Journal of Maps - CORE

Journal of Maps, 2008, 175-196

The Glacial Map of southern South America NEIL GLASSER1 & KRISTER JANSSON2 1 1Centre

for Glaciology, Institute of Geography and Earth Sciences, Aberystwyth University, SY23 3DB, Wales, UK; [email protected] 2 Department

of Physical Geography and Quaternary Geology, Stockholm University, SE-10691, Stockholm, Sweden; (Received 19th March 2008; Revised 25th June 2008; Accepted 26th June 2008)

Abstract: This paper describes The Glacial Map of southern South America. This is a new map of the glacial geomorphology of southern South America between latitudes 38◦ S and 56◦ S, approximately the area covered by the former Patagonian Ice Sheets. The map was compiled from interpretation of remotely sensed images (Landsat 7 ETM+, pan-sharpened Landsat 7 and ASTER scenes). The mapped geomorphological features include terminal moraines, glacial lineations, ribbed moraine, glacier debris stripes, trimlines, empty cirques, plateau edge, volcanoes, meltwater channels, deltas, glacial lake shorelines, alluvial fans, sandar, and glacial lake outburst flood tracks. The map also indicates the current extent of major icefields and glaciers in the area, as well as other topographic features such as lakes, rivers, shorelines, deltas, plateau surfaces and volcanoes. We briefly describe the most commonly occurring glacial landforms and provide an overview of their distribution within the mapped area.

175 ISSN 1744-5647 http://www.journalofmaps.com


1.

Glasser, N.F. & Jansson, K.N.

Introduction

The aim of this paper is to introduce The Glacial Map of southern South America, which contains the results of new mapping from remotely sensed images of the glacial geomorphology of southern South America between latitudes 38◦ S and 56◦ S, approximately the area covered by the former Patagonian Ice Sheets (Figure 1). This study marks a first attempt at geomorphological mapping of subglacial and ice-marginal landforms in this area on an ice-sheet wide scale using remotely sensed images. This work builds on the pioneering study of Caldenius (1932) who first mapped moraine systems around the contemporary icefields of South America. The palaeoglaciological significance of the mapping in terms of former ice extent, ice-sheet reconstruction and glacial chronology is provided in Glasser et al. (2008). Current understanding of the terrestrial extent of the former Patagonian Ice Sheets owes much to the pioneering study of Caldenius (1932) who first mapped moraine systems around the contemporary icefields. Caldenius (1932) distinguished four separate moraine belts to the east of the icefields. He concluded from their state of preservation that the three inner moraine systems were relatively young. In accordance with the stages of the last Weichselian Glaciation in northern Europe, Caldenius named the three moraine limits (from inner to outer) the “Finiglacial”, the “Gotiglacial”, and the “Daniglacial”. Later, the Finiglacial moraines were correlated to the last glaciation maximum (LGM) and the Daniglacial and the Gotiglacial moraine systems to the middle Pleistocene (Mörner and Sylwan, 1989). The fourth (outermost) moraine system, termed the “Initioglacial” by Caldenius (1932), is still poorly constrained in age, but is thought to be between 1.1 Ma and 2.3 Ma in age (Meglioli, 1992; Mercer, 1969; Mörner and Sylwan, 1989; Singer et al., 2004). Coronato et al. (2004) and Rabassa et al. (2000; 2005) have produced seminal reviews of glaciations in South America. These authors have argued from a synthesis of records that the oldest known Patagonian glaciations took place between approximately 7 and 5 Ma but moraine systems from these glaciations are not preserved and their occurrence is inferred mainly on stratigraphic and sedimentological grounds. Rabassa et al. (2005) suggest a minimum of eight glaciations occurred in the Middle-Late Pliocene (Oxygen Isotopic Stages 54-82). The “Great Patagonian Glaciation” developed between 1.168 and 1.016 Ma (OIS 30-34; Early Pleistocene). After the Great Patagonian Glaciation, 14-16 cold (glacial/stadial) events alternated with corresponding warm 176



Figure 1. Location map showing the area covered by the Glacial Map of southern South America.

