272 N. S. SHALER BROAD VALLEVS OF THE CORDILLERAS 



results which ma}^ in time be attained by following certain series of 

 phenomena exhibited in the valleys of the Cordilleran district. 



State of the Valleys 



It has often been observed that the valleys of the larger streams in 

 many parts of the Cordilleras — at least within the limits of the United 

 States — are prevailingly of remarkable width, their aspect being often 

 that of broad plains, sloping gently from the bases of the mountains on 

 either side to the rivers which drain them. These streams are usually 

 without definite banks and not directl}'' controlled, as regards their posi- 

 tions, by the bed rocks of the mountain ranges. Here and there, though 

 exceptionally, this great area exhibits streams which have carved very 

 deep can3'^ons — a type of valleys in singular contrast to the more common 

 broad vales. There is no definite series connecting these deepl}^ carved, 

 narrow valley's with those of the broad form. The two groups are, indeed, 

 sharply contrasted ; the variet}'' of intermediate forms seems to indicate 

 some essential difference in their histories. 



The valleys of the broad tj^pe are formed of two ver}'' distinct topo- 

 graphical elements — the mountain walls on either side and the sloi)es- 

 locally termed " benches," which extend, with gentle declivities, from 

 the base of their steeps to the central part of the trough. The mountains 

 exhibit the usual range in form, their torrent valle3'S above the zone of 

 gentle slopes having the normal shapes due to the incising action of 

 swift-flowing streams. Except that the gently sloping surfaces charac- 

 teristic of the broad vales often penetrate for some miles U])ward into 

 the troughs of these lesser streams, the}' in no noticeable wa}' differ from 

 those similarly placed in other elevated districts. Passing from the 

 steep, mountainous country into the vale, there is commonl}^ an abrupt 

 change in the angle of the slopes. The}' decline to an average of prob- 

 ably less than 100 feet to the mile. Although this slope is often quite 

 uniform for the distance — it may be of several miles — there is commonly 

 a rather gradual decline in the grade, so that it passes from about 2 feet 

 in 100 next the rocky borders to less than one-half foot in that distance 

 in the central part of the vale. The surface of these sloping bottoms of 

 the broad valleys is little varied, except where they are intersected by 

 the greater streams which make their way across them from the torrent 

 zone to the main water-courses. At these points the benches are traversed 

 by valleys, usually of considerable width, which are occupied by local 

 deposits evidently of detrital origin and of lesser declivity than those 

 they intersect as well as of more recent formation. 



Although in most instances the valleys we are considering are un- 



