Geological evidence to constrain modelling of the

The Physical Basis of Ice Sheet Modelling (Proceedings of the Vancouver Symposium, August
1987). IAHS Publ. no. 170.
Geological evidence to constrain modelling of the
Late Pleistocene Rhonegletscher (Switzerland)
W. Haeberli
Versuchsanstalt fUr Wasserbau, Hydrologie und Glaziologie
ETH Zurich, ETH-Zentrum
CH-8092 Zurich, SWITZERLAND
Ch. Schliichter
Ingenieurgeologie
ETH Zurich, ETH-Hbnggerberg
CH-8093 Zurich, SWITZERLAND
ABSTRACT By using the example of the Late Pleistocene Rhonegletscher in
the Swiss Plateau, geological and geomorphological evidence about geometry,
temperature, bed characteristics, water pressure, debris load and surface mass
balance is discussed as a basis for reconstructing and modelling vanished glaciers. A relatively non-contradictory picture can be drawn for the ablation area
around 18 ka BP. However, realistic modelling of the ice body would require
the application of a 3-dimensional, thermomechanically coupled system of time
dependent ice and permafrost over the whole glacier area. This is made
difficult by fundamental uncertainties about the ice geometry in the former
accumulation area, about subglacial water pressures and sliding velocities, and
about the appropriate time for the ice build-up. Qualitative rather than quantitative results can therefore be expected from model calculations in the foreseeable future.
Evidence géologique déterminant la modélisation du glacier du Rhbne (Suisse)
a la fin du Pleistocene
RESUME En prenant pour exemple le glacier du Rhône dans le Plateau Suisse
à la fin du Pleistocene, on discute l'évidence géologique et géomorphologique
sur la géométrie, la température, les charactéristiques du lit, les pressions d'eau,
la charge de débris et le bilan de masse à la surface de la glace comme base
pour reconstruire et modéliser des glaciers disparus. Pour la situation d'il ya
18,000 ans, une image relativement claire peut être dessinée pour la zone
d'ablation. Cependant, pour un modèle réaliste, on devrait considérer un
système à 3 dimensions et qui couple les aspects thermiques et mécaniques de
la glace et du pergélisol sur toute la région englacée en fonction du temps. Les
difficultés principales proviennent des incertitudes sur la géométrie de la surface de la glace dans la région d'accumulation, les pressions d'eau sousglaciaires et les vitesses de glissement basai ainsi que sur l'échelle de temps pour
la formation du glacier. Pour ces raisons, des calculs avec des modèles
333
334
W. Haeberli & Ch. Schlùchter
numériques apporteront des résultats plutôt qualitatifs que quantitatifs, au moins
pour un avenir pas trop lointain.
INTRODUCTION
The investigation of vanished (in most cases late Pleistocene) ice bodies is usually done either
to explain geological and geomorphological phenomena, or to derive paleoenvironmental (especially paleoclimatological) evidence. To achieve these goals, various glaciological factors may
need to be understood: for example, ice flow characteristics to understand the distribution of
erratics, mass balance patterns to calculate snowline depressions, or thermal ice conditions to
interpret observed glacier bed features. During recent years, many attempts have been made to
quantify the available information and to model vanished ice bodies. Time-dependent models
have been applied to investigate the general retreat of temperate mountain glaciers after the Little Ice Age (e.g., Smith & Budd, 1981; Kruss, 1983; Kruss & Smith, 1983); on the other hand,
steady-state models have been used for some less well documented, polythermal, late Pleistocene ice sheets and glaciers (e.g., Sugden, 1977; Boulton et ai, 1984). Simple approaches can
be useful to estimate roughly paleoglaciological and paleoclimatological parameters (e.g.,
Pierce, 1979; Kôrner, 1983; Haeberli & Penz, 1985). However, a deeper understanding of the
processes involved can be reached only with complex 3-dimensional, thermomechanical, and
time-dependent models. One basic limitation to the applicability of such models is the lack of
adequate geological and geomorphological background information that can be used as an input
or a test for the calculations. This information mainly pertains to parameters such as the
geometry, surface temperature, characteristics of the bed and the basal ice layer, subglacial
water pressure, debris load, surface mass balance, andfluctuationsof the vanished ice body.
