Surface geometry of the Last Glacial Maximum (LGM)in the southeasternSwiss Alps (Graubünden) and itspaleoclimatological significance

Using detailed field evidence provided by trimlines on former nunataks, erratic boulders and the orientations of glacial striae, the surface geometry in the accumulation area during the Last Glacial Maximum was reconstructed for the area of SE Switzerland and adjacent Italy. Collectively, the trends of trimline eleva­ tions, flowlines deduced from glacial striae and be­ drock morphology along the longitudinal valleys and their tributaries indicate that the former accumulation area consisted of an ice dome with the ice divide loca­ ted over the area enclosed by Schlarignia, Cinuos-chel, Livigno and Piz Bernina. It attained a minimum altitude of approximately 3000 m. Modelling the topography of the ice surface using a Geographical Information Sy­ stem (GIS) is consistent with these results. The paleoclimatological signal included in this surface geometry was used to draw conclusions about the main atmospheric paleocirculation patterns and to outline the principal precipitation areas for the Alps during the last glaciation. It followed from this that ice build-up was principally related to dominating precipitation by southerly circulation (foehn). The prevaleance o f foehn circulation most likely reflects a southward shift o f the North Atlantic polar atmospheric front and of the ac­ companied storm track due to the advancing margin of sea ice. There exists good agreement between these assumpti­ ons and (a) results of global circulation models for the time of the LGM; (b) estimations of basal shear stress va­ lues and flow velocities for Ice Age glaciers; and ( c ) in­ terpretations of paleowind indicators. [Die Oberf lächengeometr ie des letzteiszeit l ichen M a x i m u m s (LGM) in d e n südöst l ichen s c h w e i z e r Alpen ( G r a u b ü n d e n ) u n d i h r e paläokl imatolog ische Bedeutung] Kurzfassung: Anhand detaillierter Feldbefunde, wie z.B. Schliffgrenzen an ehemaligen Nunatakern, der Ver­ teilung erratischer Blöcke und der Orientierung von Gletscherschrammen, wurde die Oberflächengeome­ trie des Akkumulationsgebietes des letzteiszeitlichen Maximums im Gebiet Südostschweiz und Norditalien rekonstruiert. Der Höhenverlauf der Schliffgrenzen, die aus den Striemungsdaten abgeleiteten Fließbewegun*) Adress o f the Author: D. Florineth, Geologisches In­ stitut, Universität Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland. gen des Eises, und die Morphologie des Felsuntergrun­ des entlang der Hauptund Seitentäler deuten darauf hin, daß das ehemalige Akkumulationsgebiet aus ei­ nem Eisdom bestand. Die Eisscheide lag dabei über dem von Schlarignia, Cinuos-chel, Livigno und Piz Ber­ nina eingeschlossenen Gebiet und erreichte eine mini­ male Höhe von ca. 3000 m. Diese Resultate konnten durch eine Modelliernng der Topographie dieser Eis­ oberfläche mit Hilfe eines Geographischen Informati­ onssystems (GIS) bestätigt werden. Das Klimasignal, welches in dieser Gletschergeometrie enthalten ist, wurde anschließend genutzt, um vorher ­ sehende atmosphärische Zirkulationsmuster und die daraus resultierenden Hauptniederschlagsgebiete in den Alpen während des letzten Hochglazials zu bestim­ men. Es läßt sich daraus folgern, daß für den Aufbau des Eisdomes vorwiegend Niederschläge von Südstau­ lagen verantwortlich waren. Die Dominanz der Süd­ staulagen ist eine direkte Folge der vorrückenden Meer­ eisgrenze im Nordatlantik. Letztere bedingte eine Ver­ lagerung der atmosphärischen Polarfront und der damit verbundenen Hauptzugbahn zyklonaler Störungen um bis zu 20° südwärts. Diese Ergebnisse korrespondieren gut mit (a) den Re­ sultaten globaler Zirkulationsmodelle für das LGM; (b) Schätzungen der basalen Schubspannungen und der Fliessgeschwindigkeiten für die Eiszeitgletscher; und (c) mit der Interpretation von Paläowind-Indikatoren.

