Chapter 4
Glacial transport and sedimentation
1. Glacial supply and environments
Glaciers are able to transport large quantities of rocky material for considerable distances,
although not all of the mobilized particles have been produced by erosive glacial activity.
There are many other sediment sources, such as clasts formed by congelifraction processes
in the slopes around the glaciers. Other types of sources are snow and rock avalanches,
landslides and nonglacial streams that flow into the glacier. In some cases, volcanic
activity can supply pyroclastics to the ice masses, as in some glaciers that cover volcanoes
in the Andes. Finally, winds can supply small particles to glaciers (Boulton, 1978; Small,
1987; Kirkbride, 1995; Benn and Evans, 1998).
In West Greenland, discharge of debris takes place predominantly on the ice-sheet
margin, through the basal ice layer. The relative abundance of debris is an important
control of glacial sedimentary processes and features. The sediment production varies
substantially between ice-sheets and valley glaciers (Knight et al., 2002).
Glacial debris can be carried in three different environments. The material mobilized
on the ice surface constitutes supraglacial debris, which generally is angular and scarcely
modified by glacial activity. It is more abundant on valley glaciers, because their rocky
walls contribute directly to the ice surface, whereas on continental ice the only rocky
sources are the walls of the nunataks. In the accumulation area debris can be covered
by snow and, in some mountain areas, it can cover the whole ice surface, as occurs on
many of the low-albedo glaciers of the Karakoram and Himalayas (Figure 4.1) that protect
the ice against melting. In most glacial valleys, the proportion of debris increases toward
the front of the glacier terminus. The transport of supraglacial debris occurs as on a
conveyor belt, and very big blocks can be transported for large distances.
Englacial debris occur disseminated within the ice mass, and then their proportion
rarely exceeds 15 %, or in planar layers up to 5 m thick separated by clean ice, where it can
represent up to 80 % of the total mass (Embleton, 1979). These layers correspond to filled
cracks or crevasses, though in most cases they are subglacial debris displaced upwards
along flow lines. This transport occurs in the areas of compressive flow of the valley
glacier. In other cases, compression is caused by terminal moraines blocking the advance
of the ice (Goldthwait, 1951; Boulton, 1970a).
Subglacial debris come from the glacial valley bottom or from the material that
penetrates the cracks, crevasses, and tunnels. These bottom debris undergo abrasion and
fracturing, and consequently the clast roundness is greater, and the percentage of fine
material higher. Transport is by a traction mechanism (Sharp, 1988), with the particles
sliding and rolling as they are pushed by the glacier. If the subglacial bed is deformable,
then the particles will be pressed into the bottom or will become lodged in it. The clast
Glacial transport and sedimentation
1. Glacial supply and environments
Glaciers are able to transport large quantities of rocky material for considerable distances,
although not all of the mobilized particles have been produced by erosive glacial activity.
There are many other sediment sources, such as clasts formed by congelifraction processes
in the slopes around the glaciers. Other types of sources are snow and rock avalanches,
landslides and nonglacial streams that flow into the glacier. In some cases, volcanic
activity can supply pyroclastics to the ice masses, as in some glaciers that cover volcanoes
in the Andes. Finally, winds can supply small particles to glaciers (Boulton, 1978; Small,
1987; Kirkbride, 1995; Benn and Evans, 1998).
In West Greenland, discharge of debris takes place predominantly on the ice-sheet
margin, through the basal ice layer. The relative abundance of debris is an important
control of glacial sedimentary processes and features. The sediment production varies
substantially between ice-sheets and valley glaciers (Knight et al., 2002).
Glacial debris can be carried in three different environments. The material mobilized
on the ice surface constitutes supraglacial debris, which generally is angular and scarcely
modified by glacial activity. It is more abundant on valley glaciers, because their rocky
walls contribute directly to the ice surface, whereas on continental ice the only rocky
sources are the walls of the nunataks. In the accumulation area debris can be covered
by snow and, in some mountain areas, it can cover the whole ice surface, as occurs on
many of the low-albedo glaciers of the Karakoram and Himalayas (Figure 4.1) that protect
the ice against melting. In most glacial valleys, the proportion of debris increases toward
the front of the glacier terminus. The transport of supraglacial debris occurs as on a
conveyor belt, and very big blocks can be transported for large distances.
Englacial debris occur disseminated within the ice mass, and then their proportion
rarely exceeds 15 %, or in planar layers up to 5 m thick separated by clean ice, where it can
represent up to 80 % of the total mass (Embleton, 1979). These layers correspond to filled
cracks or crevasses, though in most cases they are subglacial debris displaced upwards
along flow lines. This transport occurs in the areas of compressive flow of the valley
glacier. In other cases, compression is caused by terminal moraines blocking the advance
of the ice (Goldthwait, 1951; Boulton, 1970a).
Subglacial debris come from the glacial valley bottom or from the material that
penetrates the cracks, crevasses, and tunnels. These bottom debris undergo abrasion and
fracturing, and consequently the clast roundness is greater, and the percentage of fine
material higher. Transport is by a traction mechanism (Sharp, 1988), with the particles
sliding and rolling as they are pushed by the glacier. If the subglacial bed is deformable,
then the particles will be pressed into the bottom or will become lodged in it. The clast