Fig. 469.—Diagrams to show the effect of ice shove in producing ice ramparts upon the shores of lakes (after Buckley with a slight modification).
The first lake ice of early winter forms in most cases with air temperatures a few degrees only below the freezing point of the water. When later a severe “cold wave” arrives, the ice cover is contracted and becomes too small for the lake surface. To this contraction it yields and opens cracks up which the water rises, and in the prevailing low temperature this water is quickly frozen and the lake cover again made complete. Skaters are familiar with the opening of these cracks and the loud “roaring” which accompanies it on cold mornings, the sharp skate runners sometimes starting a crack in the strained ice, as does a light scratch upon glass that is in a similar strained condition.
Fig. 470.—Various forms of ice ramparts (after Buckley).
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The original ice cover of the lake, which was formed at near-freezing temperatures, has now received a number of inserted wedges of new ice at a time when its contracted volume has made this possible. If now a “warm wave” succeeds to the “cold wave” in the air, the ice cover expands at a rate corresponding to its rate of contraction, so that a strong pressure is exerted against the shore ([Fig. 469]). Sliding up the sloping surface of the cut and built terrace, the force of this shove may be deflected upward against the cliff, and if this is of loose materials, the effect may be to ram bowlders into the bank, to push up ramparts or ridges, to overturn trees, etc. ([Fig. 470]). In marsh land the frozen surface layer may slide over its unfrozen base and be forced up into broken folds (lower diagram of [Figs. 469] and [470]).
Fig. 471.—Map of Lake Mendota at Madison, Wisconsin, showing the position of the ridge which forms from ice expansion, and the ice ramparts about the shores of the bays (based on Buckley’s map).