The traces and deposits left behind by the Late Pleistocene Rhonegletscher in the Swiss
Plateau have been studied from the very beginning of Ice Age research. It was here that the
former advances of Alpine ice lobes to the foreland were first recognized and mapped (de
Charpentier, 1841). More recent investigations have focused on local details, such as pétrographie composition, fades and stratigraphy of glacial and fluvioglacial deposits (Portmann,
1966; Van der Meer, 1982). Current studies of glacial deposits and analysis of the associations
of glacial facies demonstrate far more complexity and diversity for the Rhonegletscher area
than for more strictly defined, nearby valley glaciers (Wohlfarth-Meyer, 1986; Schlùchter &
Wohlfarth-Meyer, in press). Using the Rhonegletscher as an example of a late Pleistocene ice
body, the possibilities and limitations of efforts to reconstruct paleoglaciological information
from geological and geomorphological evidence will be discussed.
GEOMETRY
Former ice margins can usually be inferred (a) by mapping remains of lateral and terminal
moraines in the former ablation area where emerging flow must have predominated, and (b) by
tracing uppermost limits of glacial erosion within the former accumulation area, where submerging flow did not allow the formation of moraines. The basic problem is the question of
whether the information from the two areas relates to the same time period or not. In contrast
to the former ablation area, where relative age dating of moraines gives a fairly reliable picture,
dating of the uppermost traces of glacial erosion in the former accumulation area is still highly
uncertain. The possibility that these uppermost traces, which are observed today on high
mountain peaks in the Alps, date from earlier stages of the last glaciation, or even from an earlier glaciation, cannot be excluded. If so, tectonic uplift may have considerably influenced the
altitudinal position of such traces. Precision levelling across the Swiss Alps and their foreland
seems to indicate trends in recent crustal movements; the former accumulation area of Rhonegletscher in the central part of the Alps is characterized by an uplift of 1 to 2 mm a-1, whereas
the former ablation area in the Swiss Plateau and the Lake Geneva basin is subsiding slightly
Geological evidence from the Pleistocene Rhonegletscher
335
(Gubler, 1976). These movements account for a relative displacement of the foreland with
respect to the Alps of up to 3 mm/year, or about 30 m during the Holocene. This effect is
negligible for interpreting the 18 ka BP situation, but has to be taken into consideration for studies of earlier glacial events (cf. Schlùchter, 1981). In any case, the problem exists that ice
bodies that have been reconstructed by comparing erosional and depositional features may be
time-transgressive and, hence, unsuitable for model calculations.
Based on inferred ice margins, positions of the former glacier bed, as well as of the
former ice surface, can be determined. One can account for postglacial sedimentation in topographic depressions by using results of drilling and geophysical sounding. In the Plateau area,
deposits from the retreat phase of the last glacial maximum are mainly confined to overdeepened troughs. Such large-scale sediment traps are to be found in the vicinity of still-open
lake basins, where total sediment thickness can be on the order of a few hundred meters (Fig.
6) and postglacial sediment thickness can reach 60 m (Kellerhals & Trohler, 1976). This value
can be considered to be an upper limit for the deviation of the present-day surface from the
glacier bed within the former ablation area. In the lower reaches of the Wallis main valley
(upvalley from Lake Geneva, i.e., in the former accumulation area), postglacial sediment accumulation is higher and may exceed 100 m in places. Postglacial erosion is negligible with
respect to other uncertainties.
Estimation of the curvature of the contour lines at the ice surface is another problem.
Table 1 gives some maximum values of the altitudinal difference (hf-hm) between the ice surface at the central flowline (/) and at the margins (m) for some recent glaciers as a function of
the glacier width (b). The sample represents a wide variety of glacier types and shows that the
altitude difference between the central flowline and the margins varies between about 20 and
200 m; 50 to 100 m may be a reasonable average for large glaciers with more or less confined
flow. An uncertainty of up to 100 m in defining the ice thickness at the centralflowlinehas to
be accepted. This corresponds roughly to a 10-20% error in the cases of large, late Pleistocene
ice bodies in the Swiss Plateau. Such an error is about twice as much as the uncertainty of
modem thickness determinations on existing glaciers by seismic or radio-echo sounding (e.g.,
Bogorodsky et al., 1985; Haeberli & Fisch, 1984).
TABLE 1 Altitudinal difference between ice surface at the central flowline and at the margins
(hf-hm) as a function of glacier width (b)for some selected glaciers (ablation area)
Name
Aletsch
Glacier Type
temperate, valley
Hintereis
Bondhusbreen
McCall
White
Columbia
temperate, valley
temperate, valley
cold, valley
cold, valley-piedmont
temperate, valley, tidal
Siduj'bkull
temperate, piedmont
b(lcm)
.65
1.2
1.5
.7
.22
.45
.75
5.2
5.8
8.2
17.0
hf-hm (m)
35
40
30
20
30
30
30
45
60
40
200
Data are from official maps and Bfôrnsson (1986) for Sidufôkull, Blatter (1985) for White
Glacier, and Haeberli (1985) for Bondhusbreen and Hintereisferner.
