. grown from seeds of Pleistocene age. Science, 158: 113-114, Prest, V.K., Grant, D.R. and Rampton, V.N., 1968. Glacial Map of Canada. Geol. Surv. of Canada, Map 1253A. Rapp, A., 1992. K~irkevagge revisited. Field excursions on geomorphology and environmental history in the Abisko mountains, Sweden. Sver. Geol. Unders. Ser. Ca, 81: 269-276. Rodhe, L., 1988. Glaciofluvial channels formed prior to the last deglaciation: examples from Swedish Lapland. Boreas, 17:511516. Schytt, V., 1974. Inland ice sheets - - recent and Pleistocene. Geol. F6ren. Stockholm F6r., 96: 298-309. Shreve. R.L., 1984. Glacier sliding at subfreezing temperatures. J. Glacol., 30: 341-347. Sollid, J.L. and Kristiansen, K., 1982. Hedmark Fylke, KvartaergeoIogi og geomorfologi 1:250,000. Geografisk Institutt, Oslo Universitet. Sollid, J.L. and S6rbel, L., 1979. Deglaciation of western Central Norway. Boreas, 8: 233-239. Sollid, J.L. and SOrbel, L., 1988. Influence of temperature conditions in formation of end moraines in Fennoscandia and Svalbard. Boreas, 17: 553-558. Sugden, D.E., 1968. The selectivity of glacial erosion in the Cairngorm mountains, Scotland. Trans. Inst. Br. Geogr., 45: 75-92. Sugden, D.E. and John, B., 1976. Glaciers and Landscape. Edward Arnold, London, 376 pp. Sugden, D.E. and Watts, S.H., 1977. Tors, felsenmeer and glaciation in northern Cumberland peninsula, Baffin Island. Can. J. Earth Sci., 14: 2817-2823. Vorren, T., 1979. Weichselian ice movements, sediments and stratigraphy on Hardangervidda, South Norway. Norges Geol. Unders., 350:1-117.