This page is based largely on Bendle et al. (2017) and summarises the glacial geomorphology of the North Patagonian Icefield region (46–48°S).
Glaciers and the Patagonian landscape
The Patagonian Ice Sheet has expanded and contracted at least five times during the last million years1. During glacial periods, large outlet glaciers discharged along major valleys (see map below), moving mass from the ice-sheet interior to its margins2. These glaciers also eroded, entrained, transported, and deposited rock and sediment, moulding the landscape and creating glacial landforms.
The Patagonian Ice Sheet at the Last Glacial Maximum (LGM; 21,000–19,000 years ago) with extent of contemporary glaciers shown in blue. Copyright: J. Bendle.
Why study glacial landforms?
The sediments and landforms left behind by glaciers are window into the evolution of the Patagonian Ice Sheet through time. For example, ice-marginal landforms, such as moraines, tell us about the extent or thickness of former glaciers. Other landforms provide us with information about the conditions at glacier beds or margins. Glacial erosional landforms, for instance, give us clues about the bed temperature, hydrology, and flow behaviours of former glaciers, and hence glacier thermal regime. When combined with methods of dating glacial deposits, glacial geomorphology allows us to reconstruct former glacier behaviour.
How are glacial landforms mapped?
From ‘remotely sensed’ data
When at its maximum extent, the Patagonian Ice Sheet was very large2, with outlet glaciers of similar size to those draining the Greenland Ice Sheet today. Therefore, it would be almost impossible to map its glacial geomorphology solely on foot. To get around this problem, we use ‘remotely sensed’ imagery (i.e. information about Earth’s surface obtained from aircraft or satellite).
Glacial geomorphology of the Lago Buenos Aires basin, based on satellite image interpretation (Bendle et al., 2017). Copyright: J. Bendle.
Satellite images are useful because they cover large areas, making it possible to assess regional landform distribution quickly and efficiently (see image above)2,3,4. Digital Terrain Models are also valuable in areas of steep or undulating relief, as they provide 3D representations of Earth’s surface that may give clues about the origin of landforms. For example, while a moraine typically has a crest with two sloping sides, a lake shoreline has a flat upper surface with a single sloping side (see image below).
Hillshade model with draped Digital Elevation Model (DEM), showing moraine complexes and glacial lake shorelines (note shading on one side of feature only).
‘Virtual globe’ software, such as GoogleEarth, is also useful in landform mapping as it allows the mapper to view the landscape in 3D, and customise the angle of viewing (see image below)4. With GoogleEarth it is also possible to measure the size (e.g. length, width, area) and elevation of features very quickly and simply, which may help an accurate landform interpretation be made.
Oblique view generated using GoogleEarth software, and enabling the mapping of moraine ridges. Image: GoogleEarth. Compiled by J. Bendle.
On foot, in the field
Field mapping is used to check whether features mapped from satellite images have been correctly identified, and to identify additional landforms that are not easily recognised in satellite datasets, such as small-scale or low-relief features, or landforms that may be hidden beneath vegetation or clouds in satellite imagery3,4.
Field mapping of glacial landforms in the Lago General Carrera–Buenos Aires valley. Photo credit: V. Thorndycraft.
Ice-marginal landforms
Ice-marginal landforms were produced in abundance at the snout and sides of former Patagonian glaciers2,3,4,5, due to the dumping of debris from the ice surface, the pushing and squeezing of debris at the snout, or by the flow of meltwater.
Latero-frontal moraines
On the forelands of former glaciers, moraine ridges mark out former ice limits. In central Patagonia, these ridges reach 50 m relief and can run continuously for tens of kilometres in latero-frontal arcs (see image below)2,3,4. The overall distribution of moraines can be used to reconstruct the pattern of ice retreat (see image below)6.
Latero-frontal moraine arc marking the former limit of the Lago General Carrera–Buenos Aires outlet glacier during the Last Glacial Maximum (~21,000-19,000 years ago). Photo credit: J. Bendle.
Trimlines
Sharp lines on valley sides, known as trimlines, are common closer to the contemporary North Patagonian Icefield7. Trimlines mark the boundary between terrain that has been recently covered by ice, and terrain that has been ice free for a longer period of time. Therefore, trimlines provide information about glacier thickness change.
Valley side trimlines (labelled with white arrows) marking the former thickness of the Callequeo Glacier, Monte San Lorenzo. Note moraine ridges in the foreground. Photo credit: J. Martin.