Figures 1 and 2 show the presumed geometry of the ablation area of the late Pleistocene
Rhonegletscher in the Swiss Plateau as taken from Jâckli (1970) and modern topographic
maps. The accumulation area has been omitted since information on it is too uncertain; the
W. Haeberli & Ch. Schluchter
336
southern (French) lobe of the ice body is not shown either. The glacial lobe depicted was
some 120 km long and, on average, about 60 to 80 km wide. The equilibrium line must have
been close to 1000 m a.s.l.; the mean basal shear stress along the centralflowlinehas been calculated to be around 30 kPa (Haeberli & Penz, 1985). Similarly low stress values have been
calculated for six 18 ka BP piedmont glaciers of the northern foreland of the eastern Alps
(Kbrner, 1983).
FIG. 1 Ice margin and bed topography of the 18 ka BP Rhonegletscher in the
Swiss Plateau. Contour lines of the former glacier bed correspond to the generalized present day surface; altitudes above 1000 m a.s.l. are not indicated.
TEMPERATURE
In order to estimate ice surface temperatures, it is necessary to derive former mean annual air
temperatures, to extrapolate them to various altitudes using some reasonable lapse rates, and to
convert them into ice and firn temperatures. The most reliable features indicating mean annual
values of paleotemperatures are fossil permafrost phenomena such as pingos, rock glaciers, ice
wedges and (large-scale) glacitectonically deformed sediments. Ice wedge casts are sometimes
found in the Swiss Plateau, whereas glacitectonically-deformed sediments occur more frequently (e.g., Schindler et al, 1978; Van der Meer, 1980). The Rhonegletscher ice lobe was
surrounded by permafrost at the time of its maximum extent in the late Wtlrm period (18 ka
BP). In fact, during late Pleistocene time, the southern limit of continuous permafrost probably
Geological evidence from the Pleistocene Rhonegletscher
337
was close to the Pyrenees and the Mediterranean Sea (Washburn, 1979). Mean annual air temperatures in the Swiss Plateau could have been as low as about -10°C during the coldest
periods. Selecting an appropriate lapse rate is delicate, because strong temperature inversions
probably formed between the Alps and the Jura mountains. Even with a zero temperature gradient up to about 1000 m a.s.l., it is reasonable to assume that near-surface (10 m) temperatures were below freezing in the ablation area, where the difference between mean air and ice
temperature must have been only a few degrees. Due to the possible effects of percolating
meltwater, thermal conditions in the accumulation area may have been more complicated (cf.
Hooke et al., 1983).
FIG. 2 Reconstructed surface of the 18 ka BP Rhonegletscher in the Swiss
Plateau, after Tàckli (1970). See Fig. 1 for bed topography.
Figure 3 shows the assumed temperature distribution within a late Pleistocene piedmont
glacier in the, Alps. The graph is based on the results presented by Blatter & Haeberli (1984)
from a preliminary model calculation on the nearby Rheingletscher. Uncertainties in such calculations arise not only from uncertainties in estimating surface temperature conditions and in
calculating ice flow, but also from errors in the assumed value of geothermal heat flow, which
was assumed to be constant. Between the periglacial region and the head of the modelled glacier, the glacier bed first warms up in the marginal parts of the ice lobe, and then very strongly
cools down again where cold ice from the high-altitude firn zone touches the earth's surface.
This sequence would also occur at a given point in time if such a polythermal glacier
advanced. As a result, the terrestrial heat flow would be strongly reduced, if not inverted, in
338
W. Haeberli & Ch. Schlïichter
the marginal zone, with warm ice moving over cold ground, but would be strongly enhanced
where the warm bed was cooled by cold ice from the accumulation area. Such a heat-flow distribution would, at least temporarily, have cooled down the warm parts and warmed up the
cold parts of the former glacier bed with respect to the model shown in Fig. 3 and, hence, have
had a smoothing effect on horizontal temperature differences.
FIG. 3 Estimated temperature distribution within an 18 ka BP piedmont glacier of the Alps, after Blatter & Haeberli (1984). Indicated numbers are °C.
Dashed lines point to numerical instabilities of the calculation.
