112
Climatic Geomorphology
the basalt plateau of the Columbia River, and in the Columbia canyon reached 300 m in
depth. Over the plains it removed most of the loess cover and caused a landscape of bare,
channelled rocks (Channelled Scabland) (Bretz, 1969; Baker, 198 l b). Giant ripples have
been recognized in gravels, with 100 m wavelengths up to 5 m height, as well as large
plunge pools and deep rocky basins of up to 30 m depth. In the Columbia River canyon
discharges of 10 millions m 3 have been estimated (Benito, 1997), and dramatic erosive and
sedimentary landscapes were built.
2. Forms resulting from fluvioglacial erosion
Glacier melt waters are capable of carrying a large sediment load and are an important
erosive agent. The velocity of the discharge is high, with common values of 8 to 15 m s- 1.
When they are subject to high hydrostatic pressures they can erode even the rocky
substrate (Drewry, 1986). These melt waters carry great quantities of suspended sediment
and bedload. Thus, in Norwegian glaciers, suspended sediment can reach values of
3800 mg 1-l, and the bedload can be 25 % of the total sediment load during the summer
peak flows. In addition a small dissolved load is mobilized, even surpassing the suspended
load in winter, as in the Argenti~re glacier (Sugden and John, 1976).
Several microforms have been attributed to the ice melt waters. They are known as
p-forms or plastically moulded forms. They have been described in the section on glacial
erosion, because there is some discussion as to their interpretation as glacial or fluvioglacial
forms, although this latter hypothesis seems to be more accepted (Allen, 1971).
The origin of p-forms can be estimated using a series of criteria: (1) If these p-forms are
found on the glacier bed, in an improbable position for the trajectory of the subglaciar
melt water, they can thus be of mechanical origin; (2) If the glacier bed is massive and
the p-forms do not originate from pre-existing fractures, they may have a fluvial origin;
(3) If the striations around the p-forms indicate ice flowed into and along the p-forms, then
mechanical ice erosion has modified them (Rea et al, 2000).
The giant potholes developed in the bed of the ice melt channels are also characteristic
of this environment, reaching depths of up to 20 m and widths of 15 m. In general, they
are wider than deep, and show helicoidal grooves caused by gravel in a turbine-type
movement (Kor et al., 1991; Dionne, 2004).
The most outstanding features of fluvioglacial erosion are the channels incised on the
rocky substrate or on the loose material produced by glacial activity. The marginal, lateral
or terminal channels can incise in the ice or on the substratum that, in general, is a glacial
till. They use to be short, rarely exceeding 2 km in length. The channels subparallel to the
contour levels are interpreted as marginal, though this is problematic, because they can be
subglacial channels that flow close to the glacial margin (Price, 1973). The subglacial
channels can incise up to 100 m, with 1 to 2 km width and 75 km length (Selby, 1985).
They once had abrupt walls and their beds are flat and plain with some irregularities.
In general, they end suddenly, and their terminus is supposed to correspond to the outlet,
where an alluvial fan could be developed. They are interpreted as subglacial when they
occupy anomalous positions within the drainage system. Where the ice margins have
adequate gradients to evacuate the melt waters, then the channels emanate from the glacial
border. These proglacial channels spill the waters into higher-order rivers, into lakes
(Figure 5.10), or into the sea (Figure 5.11), producing considerable erosive work along
Climatic Geomorphology
the basalt plateau of the Columbia River, and in the Columbia canyon reached 300 m in
depth. Over the plains it removed most of the loess cover and caused a landscape of bare,
channelled rocks (Channelled Scabland) (Bretz, 1969; Baker, 198 l b). Giant ripples have
been recognized in gravels, with 100 m wavelengths up to 5 m height, as well as large
plunge pools and deep rocky basins of up to 30 m depth. In the Columbia River canyon
discharges of 10 millions m 3 have been estimated (Benito, 1997), and dramatic erosive and
sedimentary landscapes were built.
2. Forms resulting from fluvioglacial erosion
Glacier melt waters are capable of carrying a large sediment load and are an important
erosive agent. The velocity of the discharge is high, with common values of 8 to 15 m s- 1.
When they are subject to high hydrostatic pressures they can erode even the rocky
substrate (Drewry, 1986). These melt waters carry great quantities of suspended sediment
and bedload. Thus, in Norwegian glaciers, suspended sediment can reach values of
3800 mg 1-l, and the bedload can be 25 % of the total sediment load during the summer
peak flows. In addition a small dissolved load is mobilized, even surpassing the suspended
load in winter, as in the Argenti~re glacier (Sugden and John, 1976).
Several microforms have been attributed to the ice melt waters. They are known as
p-forms or plastically moulded forms. They have been described in the section on glacial
erosion, because there is some discussion as to their interpretation as glacial or fluvioglacial
forms, although this latter hypothesis seems to be more accepted (Allen, 1971).
The origin of p-forms can be estimated using a series of criteria: (1) If these p-forms are
found on the glacier bed, in an improbable position for the trajectory of the subglaciar
melt water, they can thus be of mechanical origin; (2) If the glacier bed is massive and
the p-forms do not originate from pre-existing fractures, they may have a fluvial origin;
(3) If the striations around the p-forms indicate ice flowed into and along the p-forms, then
mechanical ice erosion has modified them (Rea et al, 2000).
The giant potholes developed in the bed of the ice melt channels are also characteristic
of this environment, reaching depths of up to 20 m and widths of 15 m. In general, they
are wider than deep, and show helicoidal grooves caused by gravel in a turbine-type
movement (Kor et al., 1991; Dionne, 2004).
The most outstanding features of fluvioglacial erosion are the channels incised on the
rocky substrate or on the loose material produced by glacial activity. The marginal, lateral
or terminal channels can incise in the ice or on the substratum that, in general, is a glacial
till. They use to be short, rarely exceeding 2 km in length. The channels subparallel to the
contour levels are interpreted as marginal, though this is problematic, because they can be
subglacial channels that flow close to the glacial margin (Price, 1973). The subglacial
channels can incise up to 100 m, with 1 to 2 km width and 75 km length (Selby, 1985).
They once had abrupt walls and their beds are flat and plain with some irregularities.
In general, they end suddenly, and their terminus is supposed to correspond to the outlet,
where an alluvial fan could be developed. They are interpreted as subglacial when they
occupy anomalous positions within the drainage system. Where the ice margins have
adequate gradients to evacuate the melt waters, then the channels emanate from the glacial
border. These proglacial channels spill the waters into higher-order rivers, into lakes
(Figure 5.10), or into the sea (Figure 5.11), producing considerable erosive work along