234 Tin: H18TOKY OF THK NIAGARA. RIVER. 



of geolofr.V' I-'Ot us consider for a moment the tendency of stream his- 

 tories and the tendency of hike histories. Wherever streams fall over 

 rocky ledges iu rapids or in cataracts, t'.eir power of erosion is greatly 

 increased by the rapid descent, and they deepen their channels. If 

 this process continues long enough, the result must be that each stream 

 will degrade its channel through the hard ledges until the descent is no 

 more rapid there than iu other parts of its course. It follows that a 

 stream with cascades and water falls and numerous rapids is laboring 

 at an untinished task. It is either a young stream, or else nature has 

 recently put obstructions in its path. 



Again, consider what occurs where a lake interrupts the course of a 

 stream. The lower part of the stream, the outflowing part, by deepen- 

 ing its channel continually tends to drain the lake. The upper course, 

 the inflowing stream, brings mud and sand with it and deposits them 

 in the still water of the lake, thus tending to fill its basin. Thus, by a 

 double process, the streams are laboring to extinguish the lakes that 

 lie in their way, and given sufficient time, they will accomplish this. 



Now, if you will study a large map ol North America, you will find 

 that the region of the Great Lakes is likewise a region of small lakes. 

 A multitude of lakes, lakelets, ponds, and swamps where ponds once 

 were, characterize the surface from the Great Lakes northward to the 

 Arctic Ocean, and for a distance southward into the United States. In 

 the same region waterfalls abound, and many streams consist of mere 

 alternations of rapids and i)ools. Further south, iu the region beyond 

 the Ohio liiver, lakes and cataracts are rare. The majority of the 

 streams flow from source to mouth with regulated course, their waters 

 descending at first somewhat steeply, and gradually becoming more 

 nearly level as they proceed. At the south the whole drainage system 

 is mature ; at the north it is immature. At the south it is old ; at the 

 north, young. 



The explanation of this lies in a great geologic event of somewhat 

 recent date — the event known as the age of ice. Previous to the ice 

 age our streams may have been as tame and orderly as those of the 

 Southern States, and we have no evidence that there were lakes in this 

 region. During the ice age the region of the Great Lakes was some- 

 what iu the condition of Greenland. It was covered by an immense 

 sheet of ice and the ice was in motion. In general it moved from north 

 to south. It carried with it whatever lay loose upon the surface. It 

 did more than this, for just as the soft water of a stream, by dragging 

 sand and pebbles over the bottom, wears its channel deeper, so the 

 l)lastic ice, holding grains of sand and even large stones in its under 

 surface, dragged these across the underlying rock, and in this way not 

 oidy scoured and scratched it, but even wore it away. 



In yet other ways the moving ice mass was analogous to a river. Its 

 motion was perpetual, and its form changed little, but that which 

 moved was continually renewed. As a river is supplied by rain, so the 



