SUPERIMPOSED RIVERS 325 



its course. A revolving saw cuts its way through a log which is 

 pushed against it, so the river cuts its way through the rising 

 barrier. If the latter be raised faster than the river can cut, then 

 the stream will be dammed back into a lake, or will be diverted 

 to a new course. 



A famous example of what many authorities believe to be an 

 antecedent stream is the Green River in Wyoming and Utah. 

 Entering from the north, the river cuts its way in a winding course 

 through the great mountain barrier of the Uintas in a remarkable 

 series of canons, although by turning a short distance to the east- 

 ward, it might have found a path several thousand feet lower than 

 the one which it has chosen. Even more extraordinary is the 

 Yampa, which flows in from the east, and though a slight southerly 

 deflection would have brought it to the Green on the plains south of 

 the mountains, it cuts its profound gorge along the axis of the uplift, 

 meeting the Green in the heart of the range. The strange be- 

 haviour of these two streams is usually explained by regarding them 

 as antecedent, occupying nearly the same channels as they had 

 before the mountains were upheaved. The uplift was so gradual 

 that the rivers cut through the barriers as fast as the latter were 

 raised. This explanation is not accepted by all the observers 

 who have examined the region, some of whom explain the phe- 

 nomena by the theory of superimposed drainage, described in the 

 following section. Several rivers in the Alps and Himalayas, 

 which rise in the inner part of the ranges and cut their way out 

 through deep chasms, are believed to be antecedent. 



Superimposed Rivers. — A region newly upheaved from the sea 

 is covered to a greater or. less depth with marine deposits which 

 may lie unconformably upon a foundation of older rocks of en- 

 tirely different character, arrangement, and structure. The system 

 of drainage established upon the new land is at first consequent 

 upon the initial slopes of the region. As the streams cut their 

 trenches through the overlying mantle of newer strata, they en- 

 counter the older rocks below, first laying bare the higher ridges of 

 the latter, which will cause waterfalls and rapids. The upper 

 Mississippi has in many places excavated its channel through the 



