TRANSVERSE VALLEYS 317 



region above base-level is also an important factor in determining 

 the amount of work to be done. 



For reasons that will appear later, we assume that when denu- 

 dation began its work upon a region of inclined strata, that region 

 was a sloping plain, or peneplain, formed by the outcropping edges 



FlG. 133. — Mesa and round-topped buttes, exhibiting unconformity and change 

 in character of the strata. Bad lands of South Dakota. 



of the strata. The first lines of drainage established would neces- 

 sarily follow this slope, and the first valley or valleys cut would be 

 across the strike of the beds, trenching both hard and soft beds. 

 Such valleys are called transverse and the streams which flow in 

 them, transverse streams. A second series of valleys will be exca- 

 vated along the strike of the softer beds, giving longitudinal val- 



