CHAPTER XXIII
GLACIAL LAKES WHICH MARKED THE DECLINE OF THE LAST ICE AGE
Fig. 346.—The Illinois River where it passes through the outer moraine at Peoria, Illinois, showing the flood plain of the ancient stream as an elevated terrace into which the modern stream has cut its gorge (after Goldthwait).
Interference of glaciers with drainage.—Every advance and every retreat of a continental glacier has been marked by a complex series of episodes in the history of every river whose territory it has invaded. Whenever the valley was entered from the direction of its divide, the effect of the advancing ice front has generally been to swell the waters of the river into floods to which the present streams bear little resemblance ([Fig. 346]). Because of the excessive melting, this has been even more true of the ice retreat, but here when the ice front retired up the valley toward the divide. A sufficiently striking example is furnished by the Wabash, Kaskaskia, Illinois, and other streams to the southward of the divide which surrounds the basin of the Great Lakes ([Fig. 347]).
Fig. 347.—Broadly terraced valleys outside the divide of the St. Lawrence basin, which remain to mark the floods that issued from the latest continental glacier during its retreat (after Leverett).
Wherever the relief was small there occurred in the immediate vicinity of the ice front a temporary diversion of the streams by the parallel moraines, so that the currents tended to parallel the ice front. This temporary diversion known as “border drainage” was brought to a close when the partially impounded waters had, by cutting their way through the moraines, established more permanent valleys ([Fig. 348]).
Temporary lakes due to ice blocking.—Whenever, on the contrary, the advancing ice front entered a valley from the direction of its mouth, or a retreating ice front retired down the valley, quite different results followed, since the waters were now impounded by the ice front serving as a dam. Though the histories of such blocking of rivers are often quite complex, the principles which underlie them are in reality simple enough. Of the lakes formed during advancing hemicycles of glaciation, and of all save the latest receding hemicycle, no satisfactory records are preserved, for the reason that the lake beaches and the lake deposits were later disturbed and buried by the overriding ice sheets. We have, however, every reason to suppose that the histories of each of these hemicycles were in every way as complex and interesting as that of the one which we are permitted to study.