the geological evidence for glacier modelling (HAEBERLI & SCHLÜCHTER 1987) as well as (e) con clusions about prevailing paleoclimatic conditi ons (HAEBERLI 1991a(HAEBERLI , 1991b. JÄCKLI (1962JÄCKLI ( , 1970 compiled a detailed map of the Last Glacial Maxi mum (LGM) for all of Switzerland mostly from lo cal investigations. The map published by VAN HU SEN (1987) [BINI 1987;FELBER 19931) these survey maps still rank as some of the best reconstructions, particu larly with respect to the accumulation area. It is true that paleoglaciological modelling has experi enced significant progress in recent years, but the geometry of former glaciers and ice-sheets ne vertheless remains uncertain in the area above the ELA. As the accuracy of each (paleo)glaciological reconstruction is only as reliable as the geological/geomorphological input data, the basic pro blem of modelling former ice bodies is given by the limited field evidence (overprinting by youn ger glacier advances, weathering, burial by snow and sediments, etc.) in the former accumulation area. Thus, due to the results acquired in the ab lation area, the geometry of the alpine Ice Age glaciers was generally assumed to be strongly re lated to the present river system, i.e. ice-flow being the same as today's rivers. This hypothesis seems to be correct for the ablation area where minor variations in bed topography channelled the massive valley glaciers and stream flow of the ice predominated. The assumption, however, ap pears less valid at high altitudes in the central Alps, because mapping of glacial-erosion features indicate up-valley flow of some Ice Age glaciers, and ice transfluency over high altitude passes. Such features are in disagreement with existing reconstructions of the LGM. It is therefore evident that a realistic geometry of the last glacial ice bo dy cannot be achieved by applying the conclusi ons drawn from investigations in the well-docu mented Swiss Plateau and Prealpes to the poorly understood inner alpine area. The aim of this research project is to improve our understanding of the altitude and topography of the accumulation area during the LGM tising geo logical and geomorphological evidence. Special interest is focused on (a) reconstructing flowlines of the ice at the LGM by mapped ice flow patterns, (b) modelling the ice surface in the former accu mulation area by trimline evidence and (c) the possibilities to draw paleoclimatic conclusions from the shape of the former accumulation area.

Field area
Detailed mapping of glacial-erosion features in the central Alps of eastern Switzerland has alrea dy been carried out. The area described here is si tuated in the easternmost part of Graubünden (Switzerland) and adjacent Italy (Fig. 1). This region is characterised by two large, south west -northeast striking valleys (Engiadina and Valle di Livigno), emptying to the northeast and situated at a mean altitude of 1500 -1700 m. The gradient of these longitudinal valleys is low, whi le the shorter tributaries are rather steep. On both sides of these valleys the peaks of the different massifs have an average altitude of 2900 -3400 m, while the Bernina massif in the southwest reaches an elevation of 3500 -4000 m. Despite a very con tinental climate with a mean annual precipitation amounting to just some 70 -90 cm per year, the highest parts of these mountains remain glaciated today. The bedrock geology of the area, described by various authors (e.g. SPITZ & DYHRENFURTH 1914;CORNELIUS 1935;BOSCH 1937;HEIERI.I 1955;TRLIMPY I960;SPILLMANN 1993) is important to the present study because most of the glacial history is infer red from erosional traces on bedrock. This area exposes Penninic nappes in the west (Margna-Sella nappe and Platta nappe) and east (Tasna nappe), overlain by a stack of Austroalpine nap pes (Fig. 2). The latter comprise the Upper Aus troalpine Silvretta nappe (para-and orthogneisses, amphibolites) to the north, the S-charl-, Quattervals and Ortler nappes (Triassic and Jurassic sediments) to the east and the Campo/Languard nappe (gneisses) to the sotitheast as well as the Lower Austroalpine Err-and Bernina nappes (crystalline basement and Mesozoic sediments) to the southwest.

Glacial-erosion features and erratics
Fig . 3 shows the main localities of mapped glacial erosional features, including trimlines, striations, glacial polish, crescentic fractures and gouges as well as roches moutonnees. These features are scattered between the bottoms of the valleys and the trimline, superimposed on glacially-moulded bedrock. Glacial polish was found in only a few sites such as passes where ice-flow was constrict ed. The state of preservation of glacial polish and striae is highly dependent on the bedrock lithology. Fine grained igneous rocks preserve striations well, while carbonate rocks, slates and gneisses do not hold striations because of comparetively rapid physical weathering by granular disintegra tion or exfoliation.
