In order that ice ramparts may be formed, it is necessary that the winter climate of the district be severe and characterized by alternating cold and warm waves, involving considerable range of air temperature below the freezing point. If the lake is small, the push of the ice will be through so small a distance as not to yield appreciable ramparts. If, on the other hand, the lake is too large, the ice cover is not rigid enough to transmit the push to the distant shore, but, like a long beam employed in the same manner to transmit a compressive stress, it is bent out of a straight line and later broken. Thus in a broad lake, with the coming of a “warm wave”, the ice cover opens in a crack from shore to shore and finds relief from the stress by pushing up a ridge above the crack. On such lakes ice ramparts are found only about the shores of bays whose expanse does not greatly exceed a mile ([Fig. 471]).
When there is heavy snowfall, ice ramparts either do not form or are of smaller dimensions, probably in part because the ice is blanketed by the snow and so prevented from sudden elevation of temperature during the “warm wave”, but even more because the ice cover is sensibly bowed down under its load and so rendered incompetent to transmit the developed stresses to the shores.
Reading References for Chapter XXX
Lake extinction by peat growth:
C. A. Davis. Peat, Essays on its Origin, Uses, and Distribution in Michigan, Ann. Rept. Mich. Geol. Surv. for 1906, 1907, pp. 105-182; Peat Deposits as Geological Records, 10th Rept. Mich. Acad. Sci., 1908, pp. 107-112.
G. P. Burns. Bog Studies. Ann Arbor, 1906, pp. 13.
Ice ramparts:
C. H. Hitchcock. Shore Ramparts in Vermont, Proc. Am. Assoc. Adv. Sci., vol. 13, 1869, pp. 335-337.
G. K. Gilbert. Lake Bonneville, Mon. 1, U. S. Geol. Surv., 1890, pp. 71-72.
E. R. Buckley. Ice Ramparts, Trans. Wis. Acad. Sci., etc., vol. 13, 1900, pp. 141-162, pls. 1-18.