128 E. C. ANDREWS. 



head slopes downward to a narrow and steeply inclined 

 thalweg. 



This then, the force of weathering, becomes a great factor 

 in peneplanation when once the transitional stage has been 

 reached. For the channel structures of the deep valley 

 bases are attacked by the insidious force of weathering 

 and lose their coherence, and thus the streams which could 

 not corrade the fresh rock structures are enabled to 

 gradually transport their weathered fragments. 



The important point to remember about this is that once 

 the transitional stage has been reached, henceforth the 

 signs of youthful topography gradually and continually 

 decrease, never more to be revived unless by some much 

 more potent stream action or by some powerful earth 

 movement. 



Corrasion above the canon walls. — If a peneplain be 

 slightly tilted as well as locally faulted or sharply flexed, 

 then two valleys will be carved out of the upland at the 

 same time, one a canon receding from the face of the fault 

 or flex, and one a broad and shallow valley formed head- 

 wards of the canon growth. For during the peneplain stage 

 which preceded the uplift, weathering activities extended 

 to considerable depths below the peneplain surface, but the 

 streams could only feebly transport such weathered material 

 during the advanced old age stage of erosion. Upon slight 

 tilting of the uplifted peneplain, however, the streams 

 would have their velocity increased sufficiently to corrade 

 this weaker material, and thus a broad and shallow valley 

 would extend headwards of the canon growth. Even if the 

 plateau were not tilted, the very excavation of the canon 

 would yield enough stream fall to enable a new valley to 

 be formed in the weathered material above the falls. Such 

 a valley will necessarily become rapidly mature and will 

 usually be very shallow indeed owing to the downward limits 

 set upon effective weathering. 



