World Inventory - Inventaire mondial des (Proceedings of the Riedeialp Workshop, September 1978; Actes de l'Atelier de Riederalp, septembre 1978): IAHS-AISH Publ. no. 126,1980.

East-west and north-south line gradients in the northern Patagonian , Argentina

J. Rabassa, S. Rubulis and A. Brandani

Abstract The snow line has been identified on the Argentine side of the northern Patagonian Andes. A definite trend is observed in both the east-west and the north-south snow line gradients. The snow line rises abruptly from the west () to the east (Patagonia), having progressively higher positions in the landscape since it remains above the summit of the higher peaks of the Sub-Andean ranges and Patagonian volcanoes. Similarly, the snow line descends steadily, although not so markedly, from north to south, favouring the conservation of cirque and valley glaciers at lower . The east-west gradients are caused by the interference of the humid winds from the Pacific with the Andean . The north-south gradient is related to the lowering of mean annual temperatures away from the and also to the lower elevation of Andean peaks. For certain latitudes, present snow line gradients are compared with those from the Pleistocene.

Gradients est-ouest et nord-sud de la limite des neiges persistantes dans les Andes patagoniennes septentrionales Résumé. On a identifié la limite des neiges persistantes sur le versant argentin des Andes patagoniennes septentrionales. On observe un trend bien défini pour chacun des gradients est-ouest et nord-sud de la limite des neiges persistantes. La limite s'élève brusquement de l'ouest (Chili) vers l'est (Patagonie), occupant des positions de plus en plus élevées dans le paysage, puisqu'elle demeure au-dessus des sommets des contreforts des Andes et des volcans de Patagonie. De manière analogue, la limite des neiges persistantes s'abaisse de façon continue, mais toutefois moins rapidement, du nord vers le sud, favorisant la conservation des cirques glaciaires et des glaciers de vallée à des latitudes inférieures. Les gradients est-ouest sont causés par l'interférence des vents humides venant du Pacifique avec la Cordillère des Andes. Le gradient nord-sud est lié à l'abaissement des températures annuelles moyennes lorsque l'on s'éloigne de l'équateur ainsi qu'à l'altitude moins élevée des sommets andins. Pour certaines latitudes, les gradients de la limite actuelle des neiges permanentes sont comparés avec ceux du pleistocene.

INTRODUCTION

The significance of snow line position in relation to the climate and glaciation of a region, both past and present, has been stated by numerous researchers. A few papers have dealt with the Pleistocene and the modern snow line of South America, mainly in Peru, Chile, and northern Argentina (Tricart, 1965; Hastenrath, 1967, 1971a, 1971b). There are even fewer data for the Argentine side of the northern Patagonian Andes, between latitudes 39°S and 42°S, with the exception of the papers by Wilhelmy (1957), Flint and Fidalgo (1964) and Auer (1970). Field work performed during the 1978 Southern Hemisphere summer for the World Glacier Inventory provided the opportunity for a detailed, although not exhaustive, survey of the snow line position of this section of the Patagonian Cordillera of Argentina; see Fig. 1 for the location of the area studied. The aim of this paper is to present these data, explain the observed east-west and north-south gradients, and compare them with Pleistocene climatic snow line.

THE NORTHERN PATAGONIAN ANDES BETWEEN 39°S AND 42°S

South of 37° 30'S, the Andes present a significant decrease in their summit height: the elevation of the peaks drops dramatically from 5000 m to less than 1 2 J. Rabassa, S. Rubulis and A. Brandani

FIGURE 1. Location map.

