24 IOWA STUDIES IN NATURAL HISTORY 



way develop courses which would appear to be out of har- 

 mony with rock resistance and existing- topography in a 

 single cycle of erosion. 



Another case of antecedent streams involves superim- 

 position. Streams which develop courses on newly formed 

 surfaces, such as lava plains, emerging sea bottoms, or sur- 

 faces of glacial drift, may cut through superficial deposits 

 and become superimposed upon previously existing, irregu- 

 lar, buried surfaces. Such streams may have courses en- 

 tirely out of harmony with the resistance, structure and 

 topography of the old surfaces. Superimposed streams are 

 antecedent but they do not indicate that the youngest sur- 

 face degraded has been reduced in more than one cycle of 

 erosion. 



From the foregoing, it is seen that certain antecedent 

 streams are significant of more than one cycle of erosion. 

 If the main streams of a region show evidence of having 

 reached a late stage of stream adjustment but if they are 

 not in a late stage of erosion in the present cycle, they offer 

 valuable and almost indisputable evidence of more than one 

 cycle. The Susquehanna river shown in Fig. 9 practically 

 proves that the region in which it has its course is not in 

 its first cycle of erosion, if local warping under the stream 

 and superimposition can be eliminated. 



Windgaps 



It has long been the prevalent opinion that most, if not 

 all, windgaps are the result of diversions by piracy of 

 streams flowing in narrows or watergaps across hard 

 ridges, leaving the watergaps without water. If this is the 

 origin of windgaps, they have some value as evidences of 

 more than one cycle of erosion in the region in which they 

 are found. Let a region of folded strata go to old age of a 

 cycle of erosion and let the streams attain a final stage of 

 adjustment following the shortest routes to the sea parallel 

 or oblique with the dip of the strata. After uplift of the 

 surface, new streams will be started which will adjust 

 parallel with the strike on the less resistant formations. 

 These streams under the new conditions will have the ad- 