(interglacial/interstadial) equivalents (Rabassa et al., 2005). They argue that the Last Glacial Maximum occurred between 25,000 and 16,000 calendar years ago (OIS 2; Late Pleistocene), and that two readvances (or still stands) took place during the Late Glacial (15,000-10,000 14 C years BP).

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Numerous local and regional studies exist within the area covered by the former ice sheets. Studied areas include the Chilean Lake District (e.g. Porter, 1981; Bentley, 1997; Andersen et al., 1999; Denton et al., 1999), the area around the North Patagonian ice field (Glasser et al., 2004; 2005; 2006; Glasser and Jansson, 2005; Turner et al., 2005), the eastern Andes (Strelin and Malagnino, 2000; Kaplan et al., 2004; 2005; Douglass et al., 2005; 2006; Wenzens, 1999), the Magellan Strait/Fuegan area (Porter et al., 1992; Clapperton et al., 1995; Coronato, 1995; Coronato et al., 2004; Rabassa et al., 1990; 2000; Benn and Clapperton, 2000; Bentley et al., 2005) and the Torres del Paine area (Marden, 1993; 1997; Marden and Clapperton, 1995). There are also a number of palaeocological studies that indicate the environmental conditions prevailing during Pleistocene times (e.g. Heusser and Rabassa, 1987; Heusser, 1989a;b;c; 1990; 1998; 2003; Markgraf, 1993; Moreno, 1997; Bennett et al., 2000; McCulloch et al., 2000; Massaffero and Brooks, 2002; Massaffero et al., 2005). From a synthesis of key proxy records, McCulloch et al. (2000) concluded that there was a sudden rise in temperature that initiated deglaciation of the last Patagonian Ice Sheet sychronously over 16◦ of latitude at 14,600-14,300 14 C yr BP (17,500-17,150 cal. yr). There was a second step of warming in the Chilean Lake District at 13,000-12,700 14 C yr BP (15,650-15,350 cal. yr), which saw temperatures rise to close to modern values. A third warming step, particularly clear in southern Chile, occurred c. 10,000 14 C yr BP (11,400 cal. yr). Following the initial warming, there was a lagged response in precipitation as the westerlies, after a delay of c. 1600 yr, migrated from their northern glacial location to their present latitude, which was attained by 12,300 14 C yr BP (14,300 cal. yr) (McCulloch et al., 2000). A number of dating techniques have been applied to the Patagonian moraine systems and their associated glacigenic deposits, including radiocarbon (14 C) dating of moraines, peat bogs and lacustrine deposits (e.g. Mercer, 1965; 1968; 1976; Aniya, 1995; Denton et al., 1999; Hajdas et al., 2003; McCulloch et al., 2005), cosmogenic isotope dating (Kaplan et al., 2004; 2005; 2007; Fogwill and Kubik, 2005; McCulloch et al., 2005; Douglass et al., 2006), optically stimulated luminescence (OSL) dating (Winchester et al., 2005; Glasser et al., 2006), and 40 Ar/39 Ar and K-Ar dating of lava flows interbedded with glacigenic and glaciofluvial deposits (Singer et al., 2004). Numerical ice-sheet modelling experiments have also significantly increased our understanding of the extent and dimensions of the former ice sheets and their interaction with climate (e.g. Hulton et al., 1994; 2002; Sugden et al., 2002; Hubbard et al., 2005). However, despite these advances in our understanding of the form and chronology of 178



Patagonian glaciations, no-one has ever attempted to map the landforms across the entire bed of the former ice sheets. Here we present a map of glacial geomorphological features on an ice-sheet wide scale using remotely sensed images.

2.

