324 ADJUSTMENT OF RIVERS 



a stream flowing across a region, cutting its way through ridge 

 after ridge, instead of following the easy path of the longitudinal 

 valleys. This is just what the principal streams of the northern 

 Appalachians, such as the Delaware, the Susquehanna, and the 

 Potomac have done, and at first sight, their course is very difficult 

 to explain. Without going very far back into the history of these 

 mountains, we may simply state that the ridges through which the 

 rivers named have cut are the remnants of a reelevated and dis- 

 sected peneplain, across which the streams flowed to the sea, cut- 

 ting transverse valleys that were rapidly deepened into gorges. 

 On the soft strata longitudinal valleys were opened out which, 

 however, were formed after the transverse streams and could not 

 be deepened faster than they, because the main stream flowing in 

 each transverse valley gave a temporary base-level for the tribu- 

 taries flowing in the longitudinal valleys. The hard beds were 

 sawed through by the descending streams, but elsewhere these 

 beds stood up as ridges, and thus the ridges are also younger than 

 the streams. The mystery disappears at once, if we simply remem- 

 ber that the transverse streams began their flow upon a sloping 

 plain above which the present ridges did not project. 



Antecedent Rivers. — Another way in which rivers have been 

 enabled to cut their way through opposing ranges of hills and 

 even mountains, is by occupying the district before the hills or 

 mountains were made. Such streams are called antecedent and 

 are defined as " those that during and for a time after a disturb- 

 ance of their drainage area maintain the courses that they had 

 taken before the disturbance." (Davis.) The simplest case of 

 antecedent drainage is where an area is uplifted without deforma- 

 tion and without changing the direction of the slopes. Under 

 such circumstances all the streams retain their old channels, and 

 simply gain renewed power to cut them into deeper trenches, 

 down to the new base-level. Such streams are said to be revived. 

 Even if the upheaval be accompanied by folding or deformation, 

 one or more of the streams may persist in its ancient course, 

 provided the folding be very slow and gradual, so that the river is 

 able to cut down through the obstacles which are raised athwart 



