THREE NEW PHYSIOGRAPHIC TERMS 713 



But the type of youth here referred to might be superimposed on a 

 topography which was old, or on a topography which was young. 

 Thus, when the upper, but not the lower, portion of a young moun- 

 tain valley is severely glaciated, a topographic change is effected. 

 The wide, open, U-shaped or cirque-Hke upper part of the valley 

 which was glaciated, is often continuous below with a narrow valley 

 of a very different type. In spite of the fact that the glaciated part 

 of the valley is wide open, it possesses distinctly youthful character- 

 istics. Even though the valley was youthful before glaciation, its 

 youth has now assumed another phase. Here youth is superimposed 

 on youth. Examples of this phase of superimposed youth abound 

 in nearly every mountain range of the West where there were glaciers 

 during the last epoch. 



In the cases cited, the superimposition of youth has resulted 

 from glaciation. Should it become desirable to distinguish between 

 the youthfulness superimposed by different means, the type of 

 superimposition cited above might be called glacially superimposed 

 youth. 



Between the superimposed youth of the upper glaciated part of 

 the valleys cited in the above illustration, and the normal youth of 

 the parts below, there are (i) a topographic unconformity and, (2) a 

 lack of topographic adjustment. 



THE NEED OF DISCRIMINATING TYPES OF YOUTH. 



There are various types of topographic youth which need to be 

 discriminated. Take, for example, the types of youth illustrated 

 by the region about Fargo, N. D. (Fig. 4), and that shown along the 

 shore of Lake Michigan just north of Chicago (Fig. 5). The former 

 region has often been cited, and properly, as an illustration of topo- 

 graphic youth, but it hardly represents normal youth. 



The different phases of youth shown by these two areas are 

 always confusing to students at the stage when they are learning 

 to interpret topographic maps. In the one case, young valleys are 

 in process of normal development, and those which continue to grow 

 should, in the course of time, acquire permanent streams. In the 

 other, the surface came into possession of a well-developed stream 

 somewhat suddenly, after the retreat of the ice. The meandering 