Mapped Area

The Glacial Map of southern South America covers in detail the glacial geomorphology of South America between latitudes 38◦ S and 56◦ S. Two major ice masses (the North and South Patagonian Icefields) and numerous snow- and ice-capped volcanoes and mountain icefields currently exist in the region. The North Patagonian Icefield (47◦ 00’S, 73◦ 39’W) is some 120 km long and 40-60 km wide, capping the Andean Cordillera between altitudes of 700-2,500 m a.s.l. (Figure 1). The icefield covers some 3,953 km2 (Rivera et al., 2007). Annual precipitation on the western side of the icefield increases from 3,700 mm at sea level to an estimated maximum of 6,700 mm before decreasing sharply on the eastern side (Escobar et al., 1992). The South Patagonian Icefield covers an area of c. 13,000 km2 (Aniya, 1999) and runs north-south for 360 km between 48◦ 50’S and 51◦ 30’S, with a mean width of 40 km. The South Patagonian Icefield also has steep climatic gradients between the maritime west side and continental east side. Smaller mountain icefields, glacier-capped volcanoes and isolated cirque glaciers also exist in many places along the north-south spine of the Andes, as well as in the very southern part of the study area in the Cordillera Darwin.

3.

Methods

3.1

Image Interpretation

The Glacial Map of southern South America consists of glacial geomorphological features including terminal moraines, trimlines, meltwater channels, sandar, alluvial fans, glacial lineations and empty cirques, together with other related landforms such as deltas and shorelines. The map also shows non-glacial topographic features such as lakes, plateaux, 179



rivers and volcanoes. Features were mapped from Landsat 7 ETM+ scenes (30 m spatial resolution), pan-sharpened Landsat 7 scenes and ASTER (15 m spatial resolution) satellite images in ArcMap GIS software. We interpreted the entire area covered by former Patagonian Ice Sheets using 33 Landsat 7 ETM+ and pan-sharpened Landsat 7 scenes and 167 individual ASTER images. Satellite images were overlaid on a Digital Elevation Model (DEM) based on 90 m cell size Shuttle Radar Topographic Mission (SRTM) data to provide topographic context and to aid landform identification in areas of complex terrain.

3.2

Map compilation

The criteria used in landform identification and mapping for The Glacial Map of southern South America are shown in Table 1. Satellite image interpretation was performed using multiple band combinations and standard image enhancement procedures (contrast stretching and histogram equalization) to improve the landform signal strength (Jansson and Glasser, 2005; Smith and Wise, 2007). Multiple images were used for the mapping in areas where cloud cover was present. Mapping was carried out at 1:50,000 scale and landforms were manually digitized into vector layers by visual interpretation of landforms in ESRI ArcGIS (cf. Clark, 1997). The map was read into Adobe Illustrator CS3 from ArcGIS for final illustrative editing.

4.

Mapped glacial landforms

A full list of mapped landforms and their significance is provided in Table 1. Here we describe the most commonly occurring glacial landforms on The Glacial Map of southern South America and provide an overview of their distribution within the mapped area. Terminal moraines: Large moraine complexes are present to both the west and east of the Andes throughout the mapped area. Sometimes these consist of multiple moraine sets and associated glacial lineations, e.g. in the areas occupied by the large eastern former outlet glaciers (Figure 2). Large arcuate moraines mark the existence of former piedmont glacier lobes on 180



the western side of the Andes e.g. in the Chilean Lake District (Andersen et al., 1999; Denton et al., 1999). The glaciers eroded deep basins here and their terminal moraines now impound the lakes that give their name to the Chilean Lake District. The terminal moraines are generally complex features with multiple ridges and crests (e.g. at Lago Llanquihue, Lago Rupanco and Lago Ranco). On the eastern side of the Andes at this latitude, the outlet glaciers were more restricted in extent. The maximum extent of these glaciers is marked by cross-valley moraines composed generally of single ridges. Here these eastern terminal moraines are smaller and less distinct that their western counterparts but further south, to the east of the North and South Patagonian Icefields, there are again very large and well-developed terminal moraine complexes. In the very south of the mapped area are also large moraine complexes e.g. in Seno Skyring, Seno Otway, Estrecho Magallenes and Baha In´ util. Finally, throughout the mapped area are many small moraines close to contemporary ice margins. These represent late Holocene, “Little Ice Age” and Twentieth Century glacier recession (Glasser et al., 2005). 71°10'0" W