The possibility of measured features being influ enced by younger glacier advances was avoided by restricting the field survey to peaks and ridges near the trimline and to passes outside the morai nes of the Younger Dryas.
In the region of Passo del Maloja (1815 m) striati ons, chattermarks and roches motitonnees occur on the glacially-moulded gneissic bedrock of the Margna-Sella nappe, but they are not common. Directly at the hospice, P-forms as well as nume rous potholes can be found. Ice-flow direction was up-valley, towards southwest to west, as can be derived from striations and stoss-and-lee fea tures. Erratics in the pass area from the Err-and Bernina nappes, exposed more to the east, like diorite, "Saluverbreccie" and Juliergranit" (STAUB 1938(STAUB , 1951CORNELIUS 1951) confirm this flow di rection.
North of the En river, in the area between Julierpass (2284 m) and Vereinapass (2585 m) glacial ly-smoothed granitic and gneissic outcrops of the Err-, Bernina, and Silvretta nappe are common up to an altitude of 2850 m. Fine-grained granites show well-preserved striations and glacial polish, but weathering removed these features almost completely from the coarse-grained granites and gneisses. All striations give evidence of the main ice-flow towards northwest and north, with northerly ice-transfluences for all passes north of the En river. The most impressive example of such an ice-transfluence is represented by the pass south of Preda. The surface of this granitic ridge is glacially-moulded and littered with angu lar blocks of glacially transported local bedrock but mostly lacks striations. However, the orienta tion of the roches moutonnees shows an overflow of the ridge towards the north. Moreover, in some places, detailed plotting of striations clearly illu strates the deviation of the general northerly flow around nunataks, and the pattern of ice deflected around these obstacles. At Piz Kesch for example, ice-flow direction along the western slope is towards the northwest while the flow direction changes from northeastwards in the southeast to northwards in the north at Pass dal Scaletta and Sertigpass; this example illustrates how ice was deflected around nunataks on its way to the north.
The situation is different southeast of Zernez, at Pass dal Fuorn (2149 m), where glacially-smoot hed bedrock of the sedimentary S-charl nappe is exposed (Fig. 2). Here, the surface of the wide spread dolomitic outcrops suffered granular dis integration and, thus, most of the small-scale gla cial erosional features have been removed. In contrast to these carbonates, some few outcrops of Verrucano (Permian sandstones and breccias) preserved striations and can be used to determine former ice-flow. Both chattermarks and striations are rare and mainly restricted to Vernicano out crops or to recently exposed carbonate outcrops such as the occurrences along the road across Pass dal Fuorn. Scattered crystalline erratics (Juliergranit and Albulagranit) of the Err-and Bernina nappes (DÖSSEGGER 1974) and boulders of a Jurassic breccia originally exposed in the area of Livigno suggest a general flow direction from the west to the east. Combined with the trends of striations and stoss-and-lee-features, this direc tion indicates that ice flow was formerly across Pass dal Fuorn towards southeast.
At Passo del Bernina (2054 m), south of the En ri ver there is also evidence of overriding by ice. The pass area in cross section has a typical U-shape in the glacially sculptured crystalline bedrock of the Bernina nappe. Evidence of former glacier activity is the numerous striated, polished and grooved surfaces which are found all over, up to an altitude of about 2900 m. West of the pass, at 2600 m there are two generations of striations which are carved in the fine-grained granite. At the southern end of the glacially eroded basin, presently' filled by Lago Bianco, P-forms are found. All features indicate former ice-flow across the pass with a flow direction towards the south. In summary, the reconstruction of flowlines by glacial striae in the area described yield a well de fined star-like pattern of former ice flow direction similar to reconstructed flow patterns of the for mer ice sheets in North America and Scandinavia. The centre from where the ice was able to flow in all directions is assumed to have been in the area between Piz Bernina and Piz d'Urezza.
