THREE NEW PHYSIOGRAPHIC TERMS 71 1 



graphic adjustment. Its lower end is now too high, and it sets to 

 work to deepen its lower course. When the lower end of its channel 

 is brought to the level of the main at the point of junction, and when 

 its profile above is in harmony with the deepened lower end, the 

 stream is again in topographic adjustment. 



2. When falls recede past the debouchures of tributaries, the 

 tributaries are no longer in topographic adjustment to their main. 



3. The hanging valleys of glaciated mountain regions, and water- 

 falls are other cases of lack of topographic adjustment, or of non- 

 adjustment. 



Any other course of events which brings about similar results — 

 and they may be brought about in several other ways — may be said 

 to result in the topographic non- adjustment of streams. 



III. SUPERIMPOSED YOUTH. 



The terms "youth," "maturity," "old age," etc., as apphed to 

 rivers, are now in common use. There are certain phases of youth, 

 however, which the term "youth," as commonly used, does not seem 

 to define. In many parts of North America, for example, the topog- 

 raphy was mature and the rehef great when the ice of the glacial 

 period came on. Partly by erosion and partly by deposition, the 

 ice effected changes in the topography. As the ice retreated, drain- 

 age re-established itself on the modified surface. 



It often happened in such cases that the old valleys were partially 

 filled, without being obhterated. They were often filled deeply at 

 some points, while they received little drift at others. Basins were 

 developed, and in many cases these basins became the sites of lakes. 

 The drainage which established itself in such regions after the ice 

 melted has not in general had time to obliterate these marks of 

 topographic youth, especially within the area of the last ice-sheet. 

 The streams and valleys themselves have many of the characteristics 

 of youth, such as falls, rapids, lakes at high levels, and, locally, 

 narrow postglacial gorges. All this may be true while the mature 

 topography of the underlying rock is but faintly masked. Consid- 

 ering only the greater features of the topography, there is the appear- 

 ance of maturity ; but when the minor features, such as lakes, ponds, 

 marshes, falls, narrow postglacial gorges, and the pecuHar uneroded, 