Meltwater channels
Meltwater channels come in many varieties. Most commonly they follow the lateral margins of former glaciers3,4,8, and can be straight, sinuous, or meandering (see image below). They start and end abruptly, and rarely contain rivers in the present day. Like moraines, meltwater channels are useful for reconstructing former glacier extent.
Meltwater channels cut into the surface of outwash deposits, and dissecting moraine ridges (right). Image from: GoogleEarth.
Outwash plains
Outwash plains are surfaces of glaciofluvial sand and gravel that build up in front of a stable ice margin. They may slope gently away from a former ice margin to form an expansive plain (see image below), or can accumulate in topographic lows between moraine ridges (see image above)3,4. The vast outwash plains of Patagonia2 tell us that meltwater streams carrying high sediment loads were common around former ice margins3,4.
Outwash plain grading away from moraine ridges in the Lago Cochrane–Pueyrredón valley. Photo credit: J. Bendle.
Subglacial landforms
Subglacial landforms are produced at the bed of former glaciers and commonly relate to patterns of former ice or meltwater flow.
Eskers
Eskers are straight to sinuous ridges of glaciofluvial sand and gravel, which are formed in subglacial, englacial, or supraglacial channels. They can form single ridges (as shown in the image below) or consist of a braided network. Eskers give an indication of meltwater flow patterns within or beneath former glaciers9, which is important for ice dynamics (e.g. motion).
A sinuous esker ridge, and several smaller eskers (all shown in green), mapped from satellite imagery in the Lago Cochrane-Pueyrredón valley. Copyright: J. Bendle.
Ice-moulded bedrock
In the major valleys, the bedrock has been widely scoured or smoothed by subglacial erosion processes (see image below)4. This suggests that the outlet glaciers that once overrode this terrain were (at least sometimes) warm-based, fast-flowing, and carrying a significant basal debris load10.
Glacially-smoothed bedrock outcrops in the Ibañez valley, Chile. Photo credit: J. Bendle.
In some areas, the bedrock has been moulded into very long, linear ridges called glacial lineations (see image below). When mapped at a regional-scale, these landforms reveal the flow pathways of former outlet glaciers (see map below)4.
Glacial lineations formed in bedrock south of the Lago Cochrane-Pueyrredón. Image: GoogleEarth. Compiled by J. Bendle.
Map of glacial lineations (red lines) formed at the bed of the Lago General Carrera-Buenos Aires and Lago Cochrane-Pueyrredón outlet glaciers. Interpretation of glacier flow direction is shown by dashed lines. Copyright: J. Bendle.
Drumlins
Drumlins are streamlined hills formed under moving glacier ice11. They are elongated in the direction of ice flow. Like glacial lineations, therefore, they can be used to reconstruct glacier dynamics. Their length may also be related to ice velocity12, with more elongated drumlins formed under faster flowing ice. While not that common around the Northern Patagonian Icefield (see image below), drumlin swarms occur more widely around the Southern Patagonian Icefield, and across Tierra del Fuego3,13.
Drumlin swarm in the Lago Cochrane-Pueyrredón valley. Ice flow from left to right. Image from: GoogleEarth.
Glaciolacustrine landforms
These landform types are produced in association with glacial lakes, which were common around the Patagonian Ice Sheet margin14,15.
Glacial lake shorelines
Shorelines are long, continuous terraces with flat surfaces that mark the level of a former glacial lake. Around Lago General Carrera–Buenos Aires, extensive ‘flights’ of shorelines are seen (see image below) and reflect changing lake level through time4,14,15.
Top: Relict glacial lake shorelines above the present-day Lago General Carrera-Buenos Aires, marking the level of a former glacial lake. Photo credit: J. Bendle. Bottom: GoogleEarth image showing a flight of former lake shorelines.
Raised deltas
Raised deltas are flat-topped accumulations of sand, gravel, and cobbles (see image below). They mark the point at which rivers entered former glacial lakes and deposited their sediment load. Like shorelines, therefore, they provide an indication of glacial lake level14.
Raised lacustrine delta approximately 100 m above the modern Lago General Carrera-Buenos Aires, marking the level of a former glacial lake. Photo credit: J. Bendle.
What does the pattern of landforms tell us?
The geomorphology of the North Patagonian Icefield region indicates that (1) when outlet glaciers reached their maximum extent they remained stable on flat plains, forming large moraines and outwash plains. Subglacial landforms (e.g. glacial lineations) suggest that (2) the glaciers were (at least sometimes) fast-flowing and warm-based. The presence of shorelines and raised deltas show that (3) large proglacial lakes developed around the retreating ice margins during deglaciation.