CHAPTER V. 



SUEFACE GEOLOGY. 



RELATION OF THE VALLEYS TO GEOLOaiCAL STRUCTURE AND DIS- 

 PLACEMENTS OF STRATA. 



Except in such rare instances as that of astream having its course upon 

 or near a synclinal axis, nothing is more evident than that, as a rule, all 

 streams have been the instrument by which their own valleys were exca- 

 vated ; and, even in the doubtful cases referred to, it is probable that an 

 examination of other portioos of the same valley would show a confirm- 

 ation of, rather than an exception to, the general rule. This proposition 

 accepted, leaves other questions of a remarkable character, which are 

 unexplainable upon any except geological grounds. For example, in the 

 Eock3" Mountain region we not only often see a river traversing an ele- 

 vated district, cutting its course by a deep caiion in the underlying rock, 

 while the land-surface is much lower not many miles distant than that 

 which the canon traverses, but we have examples of rivers cutting 

 directly through a mountain uplift, and at right angles with its axis. 

 Furthermore, cases of this kind occur in which the mountain uplift is not 

 only a high one, composed of much harder rock than that of any of the 

 surrounding strata, but the softness of the latter and the slight elevation 

 of the surface they occupy seem to offer an especially favorable opportu- 

 nity for the river to pursue its course around, rather than through, the 

 mountain uplift. In fact, nothing is more apparent in the Eocky Mount- 

 ain region than that the courses of the rivers are independent of the flex- 

 ures which the strata have suffered over which they flow, and that their 

 courses are also independent of the present character of the surface. 



The only explanation I am able to give for this condition of things, 

 or the one presenting the fewest objections, is, that the streams were 

 established where the conditions of the surface then favored the course 

 which they pursued, and before the strata were flexed to any considera- 

 ble extent ; that all the important displacements of strata, including 

 the mountain uplifts, have taken place since the streams began to 

 excavate their valleys. Therefore the movements of these displace- 

 ments must necessarily have been exceedingly slow ; never more rapid 

 than was the deepening of the channels of the rivers by the ordinary 

 erosive action of their flowing waters, or, if so, the elevation was not 

 enough to produce a permanent damming of their waters, or a material 

 deflection of the course of the stream. When the streams were first 

 established, they must, of course, have found their way to the sea over 

 the then less elevated portions of the surface ; but during the ages that 

 have since passed, the continent has continued its elevation ; the strata 

 have been variously displaced, in some cases producing great mountain 

 flexures. Added to all this, the subaerial denudation has been so great 

 that doubtless no trace of the surface upon which the rivers were orig- 

 inally formed now remains. Indeed, it is known that, although the 

 channels of at least some of the rivers of the Eocky Mountain region 

 occupy a somewhat greater actual height above the level of the sea 

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