3000 m. This is due to the passage from the Mesozoic-Tertiary structures of the Andean Geosyncline, to the older (Variscan) structures of the northern Patagonian Andes. A general summit altitude of 2300-2600 m is exceeded only by several Upper Tertiary and Quaternary volcanoes which grew on top of the 'Gipfelflur': Volcan Domuyo (4709 m), Volcan Copahue (2980 m), Volcan Tromen (3978 m), Volcan Lanin (3776 m) and Monte Tronador (3554 m), and others in Chile. The northern Patagonian Andes, south of 39°S, are a group of relatively low ranges north-south oriented, which were heavily glaciated during Pleistocene times. Glaciers flowed away from the by means of numerous transverse valleys that had previously been fluvially eroded. Most of these valleys are now occupied by lakes of glacial origin. A great number of peaks still have glaciers. Most of these glaciers have not been studied or even visited by scientists or explorers. Moreover, most of them are still unnamed. A preliminary list of more than 200 glaciers of varied nature has been presented as our contribution to the World Glacier Inventory (Rabassa et ah, 1978). A summary of some of the data obtained is given in Table 1. Snow line gradients in Argentina TABLE 1. Summary data from the glacier inventory of the northern Patagonian Andes (235 glaciers)

Latitudinal distributior i Size distribution Sector No. % Area [km2 ] No. % 39°00' - 39°20'S 31 13.2 0.010-0.031 46 19.6 39°20' 39°40'S 45 19.1 0.031 - 0.062 37 15.7 39 40 40°00'S 62 26.4 0.062-0.124 40 17.0 40°00'-40°20'S 8 3.4 0.125-0.250 38 16.2 40°20' - 40°40'S 12 5.1 0.25-0.50 32 13.6 40°40'-41°00'S 0 0.0 0.50-1.00 18 7.7 41°00'-41°20'S 46 19.6 1.00-2.00 11 4.7 41°20'-41°40'S 6 2.6 2.00 - 4.00 6 2.6 41°40'-42°00'S 16 6.8 4.00 - 8.00 3 1.3 42°00' - 42°20'S 3 1.3 8.00-16.00 3 1.3 42°20'-42°40'S 6 2.6 16.00-32.00 1 0.4 Total 235 100.0 Total 235 100.0

Length and width distribution Mean length Mean width Size class [km] No. % No. % 0.10-0.20 84 35.7 13 5.5 0.20 - 0.40 82 34.9 66 28.1 0.40 - 0.80 28 11.9 84 35.7 0.80-1.60 18 7.7 58 24.7 1.60 - 3.20 11 4.7 13 5.5 3.20 - 6.40 10 4.3 0 0.0 6.40-12.80 2 0.9 1 0.4 12.80-25.60 0 0.0 0 0.0 Total 235 100.0 235 100.0

Classification Median glacier elevation First 2 digits No. % Contour [m] No. % 30 1 0.4 1400-1500 1 0.4 43 11 4.7 1500-1600 1 0.4 53 4 1.7 1600-1700 19 8.1 63 12 5.1 1700-1800 35 14.9 64 50 21.3 1800-1900 34 14.5 65 14 6.0 1900-2000 49 20.9 67 4 1.7 2000-2100 43 18.3 68 18 7.7 2100-2200 16 6.8 74 16 6.8 2200 - 2300 17 7.2 75 30 12.8 2300 - 2400 8 3.4 77 2 0.9 2400 - 2500 6 2.6 78 72 30.6 2500 - 2600 3 1.3 79 1 0.4 2600 - 2700 1 0.4 2700 - 2800 1 0.4 2800 - 2900 0 0.0 2900 - 3000 0 0.0 3000-3100 0 0.0 3100-3200 0 0.0 3200 - 3300 1 0.4 Total 235 100.0 Total 235 100.0 4 J. Rabassa, S. Rubulis and A. Brandani

