THREE NEW PHYSIOGRAPHIC TERMS. 



Certain phases or types of topography, some of them widespread 

 and all of them distinct, have no appropriate designation. Refer- 

 ence to them, where they '^have been recognized, has been by tedious 

 circumlocution. Names for three such phases or types of topography 

 are here proposed. 



I. TOPOGRAPHIC UNCONFORMITY. 



The essential idea underlying this term is a topography on the 

 upper part of a slope, which is out of harmony with the topography 

 on the lower part of the same slope. The slope may be that of a 

 mountain, a hill, or a valley, or it may be the slope of a larger tract, 

 such as a coastal slope. The lack of harmony may be brought 

 about in various ways. The term has to do with the result, not the 

 process by which it was brought about. Two illustrations are added. 



I. When a mountain valley, the slopes of which are well dissected 

 by rain and river erosion, is occupied by a thick and effective glacier, 

 the ice is hkely to obhterate many of the erosion features of the valley 

 slopes, up to the limit of effective ice-action. The obhteration of 

 the erosion features of the lower slopes is effected partly by erosion 

 and partly by deposition. The result is that the more open, broad, 

 and mature erosion features of the upper slopes are interrupted 

 below at the upper Hmit of effective ice-action. The more open 

 valleys and ravines above are continued below by the young, narrow, 

 gorge-Hke drainage channels developed since the glaciation of the 

 valley. These drainage lines of the lower slope are out of harmony 

 with the drainage hnes above. In such cases the topography above 

 the limit of effective glaciation is in unconformity with that below. 

 Topographic unconformity of this type is common in the western 

 mountains. It may be seen, for example, about Telluride, Colo- 

 rado, on the slopes above Lake Chelan, and in scores of other places. 

 Fig. I shows a portion of the west slope of Lake Chelan near the 

 south end. The topographic unconformity resulting directly from 

 glaciation is somewhat overshadowed by the topographic unconform- 



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