71°0'0"W

70°50'0" W

46°20'0"S

46°30'0"S

L. Buenos Aires

46°40'0"S

Legend Moraines Sandur Meltwater channels Lakes Plateau edge

Glacial lake shoreline

0

5

10 kilometres

Figure 2. Landsat subscene (left panel) and interpretation (right panel) showing moraine complexes formed by LGM outlet glaciers to the east of the North Patagonian Icefield. Note the extensive sandur development between individual moraine crests. Green colour is vegetation.

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Trimlines: Trimlines (sub-horizontal lines on valley sides separating areas of non-vegetated and vegetated land or areas covered by different types of vegetation) are developed close to the snouts of many contemporary glaciers throughout the study area (Glasser et al., 2005). In many places, they merge down-glacier with lateral and terminal moraines, marking recent (post “Little Ice Age”) glacier recession (Figure 3). These trimlines are most apparent around the snouts of the small glaciers that occur in the area between the North Patagonian Icefield and the Chilean Lake District (Figures 3 and 4). Trimlines are also developed around the snouts of the eastern outlet glaciers of both the North and South Patagonian Icefields. Meltwater channels: Meltwater channels occur throughout the study area at a variety of spatial scales. Close to the contemporary glaciers are small meltwater channels that mark recent recession of these glaciers. Further from the contemporary glaciers, meltwater channels are associated with large terminal moraine complexes, for example those that formed around the margins of the former outlet glaciers on the eastern side of the North and South Patagonian Icefields (Figure 2). Lateral and marginal meltwater channels are also common features along the lateral margins of the former outlet glaciers (e.g. the former outlet glaciers in the Magellan Strait and Seno Otway). Here the channels are sub-parallel to the former glacier margins, often merging down-glacier into moraine complexes (Benn and Clapperton, 2000). Sandur: Glacial outwash plains are closely associated with moraine ridges, and are extremely useful in determining former ice frontal positions (Figure 2). They are especially common east of the Andes, although there are also large examples west of the Andes for example in front of the moraine complex at Laguna San Rafael. In areas of contemporary glacierisation sandar still contain active braided rivers, whilst in currently non-glacierised areas they form large sediment accumulations on valley floors. Prominent terraces and breaks of slope are marked on the sandar where they are present. Glacial lineations: Glacial lineations are present throughout the mapped area. They are formed in solid bedrock as streamlined bedrock landforms and in drift (Figure 4). West and south of the main Andean watershed the lineations are bedrock features, whilst east of the watershed they are predominantly drift features. The glacial lineations are predominantly found at lower elevations, for example on valley floors (e.g. Clapperton, 1989). In many places they occur in discrete corridors, for example to the 182

Journal of Maps, 2008, 175-196 72 ° 30' 0 "W 47°20'0"S

72 ° 25 ' 0 "W

72 ° 20 ' 0 "W

72 ° 15 ' 0 "W

Glasser, N.F. & Jansson, K.N. 72 ° 10 ' 0 "W

Legend Glaciers Glacier debris Trimlines Moraines Sandur Glacial lineations Empty cirque Alluvial fan Lakes Rivers

M. San Lorenzo

47°30'0"S

47°40'0"S

47°50'0"S

0

5

10 kilometres

Figure 3. Landsat subscene (left panel) and interpretation (right panel) showing recent glacier changes in the area around Monte San Lorenzo. Glacier recession and thinning is shown by the moraines and trimlines developed around the contemporary glaciers. In the top centre of the image is a moraine and sandur complex and in the lower right of the image are glacial lineations. These features are presumed to be LGM in age. Isolated cirques also occur.