TABLE 1 continued Orientation Quadrant No. % N 5 2.1 NE 32 13.6 E 65 27.7 SE 49 20.9 S 59 25.1 sw 14 6.0 8 3.4 NwW 3 1.3 Total 235 100.0 The climate of the region is temperate, humid to perhumid. The following data have been obtained from De Aparicio (1958). The mean annual precipitation is more than 3000 mm at the international border; it becomes gradually drier to the east but the area studied lies totally within the 800 mm isohyet. Precipitation is concentrated in winter, the rainfall then being four times higher than during the summer. Snow storms are frequent, more than 20 per year. The whole area is rain-shadowed by the Andean ranges. The humid winds from the Pacific Ocean drop most of their moisture on the western slopes of the Andes (Chile) as they rise over the chains. The westernmost peaks of the Argentine side still receive some of the rain but the eastern sides of the ranges and the lowlands of Patagonia become progressively drier. A strong rain gradient is evident from the meteorological data of our own stations at Mt Tronador: 1900 mm/year at Pampa Linda Station and 1450 mm/year at Lake Mascardi Station, the latter located 15 km to the east at the same latitude. This gradient is similar throughout the region under study. The temperature is moderate, the whole area being enclosed by the 6°C annual isotherm. Most of the region lies within the 2°C July isotherm, but the 0°C July isotherm is located more than 300 km to the south. METHODOLOGY Porter (1975) enumerated the reasons that make a strict comparison of present and past snow line data between different regions of the world very difficult. Two of them are the different definitions of snow line and methods of surveying it employed by various workers. Thus, it is fairly important to state clearly the definitions and methods that we have used in this investigation. Definitions have been taken from Péwé and Reger (1972) and TTS (1977). Snow line: temporary line delimiting an area or altitude with complete snow cover or, in a zone of patchy snow, the area or altitude of more than 50 per cent cover of snow; this is the transient snow line. The highest position on glacier surfaces of the transient snow line at the end of the budget year is called the firn line. Climatic snow line: the average line or altitude delineating, at a specified time, the area with more than 50 per cent snow cover on horizontal surfaces, averaged over a long time period of climatic significance. In our case, climatic snow line will refer to the Pleistocene snow line, although the period represented may be only a part of it. Equilibrium line: the elevation on a glacier where the net balance equals zero and accumulation equals ablation; the line separating the superimposed ice zone from the ablation area which lies below (Miiller, 1962). All the measurements of snow line in this paper have been done on glacier surfaces; thus, we have actually measured the distribution of firn line, which is in turn roughly coincident with the equilibrium line for mid-latitude, temperate glaciers (Péwé and Reger, 1972). Snow line gradients in Argentina 5 No mass balance measurements are yet available for this pari>of the Andes. Snow line observations were made in the field on every observed single glacier by two of the authors (Rabassa and Rubulis) during field work for the World Glacier Inventory. Measurements were made between February and April, 1978. The hydrological year of 1977-1978 was exceptionally warm (Rabassa and Rubulis, unpublished data) and it is assumed that the snow line position was the highest for the last ten years. The great extent of the area studied, the inaccessibility of most of the glaciers and the lack of recent aerial photographs (the last survey was performed in 1970-1972) made it impossible to take a large number of observations over a very short span of time. However, some glaciers that were observed in February and re-visited in April did not show a change in the snow line position larger than the precision of the measuring techniques. Some of the data were directly obtained by standing on the firn line with a hand altimeter (error: ± 5 m). On other glaciers of difficult access the vertical angle from a point of known altitude (either by hand altimeter determination or by observation on a topographical map) to the snow line position was measured using a plane table Wild RK-1 (vertical angle error: ±10 min). Distances were measured on topographical maps, scale 1:50 000 and contour interval of 25 m, and the snow line elevation was computed. The elevation error was maintained below ± 25 m by discarding those measurements where distance and/or vertical angle went beyond the desired error. For large glaciers, the snow line was measured strictly following the definition stated above. For cirque and niche glaciers or glacierets, the snow line was taken as an average contour, because the firn line is usually very contorted on these small ice bodies. More than 200 glaciers have been investigated for this purpose. Only 15 per cent of them are large, outlet or valley glaciers; the rest are composed of cirque and niche glaciers, glacierets, snowfields or group-remnants (terminology by TTS, 1977); ad of them are larger than 0.01 km2 and most are larger than 0.1 km2 (see Table 1).