It is evident that correct modelling of realistic conditions can only be achieved by a
thermo-mechanically coupled system of time dependent ice and permafrost over the whole glacier area. Nonetheless, it seems clear that large parts of the glacier bed must have been at
sub-freezing temperatures and that sliding processes could hardly have played a predominant
role for the overall glacier behavior around 18 ka BP. Different conditions may have existed
during the earlier advance period.
Temperature modelling of Pleistocene glaciers and permafrost can be checked against
information about terrestrial heat flow. Haeberli et al. (1984) discussed geothermal effects of
18 ka BP ice conditions in the Swiss Plateau. The greatest effects due to heat conduction have
to be expected at a depth of roughly 1-2 km (Fig. 4). The depression of the temperature at this
depth is estimated to be about 5°C for formerly temperate glacier beds, some 6 to 8°C for
formerly periglacial regions and up to perhaps 10°C for very cold parts of the former glacier
bed. The corresponding heat flow reduction within the uppermost 1000 m depth is on the
order of about 10-25%. Effects due to latent heat exchange during the melting of ice-rich, late
Pleistocene permafrost are assumed to be insignificant in most cases.
The most recent compilations of terrestrial heat flow data in Switzerland are given by
Bodmer (1982) and Bodmer & Rybach (1984). Their heat flow values are corrected for topographical effects, and were calculated from temperature measurements in deep boreholes and
from thermal conductivity determinations using a representative sample of rock specimens.
Figure 5 shows the distribution of heat flux in the region of the late Pleistocene
Geological evidence from the Pleistocene Rhonegletscher
339
Rhonegletscher. With an average value of 80-85 mW m~2 for the present-day heat flux in the
region under study, paleoglaciologically induced variations of about 70-95 mW m~2 could
theoretically be expected. The highest heat flow values are indeed observed in areas where the
former glacier bed could have been at or very close to the phase-equilibrium temperature (cf.
Harrison, 1975). The observed regional variations in heat flux, however, cannot be easily
explained in detail by paleoglaciological effects, and are larger than anticipated; they probably
include other effects, such as forced water convection or erosion/sedimentation effects.
0
-2
-4
-6
-8°CAT*
FIG. 4 Perturbation AT* of geothermal profiles beneath the Swiss Plateau, as
a function of depth z, for various Ice Age surface temperatures in °C. (From
Haeberli et al, 1984.)
BED CHARACTERISTICS
The currently exposed beds of late Pleistocene glaciers may furnish information about (a) subglacial erosion and. sedimentation, (b) the distribution of deformable and nondeformable substrates, (c) the debris load within and at the surface of the ice, (d) effective normal stresses and
(e) total displacements of morainic material at the sole of the former ice body. The total debris
input into the 18 ka BP Rhonegletscher was probably small, because the occurrence of rock
walls capable of producing debris must have been limited. At the same time, the transport
capacity of the meltwater stream must also have been low, due to the reduced precipitation and
the small inclination of the sub- and proglacial topography (cf. Haeberli, 1986). As debris
from surrounding rock walls was not able to reach far into the accumulation area, or even
directly into the ablation area, the debris content of the ice must have been strongly concentrated near the base of the glacier, with a thin, if not almost non-existent moraine cover on the
surface.
340
W. Haeberli & Ch. Schlïïchter
FIG. 5 Heat flux, in mW m 2, in the region of the 18 ka BP Rhonegletscher in
the Swiss Plateau (after Bodmer & Rybach, 1984).
Figure 6 shows the distribution of rocky and sedimentary parts of the late Pleistocene
Rhonegletscher bed. Two main zones must be discriminated: (a) the high area to the north of
Lake Geneva, i.e., the region of ice-transfluence to the Plateau, where bedrock with a thin till
cover predominates; and (b) the low-lying plateau area with two main erosional troughs of considerable overdeepening containing extensive Pleistocene (partially pre-Wùrm) deposits, the
troughs being separated by ridges of bedrock that are, in places, covered by thin till layers.