The trimline Trimlines are glacial-erosion features etched into bedrock, marking the maximum erosion level of the former ice body into pre-existing weathered bedrock. They describe the boundary between an ice-moulded downslope area and a frost-affected upslope zone (THORP 1981;BALLANTYNE 1990 North of the En river a conspicuous trimline occurs on almost all peaks and steep ridges and shows consistent elevations across lithologic and stnictural breaks. It is best developed and coher ent in crystalline bedrock of the Err-and Bernina nappes. South of the En river trimline data are ra re because of unfavorable bedrock such as slates and fine-grained sediments. However, a trimline is still visible at Passo del Bernina and in the area of Pass clal Fuorn. The shape of the former ice surface is best illu strated by plotting the mapped trimline on (a) a SW -NE projection plane representing an 80 km long transect from Piz Lunghin to Piz Minschun aligned parallel to the Engiadina and (b) on a 35 km long transect from Flüelapass to Pass dal Fuorn aligned NW -SE, perpendicular to the first transect (Fig. 1). The result (Fig. 4A) Fig. 4 B, with a progressive decline in trimline altitude towards the southeast and nor thwest, away from its maximum elevation of ab out 2900 m located right above the bottom of the Engiadina. Again, this trend is consistent with in formation on ice flow directions inferred from in dependently mapped glacial erosional features (SPITZ & DYHRENFURTH 1914;BOSCH 1937;HEIERLI 1955).
The intersection of the different transects allows a reconstruction of the regional trend of trimline elevations. As described earlier, the trimline at tains its highest elevation of 2950 m or even more in the area between Piz Bernina and Piz d'Urezza and smoothly slopes northeastward to 2600 m and southwestward to 2720 m. A steeper decline of trimline elevations is found across the ridges to the northwest and to the southeast. This pattern displays a gradual decline of trimline altitudes in all directions from the centre in the upper part of Engiadina, and is consistent with the conclusions drawn from flowlines determined by striation trends. Again, the reconstmction of the ice surfa ce, when ice stood at the trimline, indicates that there was a high elevation zone in the area enclo sed by Piz Bernina, Piz Kesch, Piz d'Urezza, Piz Quattervals and Monte Forcellina (Fig. 3). The ice was drained predominately downstream towards the northeast and up-valley towards the south west by ice streams but, as documented by striati on measurements, it also spilled over the ridges to the northwest and to the southeast of the Engiadi na and flowed into the ice streams of the Rhein and Etsch glaciers.

Bedrock morphology
With the exception of some nunataks, ice age gla ciers have reshaped the pre-existing relief to a glacially moulded landscape. The result is the ty pical U-shaped cross-section of the valleys. Most of the short, steep tributaries end today as han ging valleys because the large longitudinal valleys draining most of the ice experienced much stron ger glacial erosion. As far as Engiadina is concer ned, all tributaries between Passo del Maloja and Schlarignia are hanging valleys and then again from Cinuos-chel on downstream. Tributaries between Schlarignia and Cinuos-chel however, end at the level of the main valley floor and the lo wer section of these valleys is characterised by a very small gradient. This is also true for tributaries of Valle di Livigno upstream from Livigno. Glacial erosion did not only shape the cross-sec tions, but also modified their longitudinal profiles forming basins and troughs. On a profile of the Engiadina a distinct change in slope occurs at Ci nuos-chel (Fig. 4 A ). Whereas the elevation of the valley floor constantly decreases downstream from Cinuos-chel, it displays a nearly consistent elevation of about 1700 m on the section between Cinuos-chel and Passo del Maloja, with a step of about 50 m in the area of Schlarignia and an ab rupt end of the valley at Passo del Maloja. The si tuation along Valle di Livigno with Livigno as a hinge, is even more pronounced.
Several possible interpretations can be made for the reduced basal erosion in particular sections of the valleys mentioned. One explanation is tecto nic, as Cinuos-chel and Livigno lie at the bound ary between a crystalline nappe (Silvretta and Languard) and a sedimentary nappe (Quattervals nappe). Additionally, this nappe boundary is cut by the younger sinistral transform fault (Engadine line) which runs parallel to the floor of the Engia dina (Fig. 2). Both, the nappe boundaiy as well as the transform fault may have resulted in differen tial basal glacial erosion. Second, the area of reduced erosion formerly may have been covered by ice of a cold based glacier. In this scenario, there would have been no basal erosion as the cold based ice was frozen to its bed and, therefore, permitted no slip between the ice and its bed. The third interpretation is that it represents an area which was at or beneath the ice divide of a former dome-type glaciation. Observations from regions formerly covered by ice sheets such as North America or Scandinavia 1 show that very litt le or even no basal erosion may take place below the central part of an ice dome (DENTON & HUGHES 1981).