PREVIOUS WORK Present and Pleistocene snow lines in some parts of the South American Andes have been determined by Wilhelmy (1957), Tricart (1965) and Hastenrath (1967,1971a, 1971b). A review of previous, more or less isolated observations may be found in Hastenrath's papers. Hastenrath (1971a and 1971b) presents a series of transects along and across the Andes of Peru, Bolivia, Chile and Argentina up to latitude 32°S. He recognizes that the meridional pattern of the Pleistocene snow line is quite similar to that of the present, reaching its highest elevation around the latitude of most extreme aridity, i.e. 18-26°S and with a sharp decrease in altitude from there southwards. East-west transects show that the modern snow line elevation decreases towards the east, from latitude 14° S to latitude 30° S whereas southward from this latitude it rapidly increases towards the east. Hastenrath (1971a, p. 259) relates this reversal of zonal variation to the change from an easterly to a westerly wind regime at the latitude of the subtropical high pressure cells. Furthermore, he states that the zonal variation of Pleistocene and modern snow lines is very similar in general terms, although the reversal of the zonal variation seems to have been located around latitude 25°S during Pleistocene tiroes, i.e. 550 km nearer the equator. This reversal zone had previously been observed by Wilhelmy (1957) although he located it at latitude 27°S. In the northern Patagonian Andes we have just a few isolated observations about snow line position (Iliboutry, 1956; Sabor, 1950; Mercer, 1967; Colqui, 1970), except for the discussion of a general diagram by Wilhelmy (1957) in Auer (1970) and an evaluation of the climatic (Pleistocene) snow line by Flint and Fidalgo (1964). Wilhelmy's (1957) diagram presents the distribution of the snow line in Central America (latitude 12°N 6 J. Rabassa, S. Rubulis and A. Brandani southwards) and South America, for the Eastern, Central and Western , in Wurmian and modern times. For the region studied in our paper, Wilhelmy (1957) presents data only for the modern snow line. His longitudinal transect exhibits a moderate north-south snow line gradient of 1.2 m/km for the Western Cordillera and 1.4 m/km for the Eastern Cordillera, between latitudes 38°S and 42°S. Wilhelmy (1957, p. 281) finally came to the conclusion that during the 'Wiirm Glaciation' the winds had blown from the same directions as today. Flint and Fidalgo (1964) present a series of seven east-west profiles of the northern Patagonian Andes between latitudes 39°S and 41°20'S, where the altitude of cirque floors has been plotted against distance from the international border, which is essentially north-south oriented. Their profiles show a marked increase of the cirque-floor elevation towards the east, with gradients varying from 9 to 16 m/km. It is even assumed that the altitude of cirque floors is roughly approximate to the elevation of the climatic snow line. Porter (1975) has indicated that the cirque floor method is applicable only to areas of former cirque glaciation and that in heavily glacier covered areas, such as the Southern of (and, by extension, the Patagonian Andes), firn limits of former valley glaciers lie well below the cirque floors at the source of the glaciers. Nevertheless, we think that the technique used by Flint and Fidalgo provides a maximum elevation for the Pleistocene snow line and gives a reasonable idea of its east-west gradient, although the actual elevation of the climatic snow line may have been lower than is shown by the profiles. Flint and Fidalgo reach a similar conclusion to that of Wilhelmy (1957); the westerly wind pattern affecting this section of the Andes was not much different during Pleistocene times. They also calculate a snow line depression of 950 m for Mt Tronador, corresponding to a mean annual temperature for the region of about 6°C lower than today.

DATA AND RESULTS A total of 229 observations of firn line position have been performed along a sector of the northern Patagonian Andes from latitude 39°00'S to latitude 42° 20'S (Fig. 1). These measurements have been distributed in five sections of 20' of latitude and two sections of 40' of latitude. This has been done by unification of two consecutive sections of 20' latitude where the small number of observations was considered unsatisfactory. However, no observations were made in the latitude 40° 40'-41°00'S section; in this region glaciers are located at very inaccessible places and it was decided not to depend exclusively upon the 1971-1972 aerial photographs. A map of snow line altitude has been presented to the World Glacier Inventory. Data for each latitudinal portion have been processed to obtain linear regression equations

