Geography of Ohio 385 



It would be erroneous to infer that rock structure and attitude 

 alone account for marked relief. To appreciate this fact, we have 

 only to recall certain arid parts of the world, desert plains, where 

 weathering is due mostly to temperature changes; there the land- 

 scape is monotonous, almost devoid of relief; the slight irregulari- 

 ties present usually represent wind deposits. 



River and valley development give pleasing details to relief 

 features ; the beautiful landscapes haA^e been thus produced . Our 

 range of vision from any particular point seldom grasps more 

 than the irregularities of stream erosion. So, while the basal 

 control in physiography is the rock structure, the details that 

 are within our visual grasp depend largely upon stream work. 

 River history. There are a few terms that we should understand 

 in any discussion of stream work. All apprehend the fact that 

 if the agents of weathering are given time enough in a land area 

 that continues stable, that is, is neither uplifted nor depressed, 

 these agents will bring the area down to the level of the sea; the 

 agents of disintegration will have crumbled the rocks, and streams 

 will have distributed the products. When, in a given region, the 

 streams have accomplished this, that region is said to have been 

 base-leveled. A requisite condition, then, for base-leveling, is a 

 static position of the land area. Whenever earth movements 

 tend to uplift or to depress a region, the stream history becomes 

 complicated and the normal process of base-leveling is either 

 checked or accentuated. X particular river in a base-leveled 

 country may be said to have completed a cycle. 



The river started at the ocean border as a notch in the rin) of 

 the land; its course gradually lengthened; tributaries developed 

 on either side whose courses also lengthened, and widening of the 

 basin continued until no more area could be reached. 4 stream 

 system, with the aid of the weathering agents at work between 

 its several tributaries, in time reduces its basin approximately to 

 sea level. At first it was a youthful stream, having a steep gradi- 

 ent and a narrow valley; next it passed through the period of 

 maturity; and the last stage of its history was that of old age. 

 In river development these three stages constitute a cycle; but 

 you observe at once thatfor the regular development of thec> cle, it 

 is absolutely necessary that a static relationship exist between the 

 land area and sea level. This is a theoretical consideration, the 