This third hypothesis is favoured for the following reasons: The occurrence of striated bedrock and chattermarks all over the study area up to the trimline clearly show that the ice was at pressuremelting point, permitting basal sliding even at high altittides. Therefore, the possibility of cold based ice frozen to its bed can be excluded. The tectonic interpretation also is not very plausible because at Cinuos-chel it is the sedimentary Quat tervals nappe which is less eroded than the crystalline Silvretta nappe but at Livigno it is the

Reconstruction of glacier geometry for the LGM by GIS
In addition to transects with the regional trend of the trimline on well defined projection planes, a three-dimensional ice surface for the whole area has been computed with a geographical informa tion system (GIS). This study describes a first at tempt to make use of the techniques for surface modelling and analysis provided by GIS (WEIBEL & HELLER 1991) in order to assist the reconstruc tion of a three dimensional ice surface of the ac cumulation area during the LGM.
Modelling of the ice surface by GIS was done as follows: First, a TIN (triangulated irregular net work) data model has been computed from the point data containing all mapped field evidence for surface elevation with x, y coordinates and z values and line data representing the digitised contours of the map published by VAN HUSEN (1987). In order to ascertain that the modelling is not affected by "rough" or inexact data, digitised isolines have been considered only outside the mapped area. A common digital format for repre senting surfaces in a matrix of equally spaced sample points are digital elevation models (DEM or DTM). A first interpretation of a DEM is refer red to as a lattice and therefore the TIN surface da ta which is based on irregularly spaced point and line data was converted to a lattice. A linear inter polation method was used to calculate missing points inside the triangles. In the last step, this lat tice containing the elevation values for a conti nuous, smooth ice surface was intersected with a digital terrain model (DTM) so as to clip the peaks protruding the ice surface as nunataks. Fig. 5 shows the ice surface dtiring the LGM as reconstructed by GIS. For clarity, the surface altitude is displayed with contours at intervals of 200 m.

Paleoclimatic implications
Located in the central part of the Alps, the study area represents a key location concerning atmo spheric circulation patterns; this is true not only for the present climate but also for paleoclimates. Consequently, the results provided by the recon struction of the LGM ice surface in this area are crucial for understanding atmospheric circulation and the precipitation patterns in the period of massive ice build-up from 28000 BP to 23000 BP Before turning to former climatic conditions, the characteristic precipitation patterns of today's most important weather situations are discussed (Fig. 6). At present, in the Alps there are two different weather systems responsible for the principal supply of precipitation. Moist air masses from the precipitation-bearing cyclones produced in the North Atlantic are brought to the Alps either by westerly winds across central France or via sou thern France and northern Italy by the southerly winds (foehn weather situation). The cooling ef fect of the Alps on the incoming moist air masses causes different precipitation patterns, either with elevated precipitation in the northwest and a pre cipitation shadow in the south(east), or the oppo site. Both of these situations can be observed in the study area: during northwest winds which prevails on more than 50% of all days, precipitati on is highest in the north and northwest of the En giadina. In contrast, during foehn weather (25% of all days) precipitation is highest in the south (Fig 6). The boundary between these two diffe- rent precipitation areas is marked on the precipi tation charts by the isoline of 70 rnm/m 2 and coin cides with the ridge from Julierpass, Piz Kesch, Flüelapass to Pass dal Fuorn. Additionally, there is a distinct difference in the total amount of preci pitation. The maximal precipitation values during a typical foehn weather situation are in general lo cally restricted but significantly higher than during (north-)west weather situations when precipitation extends over a wider area. This is important, when taken into account, as shown in figure 6, that the former situation lasted only half as long as the latter.
At present westerly circulation is about 50% mo re frequent than southerly circulation. From this we may conclude that during about 60% of all days in an average year the weather in Graubün den is dominated by westerly winds and conse quently the total annual precipitation must be considerably higher in the northern parts of Graubünden than in the Engiadina and adjacent valleys to the south.
Prevailing paleocirculation patterns during the last glaciation can be determined by comparing these modern precipitation patterns to the recon structed Ice Age topography. The configuration of the ice surface has the potential to provide in sight into past precipitation patterns and to outli ne the main precipitation areas. Like the isolines of precipitation, the contours of the ice surface roughly represent the main precipitation areas and approximate the precipitation pattern res ponsible for the ice build-up, in spite of deforma tion by ice flow (Fig. 5).