Extensive accumulations of water-laid tills can be found in the refilled parts of the overdeepened troughs which are today partially occupied by the lakes of Murten, Neuenburg and
Biel. Subglacial ponding must have occurred here and even seems to have extended several
hundred meters beyond the actual lake basins around the time of maximum ice extent. Similar
conclusions were drawn from the Lake Zurich drilling project, which recorded up to 90 m of
water-laid tills within the reach of the nearby Linthgletscher (Schlûchter, 1982a, 1984). The
fact that these water-laid tills within Lake Zurich may have been glacitectonically deformed
supports the idea that the ice was indeed cold-based during its final advance (Hsti et al. 1984)
and, hence, that beds of the last Pleistocene ice advances in the Swiss Plateau probably
changed from temperate to cold between the build-up and the maximum stage. Outside the
valley floors, longitudinal ridges of bedrock (rock drumlins) and composite ridges (bedrock
with a cover of deformed sediments and basal till) occur depending on the nature of bedrock
and on the scale of the observations. Depositional forms clearly indicative of formerly warmbased ice, such as eskers or fluted moraines (Sugden & John, 1976), have not been reported so
far and may not have survived the final period of cold-based ice.
Geological evidence from the Pleistocene Rhonegletscher
341
FIG. 6 Bed characteristics of the 18 ka BP Rhonegletscher in the Swiss Plateau. Thick layers of unconsolidated sediments formed the glacier bed within
the areas of overdeepened basins and actual lakes.
One distinct feature of the sediments related to the ice advances into the Alpine foreland
is the conspicuous sequence of fluvioglacial gravels often encountered. These coarse elastics
belong to the meltwater systems which must have been active during the early phase of ice
build-up. Their advance-related characteristics have been demonstrated by Schlùchter (1973,
1976, 1979), Van der Meer (1982) and Wohlfarth-Meyer (1986). The most striking aspect is
the almost complete absence of equivalent fluvioglacial elastics from the retreat period
(Schlùchter, 1983). Debris of supraglacial origin is very sparse within the present surfaceforming till cover from the final advance. These observations seem to reflect the fact that
down-wasting of rather clean ice took place under conditions of a dry, continental climate after
18 ka BP. The grain-size distribution of a typical basal lodgement till from Rhonegletscher
reflects a distinct percentage of sand from incorporated molasse sandstone, indicating relatively
short transportation distances at the ice base. In addition, where high-rising bedrock ridges
occur, an upward-progressive reworking and mixing of local bedrock with far-travelled Alpine
components can be observed. The pebble fraction is clearly dominated by Alpine rock types
(up to 75%) in the area to the east of the Lake of Biel (Wohlfarth-Meyer, 1986); the amount of
reworked molasse varies considerably. The fact that a remarkably large fraction of these tills
(up to 30%) consists of lithologies from the Jura Mountains remains to be explained. Tracer
boulders, i.e., specific lithologies from the Alpine drainage basins, are restricted to certain
regions and are absent in others. This implies a well defined flow pattern within the ice lobe,
probably persistent over a considerable length of time.
342
W. Haeberli & Ch. Schlïichter
The degree of consolidation of formerly subglacial sediments could furnish information
on effective normal stresses and subglacial water pressures in the past. In principle, compaction tests only allow speculation about the maximum load (or minimum water pressure) ever
attained during or after the deposition of the sediment. Maximum load is most likely to occur
within marginal areas of temperate glaciers, where water flows in open channels. Strong compaction of unconsolidated sediments is thereby possible if the pore water can drain during the
lodgement process. This is not the case with a cold-based glacier and subglacial permafrost,
nor with a floating ice body and deposition of water-laid till. Due to the fact that such conditions may have predominated around 18 ka BP, interpretation of results from compaction tests
and from measurements of moisture content on samples from Rhonegletscher sediments
remains difficult. So far, it has only been possible to discriminate between normally consolidated and overconsolidated sequences. Overconsolidated sediments generally occur outside the
overdeepened troughs. The question of whether this pattern has been influenced by thaw settlement due to permafrost degradation has not been investigated so far.
MASS BALANCE
The equilibrium line altitude and the mass balance gradient determine the mass balance distribution at the surface of an ice body. Equilibrium line altitudes are commonly derived from an
average accumulation area ratio, or from the elevation at which lateral moraines first start to
show (Gross et al, 1977). Errors are insignificant for small glaciers but considerable for large
ice masses.
The determination of a reasonable balance gradient is less direct The mass balance gradient is known to be related to the continentality of the climate (Kuhn, 1981). Low gradients,
a low mean annual air temperature at the equilibrium line and, hence, a low mass turnover
within cold glaciers, seem to be typical for dry, continental conditions, whereas high gradients
with a correspondingly important mass turnover in temperate glaciers occur in wet, maritime
climates with a high mean annual air temperature at the equilibrium line (Haeberli, 1983).