east of the North Patagonian Icefield, where the glacial lineations indicate that ice discharge was concentrated into large, fast-flowing, topographically determined outlet glaciers (Glasser and Jansson, 2005). There is a similar situation in the Fuegan Andes where glacial lineations aligned NW to SE indicate sustained and vigorous ice flow towards the SE through Seno Almirantazgo towards Lago Fagnano. Empty cirques: Glacial cirques are common in the mountainous regions throughout the mapped area. Cirques are particularly well-developed in the Andes in the Northern and Central sectors in the area between the North Patagonian Icefield and the Chilean Lake District (Figure 3). Here cirques are developed on both the eastern and western flanks of the Andes. Cirques face generally east or south-east on the eastern flank of the Andes and west or south-west on the western flank of the Andes, although there is considerable local variation because of the influence of topography (e.g. Ljungner, 1948). Further south, there are concentrations of cirques in the area to the west of the South Patagonian Icefield. In the extreme southern part of South America south-facing cirques are developed on the mountains 183



that flank Seno Almirantazgo and Lago Fagnano. 70°56 ' 0 "W

70°52 ' 0 "W

70°48 ' 0 "W

Legend Moraines Sandur Glacial lineation Lakes

52°40 ' 0 " S

52°44 ' 0 "S

Seno Otway

52°48 ' 0 "S

0

4 kilometres

Figure 4. Landsat subscene (left panel) and interpretation (right panel) showing the glacial geomorphology of the area around Laguna Cabeza del Mar (to the east of Seno Otway in southern Patagonia). At least two sets of glacial lineations can be traced to ice margins in the north east of the image. The ice margins are marked by moraines, sandar and meltwater channels.

5.

Conclusions

In this paper we have presented a new map of the glacial geomorphology of the whole of southern South America (The Glacial Map of southern South America) between latitudes 38◦ S and 56◦ S, approximately the area covered by the former Patagonian Ice Sheets. Features presented on the map include moraines, trimlines, meltwater channels, sandar, glacial lineations and cirques, together with other related landforms and topographic features such as lakes, shorelines, deltas, plateaux, rivers and volcanoes. From the distribution of landforms we draw the following conclusions.

1. There are large contrasts between the eastern and western sides of the former ice sheets. To the east the landscape is dominated by large 184



moraine systems and associated sandar, whilst to the west there is a landscape dominated by fjords and ice-scoured bedrock. 2. Moraines and associated sandar typically mark the former extent of outlet glaciers of the Patagonian Ice Sheets. The largest moraines systems are those developed in the central and southern sectors of the mapped area to the east of the Andes around Lago Buenos Aires, Lago Argentino and Lago Viedma, but there are also large moraines present in the Fuegan Andes around Bahia Inutil and the Magellan Strait. 3. Trimlines are developed close to the snouts of many contemporary glaciers throughout the study area. In many places, they merge down-glacier with lateral and terminal moraines, marking recent (post “Little Ice Age”) glacier recession. These trimlines are most apparent around the snouts of the small glaciers that occur in the area between the North Patagonian Icefield and the Chilean Lake District but trimlines are also developed around the snouts of the eastern outlet glaciers of both the North and South Patagonian Icefields. 4. Meltwater channels occur throughout the study area at a variety of sizes. They are often associated with large terminal moraine complexes, for example those around the margins of the former outlet glaciers on the eastern side of the North and South Patagonian Icefields. Lateral and marginal meltwater channels are also common features along the margins of the former outlet glaciers (e.g. in the Magellan Strait and Seno Otway). 5. Glacial lineations are developed in many areas of the bed of the former ice sheets. These glacial lineations indicate that in these areas the ice sheets were largely warm-based. The distribution of glacial lineations also indicates that ice discharge from the former ice sheets was concentrated into large, fast-flowing, topographically determined outlet glaciers. This is especially the case to the east of the Andes. 6. Cirques are found in the mountainous regions throughout the study area on both the eastern and western flanks of the Andes. Cirques face generally east or south-east on the eastern flank of the Andes and west or south-west on the western flank of the Andes, although there is considerable local variation because of the influence of topography.