y = axx +a0 where y is snow line altitude, x is longitude in Gauss-Kruger coordinates [km], fix is the slope of the regression [m/km], and a0 is the intercept on they axis at x = 0: a i and a0 are empirical parameters. Exponential, logarithmic and power regressions were computed but no better fit was observed; thus we preferred to use linear regressions to present our data. Computed slope parameters and the coefficient of determination (r2 ) are presented in Table 2. Although the results differ from section to section, it is clear that a significant east-west gradient exists in all of them. In every section of this 360 km-long portion of the northern Patagonian Andes, snow line altitude increases steadily towards the east. The slope of this tendency varies from 4 m/km to 35 m/km, but this variation may be related to sampling deficiency more than to geographical setting. The possibility of a north-south gradient has also been investigated. In this case, Snow line gradients in Argentina TABLE 2. Slope of snow line positions in east-west transects of the northern Patagonian Andes

Slope Sector n [m/km] r2 39°00' - 39°20'S 31 7.44 0.822 39°20' - 39°40'S 40 34.68 0.552 39°40'-40°00'S 63 16.65 0.805 40°00' - 40°40'S 16 7.16 0.808 40°40'-41°00'S no data available 41°00'-41°20'S 46 10.34 0.438 41°20'-41°40'S 7 4.01 0.724 41°40'-42°20'S 23 9.42 0.834

all the observations of snow line altitude were plotted against latitude in Gauss-Kriiger coordinates [km]. The following linear regression was obtained: y = 0.6723*-1790.96 where y is snow line altitude [m] and x is latitude in Gauss-Kriiger coordinates [km]. The slope of the linear regression is quite small, only 0.672 m/km. The total number of observations is 226. The coefficient of determination (r2 ) is only 0.106; therefore, the values obtained should be regarded carefully. However, the existence of a gentle north-south snow line gradient may be accepted, although our results are substantially lower than those presented by Wilhelmy (1957).

A COMPARISON WITH THE PLEISTOCENE SNOW LINE The Pleistocene snow line for this region has been investigated by Flint and Fidalgo (1964), using the altitudinal distribution of cirque floors. They present a series of east-west transects, the number of observations and regression slope values being summarized in Table 3 and compared with our own results. TABLE 3. Comparison of Pleistocene snow line according to cirque floor altitudinal distribution (from Flint and Fidalgo, 1964) and present snow line (this study)

Pleistocene snow Present snow line Difference line slope slope Sector n [m/km] [m/km] [m/km] 39°00' - 39°20'S 15 15 7.44 -7.6 39°20' - 39°40'S 8 16 34.68 +18.7 39°40'-40°00'S 29 14 16.65 +2.6 40°00' - 40°20'S 13 9 7.16 -4.9 40°20'-40°40'S 25 ^ 40°40'-41°00'S 27 1141 f no data ? 41°00'-41°20'S 23 12 10.34 -1.7

No data are available for other sections south of 41°20'S; Flint and Fidalgo (1969) were not able to identify correctly the altitudinal location of cirque floors in this southern portion, due to the lack of good quality topographical maps. The data in Table 3 suggest that, with the exception of the latitude 39°20'S-39°40'S section, the position of the modern snow line is similar or a little less inclined than the Pleistocene snow line. As suggested by Porter (1975), Pleistocene equilibrium line altitudes for a region like this should be lower than the altitudes suggested by cirque floor distribution. Thus, it should be possible to determine steeper positions for the Pleistocene equilibrium line if a regional study based on Porter's methodology were achieved. This 8 J. Rabassa, S. Rubulis and A. Brandani would probably increase the observed differences between Pleistocene snow line slopes, as studied by Flint and Fidalgo (1964) and the present gentler snow line slopes.

