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meltwater discharges eroded spillway channels, deposited broad alluvial fans, and pro-
vided the source waters for major river systems. Further away from the ice margin, snow-
melt and groundwater discharges effected the asymmetrical modifi cation of valley-side
slopes and the erosion of periglacial dells and valleys. Whether or not runoff was upon
seasonally- or perennially-frozen ground is diffi cult to determine.
13.5. SLOPE MODIFICATION
Here, three examples of cold-climate slope modifi cation are described from the lowlands
of Europe. They illustrate the importance of mass-wasting processes, the infi lling of
valleys with both fl uvial sediment and mass-wasted material, and an overall lowering of
relief.
13.5.1. Mass Wasting on Slopes
Blockfi elds have already been discussed in the context of frost-weathered bedrock (see
earlier). Where concentrations of blocks occur on lower slopes and in valley bottoms, they
have clearly been moved to their present positions. These accumulations, termed “stone
streams,” “rock streams,” or “coulées pierreuses,” occur at a number of localities in
western Europe and North America. They are interpreted to be the result of cold-climate
mass wasting (Péwé, 1983b; Pissart, 1953; Smith, 1962).
Periglacial “rock-streams” that occur within the Chalk valleys of southern England
(Small et al., 1970; Te Punga, 1957; Williams, 1968) serve to illustrate the role played
by mass-wasting in slope evolution during the cold periods (Figure 13.10). Thaw-
consolidation and solifl uction would have been the mechanisms of transport.
The stone streams of the English Chalklands consist of large blocks of silicifi ed sand
and fl int conglomerate (“sarsens”) that presumably originated on the upland surfaces of
the Chalk as a form of “duricrust” or silcrete in Late Tertiary times. During the Quater-
nary, these heavy blocks moved, fi rst downslope and then downvalley, some as much as
4000 m on an average gradient of between 1° and 3°. Any explanation involving sarsen
movement by early man is highly unlikely because many stones weigh several tons. It is
likely that there were several episodes of movement during the Pleistocene. On each occa-
sion, the valley became plugged with material and the stream was pushed laterally to
undercut the opposing slope. It has yet to be established whether the mass wasting was
associated with either seasonal or perennial frost. If it were the latter, the sarsen stones
probably slid across the base of the active layer as a form of plowing block. If it were sea-
sonal frost, the stones were probably rafted downslope within the waste mantle. It is likely
that both mechanisms operated at different times.
In terms of landscape modifi cation, rocks streams indicate denudation from upland
surfaces and the transport and deposition of debris on lower slopes and in valley
bottoms.
13.5.2. Valley-Bottom Aggradation
Mass wasting on slopes and the movement of material towards the valley bottom was often
accompanied by the aggradation of fl uvial sediment within the actual stream channel. It
must be remembered that the rivers that fl owed away from the ice margin carried high
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