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Software All mapping for The Glacial Map of southern South America was conducted in ESRI ArcGIS software. Vector layers were stored as ArcGIS Shapefiles. Final map compilation was completed in Adobe Illustrator CS3.

Acknowledgements The Glacial Map of southern South America described in this paper was compiled between July 2006 and February 2007 whilst NFG was supported by a Leverhulme Trust Research Fellowship. We thank the journal referees Mike Sigouin, David Sugden and Tim Fisher for their helpful comments on the map and text.

Table 1 (following page) The criteria used to identify the landforms included in the Glacial Map of southern South America.

186

Moraines and moraine complexes

Glacial lineations

Trimlines

Ice-scoured bedrock

Contemporary glaciers and glacier debris

Landform/feature

Morphology Bare ice, snow and debris. Surface structures (e.g. crevasses, foliation) are common. Widespread exposures of bare or lightly vegetated bedrock. Numerous small lake basins and open joints visible. Sub-horizontal lines on valley sides separating areas of non-vegetated and vegetated land or areas covered by different types of vegetation. Parallel features indicating ice-flow direction. Formed in bedrock by glacial erosion or by sediment accumulation. Prominent cross-valley single or multiple ridges with positive relief. Linear, curved, sinuous or saw-toothed in plan. May occur as single moraines or in more complicated moraine systems. Often associated with other ice-marginal features such as meltwater channels, ice-contact landforms and sandar. Shadowing due to change in height or relative relief. Change in colour because of different soil and vegetation cover compared to surrounding terrain.

In bedrock, change in surface structure compared to surrounding terrain. In debris, different colour compared to surrounding terrain due to change in vegetation cover.

Colour/structure/texture Snow and ice appear white to light blue. Surface smooth to rough. Debris appears as grey to black in colour. Grey to light pink when vegetation cover is present. Bedrock structures and faults often present. Upper surface often has a rough and irregular texture. Sharp altitudinal change in surface colour and texture due to change in vegetation cover.

Identification criteria

Possible under-estimate in areas of thin debris cover. Bedrock landforms may be confused with bedrock structures in certain lithologies. Possible, but unlikely, confusion with trimlines where moraines have low relative height. Possible confusion with shorelines around lake margins and in coastal areas. Possible confusion with landslip material beneath volcanic plateaux.

Possible, but unlikely, confusion with other subhorizontal or horizontal features such as glacial lake shorelines.

Minor over-estimate in glacier extent possible where confused with snow cover. Possible under-estimate where bedrock obscured by vegetation cover.

Possible identification errors

Moraines mark the former terminal position of glaciers.

Show ice-flow direction and may indicate high former ice velocities, when highly attenuated.

Former vertical dimensions of glaciers. Possible englacial thermal boundary.

Evidence for extensive areas of former glacier ice at its pressuremelting point.

Foci for ice discharge from the contemporary icefields.

Significance

Journal of Maps, 2008, 175-196 Glasser, N.F. & Jansson, K.N.

187

Sub-horizontal fans on valley sides. Typically fed by a meltwater channel or stream.

Isolated conical peaks associated with former or contemporary volcanism.

Breaks of slope running parallel or subparallel to coastline in coastal areas (marine shorelines) or around lakes (lake shorelines).

Alluvial fans

Volcanoes/cinder cones

Shorelines

sedi-

Valley floor accumulations of sediment, commonly dissected by a braided stream pattern. Terrace edge in sandur Break of slope in sandur deposits. Highlevel terraces may be graded to same elevation as moraines. Delta or ice- contact Flat-topped sediment deposits accumulations above the present day valley floor, commonly with a steep delta front.