The most striking feature is that the contours of the ice surface computed by GIS and the isolines of foehn weather correspond very well with one another, i.e. the ice surface reaches its maximum altitude in the areas with maximum precipitation due to southerly circulation. On the other hand, there is a marked discrepancy with respect to the isolines of the northwest wind weather conditi ons. It appears, therefore, that the build-up of the ice was related mainly to precipitation by sou therly winds similar to today's foehn. On the con trary, the influence of the westerlies was much less important for precipitation in the Swiss Alps, at least during the last phase of the Würm glacia tion. The most reasonable explanation for the do minance of southerly circulation during isotope stage 2 is a displacement of the North Atlantic po lar atmospheric front towards the equator due to the advancing margin of pack ice. In such cir cumstances, the build-up and prevailing tracks of the cyclones likewise moved southwards to a new position south of the Alps; this would have promoted southerly circulation and thus a decline in precipitation away from the southern fringe of the Alps and the development of permafrost con ditions in the northern part of the Central Plateau (Fig 7).
The  (WASHBURN 1979;FRENZEL 199D. Such changes of atmospheric cir culation were associated with changes in oceaniccirculation, and the extent of sea ice increased markedly: For example, the southern limit of per manent pack ice during winters in the North At lantic reached as far south as 40 -50°N, and 60° N during summers (CLIMAP 1976). The displace ment of the polar oceanic front southward to the latitude of New York and Spain (RUDDIMAN, et al. 1980) was accompanied by an equivalent shift of the polar atmospheric front and the associated mid latitude cyclones of about 10 -20°; this cau sed the related storm tracks to move south and to flow from west to east across the Mediterranean. As a result the Mediterranean area and the Sou thern Alps were subjected to increased rainfall and decreased evaporation (BONATTI 1966;BARTO-LAMI, et al. 1977).
Paleowind indicators such as sedimentary struc tures in Loess sediments (MEYER & KOTTMEIER 1989) and the orientations of dunes (POSER 1948) also have been used to reconstruct past atmo spheric circulation in Europe during the LGM. These studies provide additional evidence of an ticyclonic circulation in the immediate vicinity of the Eurasian ice sheet, severe periglacial condi tions in Central Europe, and a displacement of the main track of the mid latitude cyclones towards Fig. 7: Assumed characteristics of a typical synoptic weather chart during isotope stage 2. The position of the reconstructed LGM polar atmospheric front (PF) in the North Atlantic during winters according to RUDDIMAN, et al. (1980 also indicate climatic conditions south of the Alps which were warmer and more humid than in the north.

Conclusions
The reconstruction of the ice surface by paleoglaciological field data shows that during the LGM ice poured northward through the passes of Julier, Albula, Scaletta, Füela and Vereina as well as southwards through Pass clal Fuorn and Passo del Bernina. Flowlines derived from glacial ero sional features below the trimline, the distribution of erratics and the trends of trimline elevations indicate that the last glacial ice body consisted of a dispersal area with the ice divide situated in the triangle Schlarignia -Cinuos-chel -Livigno and attained a minimum altitude of approximately 3000 m. Bedrock morphology which cannot be explained satisfactorily by tectonic features also supports the reconstruction of an ice dome with radial outflow. A three-dimensional reconstruc tion of the ice surface using GIS supports the in terpretation of this ice dome. The configuration of the ice surface during the LGM has the potential to provide insight into past climatic conditions and therefore allows the pre vailing precipitation pattern which led to the ice build-up to be reconstructed. When modern pre cipitation patterns are compared with the shape of the reconstructed accumulation area for the LGM, it becomes evident that the configuration of the past atmospheric pressure system during the last glaciation was different to today's, with south erly circulation prevailing at that time. The predo minance of foehn is correlated with the south ward displacement of the polar atmospheric front that caused the related mid latitude cyclones ha ving tracked eastwards across the Mediterranean. The results from this study fit well with the report ed evaluations of paleowind indicators, the re sults of simulations of global ice age atmospheric circulation as well as calculations of basal shear stress values and surface velocities of Alpine Ice Age glaciers.