CONCLUSIONS A regional survey of the present snow line of this part of the northern Patagonian Andes has proved the existence of a strong east-west gradient and a much gentler north-south gradient. East-west gradients have been investigated in several transects; their slope values vary from 4 m/km to 35 m/km, but in all cases snow line altitude increases rapidly towards the east. This is due to the significant influence of the humid winds blowing from the Pacific Ocean onto the continent. When these winds are intercepted by the mountain ranges, they drop their moisture on the western slopes as they rise. Only a small part of the rain effectively falls on the easternmost slopes and the winds passing to the Patagonian uplands are dry, cold and strong. Although there are peaks in Patagonia high enough to receive some snowfall during the winter, precipitation is very small and snowmelt is complete, even at the beginning of summer. The north-south gradient is much more poorly defined, with a very gentle slope of only 60 cm/km. This gradient is related to the normal decrease of mean annual temperature polewards, which enables the persistence of glaciers and snowfields very close to, or at, sea level in southernmost Chile and (latitude 52°S to latitude 55°S). A comparison with the Pleistocene snow line estimated from cirque floor distribution has shown that the east-west gradient was already existent in Glacial times. This has been pointed out by Flint and Fidalgo (1964) who stated that planetary circulation of westerly winds for this part of the Andes has not changed significantly since Pleistocene times. It has also been noted above that the slope of the present snow line altitudes is not exactly the same as the Pleistocene snow line. Although the methodology used is not perhaps the best, a general conclusion may be suggested: in addition to the general increase of elevation of snow line since Late Glacial times, estimated as 950 mby Flint and Fidalgo (1964) for Mt Tronador (latitude 41°10'S), there is also a variation in the slope of snow line surfaces, which seems to be gentler now than during the Pleistocene, with the possible exception of the latitude 39°20'S- 40°00'S section. The influence of this decrease (which may even be an oscillation through time and space) of snow line slope upon the vegetation is still not clear. The descent of the snow line during Neoglacial times must have had a significant impact on the Patagonian rain forest. Old, mature trees that show a strong curvature at their base have been found at relatively very low altitudes and may be interpreted as a result of a 'fossil' snow line, namely the Third Neoglacial snow line (1600-1850 AD; Mercer, 1976). Similarly, the elevation of the snow line and the gradient modification must have had a decisive influence on the vegetation cover since Late Glacial times. Generally, plant communities forming part of the Patagonian forest ecosystem have been found to be stratified and geographically distributed following snow line position. A detailed review of this may be found in Brandani (1977). The modification of snow line position in time, both in altitude and east-west gradient, provided new land areas to be occupied by the plant cover, as the snow disappeared and the climatic conditions improved. In the sections where a change in gradient accompanied the general elevation, more extensive portions of the landscape were abandoned by the snow in the western parts compared to the eastern areas of the same latitudinal stretch. Thus, more physical space would have been available for plant colonization and this could be responsible for the anomalies in plant community stratification and the structural diversity found in some areas. Ecomorphological characteristics and diversity should be more complex in communities located at western areas (i.e. closer to the higher peaks) and higher Snow line gradients in Argentina 9 altitudes, especially between Pleistocene and modern snow line positions, and perhaps also between Neoglacial and present snow line altitudes. In these portions, the relative ascension of the snow line since Pleistocene times and its oscillations may have been more significant. Thus, another gradient may be regarded as superimposed upon the gradient, and it should be carefully considered in further investigations of the ecology of plant communities of this mountainous region of southwestern Argentina.

Acknowledgements. The authors are peatly indebted to Professor Francisco Fidalgo for his valuable criticisms on the ideas expressed in this paper.

