Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

Lake Warren, “thumb” outlet.

Atlantic Drainage

Lakes Iroquois and Algonquin (early), Trent and Mohawk outlets.

Lakes Iroquois and Algonquin (late), Port Huron and Mohawk outlets.

Nipissing Great Lakes, North Bay outlet.

Permanent changes of drainage affected by the glacier.—While the lake history which we have sketched is made up of episodes which endured only while the ice front lay between certain stations upon its retreat, there were none the less brought about the profoundest of permanent modifications in the drainage of the region. It is possible to restore upon maps in part only the preglacial drainage of the north central states, but we know at least that it was as different as may be from that which we find to-day. The Missouri and the Ohio take their courses to-day along the margin of the glaciated area as an inheritance from the border drainage of the ice age. Within the glaciated regions rivers have in many cases been compelled by morainal obstructions to enter upon new courses, or even to travel in the opposite direction along their former channels. In districts of considerable relief these diversions have sometimes caused the streams to plunge over the walls of deep valleys, and it may truthfully be said that we owe much of our most beautiful scenery in part to the carving and molding of glaciers, but especially to the cascades and waterfalls directly due to their interference with drainage.

Fig. 366.—Probable preglacial drainage of the upper Ohio region (after Chamberlin and Leverett).

Many diversions or reversals of former drainage lines, through the influence of the continental glacier, are at once suggested by the abnormal stream courses, which appear upon our maps, and the correctness of these suggestions may often be confirmed by very simple observations made upon the ground. The map of [Fig. 366] shows how different was the preglacial drainage of the upper Ohio region from that of to-day.

An interesting additional example is furnished by the Still River which in Connecticut is tributary to the Farmington, and is no less remarkable for its abnormal northerly course and sluggish current perpetuated in its name, than for the way in which it is joined to the Farmington system ([Fig. 367 A]). A careful study of the district has shown that the Still River was once a part of the Naugatuck and flowed southward toward Long Island Sound like other rivers of the district ([Fig. 367 B]). It possessed, however, an advantage in a narrow belt of softer rock along its course, and because of this advantage it captured a portion of one of the tributaries to the Farmington ([Fig. 367 C]). The continental glacier later covered the region, and on its retreat laid down morainal obstructions directly across this river and also at the head of the severed arm of the Farmington tributary ([Fig. 367 D]). The now impounded waters found their lowest outlet near Sandy Brook, and in waterfalls and cascades the now reversed river falls one hundred feet to the bed of that stream. With the aid of the excellent topographic maps which are now supplied by a generous government at a merely nominal price, such bits of recent history may be read at many places within the glaciated region.