Rit (Vísindafélag Íslendinga) - 01.06.1975, Blaðsíða 25

and pools above them, and the side which faces the ponds is steep
and eroded by the water. In some areas water only collects in
winter, it drains or it dries up in early summer and then patches
appear, which are reminiscent of irrigation meadows (Fagridalur
and Siðuafréttur type). In both these types I believe sohfluction to
be the primary cause of the ridge formations, but doubtless in con-
nection with frost effect. Otherwise it would hardly be comprehen-
sible that these ridges only form in colder areas. In the plain type
I believe that solifluction occurs primarily in spring, when the top
layer of the soil has thawed out leaving frozen soil undemeath.
Although the slope is quite slight, it suffices to set the thawed out
layer in motion, causing it to slide as soon as a smooth surface has
formed on top of the frozen layer. The motion is slow, however,
and presently small ground protrusions get in the way, stopping
the slide, whereby a slight ridge forms, but little alteration need
occur to set off changes in vegetation on the ridge. Moss increases
and shrubs take root, creating an insulating layer on top of the
soil. This layer preserves the ice core in the soil into late spring
and causes more resistance, so that the ridge constantly increases
m height, both because of continuing solifluction and by virtue of
the binding action of the vegetation on material transported by
wind and water. In the Fagridalur type where ridges are in con-
tact with water for longer or shorter periods of time the current
deposits additional material on the ridges. In addition solifluction
always occurs when the ground is in thaw because of the increase
m slope. It is also almost certain that ice expansion in the water
pushes the ridge upwards in a fashion seen everywhere on banks
of mire ponds and lakes. There the banks often appear to be built
up into small levees which are most probably created by current
and ice pressure, which results when the water in the pond freezes
over before the surrounding soil. In bigger lakes pressure from
drifting ice may also be taken into account. The ridge formation
ts thus largely a climatic phenomenon, although solifluction has a
bearing on it, the effect of which is heightened by frost effect,
ctnrents, water motion and finally by the vegetation, when the
ridges are high enough to permit special vegetation.
The mound formation is less pronounced in the flói area than in
other mire types, which is in my opinion due mainly to the follow-
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