REFERENCES Auer, V. (1970) The Pleistocene of Fuego and Patagonia. Part V: Quaternary problems of southern South America. Ann. Acad. Sci.Fenn. Series A. 3, no. 100,1-194. Brandani, A. (1977) La distribution de nichos ecolôgicos de las especies végétales en un ecosistema de bosque de la region Andino-Patagônica. PhD Thesis, University of Buenos Aires, Argentina. Colqui, B. (1970) Estudio glaciolôgico de la Cuenca del Rio Manso. Ann. Soc. Argent. Estud. Geogr. 14,13-24. De Aparicio, F. (1958) La Argentina, Suma de Geografia, tomo II: Peuser, Buenos Aires. Embleton, C. and King, C. A. M. (1975) Glacial Geomorphology: John Wiley, New York. Flint, R. F. and Fidalgo, F. (1964) Glacial geology of the east flanks of the Argentine Andes between latitude 39°10 S and latitude 41°20 S. Bull. Geol. Soc. Amer. 75, 335-352. Flint, R. F. and Fidalgo, F. (1969) Glacial drift in the eastern Argentine Andes between latitude 41°10'S and latitude 43°10'S. Bull. Geol. Soc. Amer. 80, 1043-1052. Hastenrath, S. L. (1967) Observations on the snow line in the Peruvian Andes. /. Glaciol. 6, no. 46, 541-550. Hastenrath, S. L. (1971a) On the Pleistocene snow line depression in the arid regions of the South American Andes. /. Glaciol. 10, no. 59, 225-267. Hastenrath, S. L. (1971b) On snow line depression and atmospheric circulation in the tropical Americas during the Pleistocene. S. Afr. Geogr. J. 53,53-69. Lliboutry, L. (1956) Nieves y glaciares de Chile. Universidad de Chile, Santiago, Chile. Mercer, J. H. (1967) Southern Hemisphere glacier atlas. Technical report no. 67-76-ES, US Army Natick Laboratories, Natick, Massachusetts. Mercer, J. H. (1976) Glacial history of southernmost South America. Quatern. Res. 6, 273-279. MùËer, F. (1962) Zonation in the accumulation area of the glaciers of Axel Heiberg Island, NWT, Canada. /. Glaciol. 4, no. 33, 302-318. Péwé, T. L. and Reger, R. D. (1972) Modern and Wisconsinan snow lines in Alaska. 24th International Geological Congress, Section 12,187-197. Porter, S. C. (1975) Equilibrium line altitude of Late Quaternary glaciers in the Southern Alps, New Zealand. Quatern. Res. 5, 27-47. Rabassa, J., Rubulis, S. and Suârez, J. (1978) Glacier inventory of the northern Patagonian Andes, Argentina, between latitude 39°S and latitude 42°20'S. Internal report to the Temporary Technical Secretariat (TTS) for World Glacier Inventory, Swiss Federal Institute of Technology, Zurich. Sabor, J. E. (1950) Altura promedial y limites de las nieves eternas en la Cordillera. Anu. Club Andino Bariloche, no. 18,111-112. Tricart, J. (1965) Observations on the Quaternary firn line in Peru. J.Glaciol. 5, no. 42, 857-863. TTS (1977) Instructions for Compilation and Assemblage of Data for a World Glacier Inventory: Temporary Technical Secretariat (TTS) for World Glacier Inventory, Swiss Federal Institute of Technology, Zurich. Wilhelmy, H. (1957) Eiszeit und Eiszeitklima in den feuchttropischen Anden. Petermanns Geogr. Mitt., Erganzungsheft no. 262,281-310.

DISCUSSION

Chinn: Did you make corrections for aspect to your snow line elevations before plotting a trend surface? 10 J. Rabassa, S. Rubulis and A. Brandani Rabassa: No. The snow line elevations were plotted without taking the orientation into account.

Chinn: Did you compare the trend surface slopes with any known precipitation gradients?

Rabassa: No. The available precipitation data are not sufficient to prepare rainfall-gradient profiles for this region.

Ommanney: Why do you feel it necessary to name every glacier? The inventory number should be sufficient identification.

Rabassa: The inventory number does not seem to be sufficient to identify glaciers included in this inventory. We have used official names for peaks and/or streams taken from the 1 : 50 000 topographic maps of the Instituto Geografico Militar of Argentina. The glaciers were designated according to these names. If more than one glacier was identified on a single peak, then literal digits were used to designate them, starting with 'A' for the northernmost facing glacier and proceeding clockwise. These names complement the inventory number and do not interfere at all with it.

Corte: Southwest storms produce the precipitation which is responsible for the gradient towards the Pacific. But there are southeast storms which could produce a snow line with a large gradient to the east. For example, in Mendoza there are southeast storms producing high precipitation in the eastern Andes (El Plata).

Rabassa: In the area studied, precipitation usually comes from the west. East storms are rare events.