Sandur/fluvial ment

Fan shaped accumulation with sharp boundary with terrain due to change in surrounding vegetation cover. Pattern of braided streams on upper surface. Volcanoes rise sharply above surrounding terrain, are snow- or icecapped and contain open surface craters. Flanks may show evidence of lava flows with sharp boundaries with surrounding terrain. Shadowing due to change in height or relative relief. Change in colour if former shorelines are vegetated. Many shorelines mirror the shape of existing coastline or lake margins.

Homogeneous surface texture with flat upper surfaces, erosional scars and sharp boundaries with surrounding terrain.

Flat, mainly light red areas with medium grey where there is thin vegetation cover. Erosional scars and sharp boundaries with surrounding terrain. Terrace edges have sharp boundaries. Obvious break of slope with surrounding terrain.

Possible confusion with moraines, especially around major lakes where both shorelines and moraines may be present.

Indicates former lake or sea levels. Some lake shorelines indicate the presence of former icedammed lakes.

Sediments deposited by meltwater streams draining tributary valleys onto/against glacier ice in main valleys. Indicates thickness and extent of ice in the main valley. Shows former glacial lake levels. Possible to misinterpret as Reworking of unconfossil delta or ice-contact solidated material by deposit. contemporary meltwater channels and streams. Possible, but unlikely, Indicates former or confusion with non- contemporary volcanvolcanic mountains. ism.

Individual terraces indicate down-cutting and aggradational events.

Possible, but unlikely, confusion with shorelines near lakes.

Possible, but unlikely, confusion with sandur or alluvial fans.

Marks major drainage routes from contemporary glaciers and other glacier-fed streams.

Possible, but unlikely, confusion with deltas or ice-contact deposits.


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Drift limit

Plateau edges

Catastrophic failure of moraine-dammed or glacier-dammed lakes.

Indicates presence of localised or restricted mountain glaciations.

Sediment accumulation with sharp Possible, but unlikely, boundary with terrain due to confusion with sandur or change in surrounding vegetation alluvial fans. cover. Often located below a breached moraine or drained lake.

Sharp boundaries with surround- Possible, but unlikely, ing terrain, including cliffs. confusion with massmovement or landslip scars especially beneath volcanic plateaux. Well-defined channel edges and Possible, but unlikely, sharp boundaries with surrounding confusion with contempoterrain. Shadowing due to change rary drainage routes. in height or relative relief. Channel floors may be different in colour to surrounding land.

Indicates large volumes of meltwater production. Channels may indicate position of former ice margin, especially when viewed in association with moraines and sandar. Lakes appear as blue, black or Areas in shadow within Lakes indicate imother dark colours due to high re- regions of high relative re- peded drainage. High flectance. Sharp boundaries with lief or cloud shadows may frequency of lakes may surrounding terrain. Variety of be mistaken for lakes. indicate presence of shapes possible. rock basins formed by glacial over-deepening. Break of slope around Plateau edges have sharp boundIndicates scarp retreat? plateau edges; partic- aries. Obvious break of slope with ularly common around surrounding terrain. basal plateaux. Sub-horizontal or gen- Sharp vertical or horizontal change Possible, but unlikely, Former vertical and tly sloping lines on in surface colour and texture due confusion with other subhorizontal dimensions valley sides separating to change in vegetation cover, or horizontal or horizontal of glaciers. different types of drift, change from drift-covered slopes to features such as glacial or separating drift- bedrock. lake shorelines. covered slopes from bedrock.

Glacial lake outburst Prominent accumuflood (GLOF) tracks lations of sand and gravel, often on steep slopes and located below contemporary glaciers or breached moraines. Cirques Large amphitheatreshaped hollows on mountain flanks or incised into plateau edges. Meltwater channels Linear features with abrupt inception and termination. Often contain no contemporary drainage. Channels may follow or cut across local slope direction. Lakes Large freshwater bodies within enclosed basins.


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