Boulton et al. (1984) have schematized this pattern for the purpose of modelling. The best
indicators of paleoprecipitation are pollen records and permafrost features, the latter of which
are limited to dry, continental conditions. The occurrence of such features in the Swiss Plateau
indicates that a climate favoring the existence of low balance gradients must have prevailed on
and around Rhonegletscher during late Pleistocene time (Haeberli, 1983). The same conclusion
can be drawn from pollen records. In his compilation of palynological evidence on paleoclimate during the last Ice Age, Frenzel (1980) indicates that precipitation was markedly reduced
with respect to today's values in Central Europe during the whole Wùrm period, especially
during the time of maximum glaciation; this was a time when annual precipitation must have
been far below 500 mm and when the difference between summer and winter temperatures was
almost twice as large as today's. Low balance gradients lead to the formation of relatively
long and thin glaciers (Boulton et al, 1984) with correspondingly low basal shear stresses.
The low value of the mean basal shear stress calculated for the late Pleistocene Rhonegletscher
is therefore not astonishing, and probably needs no explanation in terms of extraordinary sliding processes (cf. Beget, 1986).
The reverse procedure, i.e., the calculation of mass balance characteristics starting from
the reconstructed geometry of the vanished glaciers has been attempted by Haeberli & Penz
(1985). Despite the great uncertainty of such calculations, it seems to be quite obvious that the
late Pleistocene Rhonegletscher was very weakly active and that the ice flow was driven by a
low mass balance gradient Conditions probably resembled those of a cold desert.
Geological evidence from the Pleistocene Rhonegletscher
343
FLUCTUATIONS
The last main advance as depicted in Fig. 1 is commonly correlated with global cooling around
18 ka BP. However, it has not yet been dated directly, and it has even been proposed that the
Rhonegletscher did not extend out of the Lake Geneva basin during Wiirm time (Burri, 1977).
Disregarding this isolated opinion, it remains uncertain whether the ice build-up to the maximum position should be represented as one single event or rather a multistadial evolution
which started already around 85 to 90 ka BP (cf. Van der Meer, 1982). The last glacial maximum in the Lake Geneva basin is assumed to have occurred around 20 ka BP and to have
been preceded by a major stadial around 70 ka BP which covered, at least partially, the
western part of the Plateau (Am, 1984). Due to warming after 70 ka BP, the Plateau area
would have become ice free again and the 20 ka BP ice lobe would have built up within less
than about 30 ka after about 45 ka BP (Arn, 1984). The neighboring Aaregletscher displays a
more clearly pronounced two-phase chronology for the last glaciation (Schlùchter, 1982b).
Following Welten (1982) and his interpretation of the important Meikirch section, a model with
a quick build-up within about 30 ka seems to be most realistic. A slower, multistadial evolution within about 60 ka, however, cannot be strictly ruled out. The final downwasting and
decay of ice in the Plateau can be placed at around 14 ka BP on the basis of radio-carbon dating (Ammann, 1985; Gaillard, 1985).
CONCLUSIONS
Geological and geomorphological evidence allow a relatively non-contradictory picture of the
former ablation area of the late Pleistocene Rhonegletscher in the Swiss Plateau to be drawn.
The flat, and probably predominantly cold ice lobe present in the latest phase of maximum
extent terminated finally within continuous permafrost. It flowed under low basal shear stresses
and, correspondingly, deformed very slowly. No firm evidence exists that extraordinary sliding
processes (surges) or large scale deformation (over many kilometers) of "soft" beds took
place. Mass turnover must have been low, and the supraglacial debris load almost negligible.
Length variations of the order of 100 km happened over time periods of some 10,000 to less
than 100,000 years. During the ice build-up before 18 ka BP, climatic conditions of a less
continental type, with more active and warm-based ice, probably prevailed.
Realistic modelling of such ice bodies requires the application of a 3-dimensional and
thermomechanically coupled system of time-dependent ice and permafrost over the whole glacier area (cf. Hutter & Vuillet, 1985; Hutter et al, 1986). Such an attempt is made difficult by
the severe limitations of geological and geomorphological input and test data. The most
important gaps are the uncertainties concerning the reconstruction and estimation of the ice
geometry in the former accumulation area, subglacial water pressure and sliding velocities, and
the appropriate time scale for the ice build-up. In the foreseeable future, therefore, qualitative
rather than quantitative results can be expected from model calculations about Ice Age glaciers
in the Swiss Plateau. In the meantime, more quantitative information about past ice conditions
should be collected and derived from geological and geomorphological observations, overdeepened basins and actual lakes.
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