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POPULAR SCIENCE MONTHLY 



deeper submarine valleys was thus still left unexplained, as if they 

 were either unimportant phenomena or as if they had been formed 

 by causes not in operation at the present time. 



Chaeacter of Land Valleys. — Ordinarily speaking, valleys 

 are produced by the chemical and physical action of the rains, rills, 

 and rivers denuding- the surfaces of the land, while the character of 



the rocks gives rise to modified fea- 

 tures. Streams flowing through old 

 valleys are usually insignificant, if 

 compared with the size of the val- 

 leys. When a stream commences 

 its work, it gradually deepens its 

 channel until it is reduced to the 

 base level of erosion — that is to say, 

 when the slope of the floor of the 

 valley rises so gently from the sea 

 level, or from some other barrier, 

 that it does not permit any fur- 

 ther deepening of the river channel. In the first stage, the 

 stream only excavates the narrow gorge or caiion. Upon the base 

 level being reached, the rains and rills widen the valley, while the 

 work of the river itself is chiefly that of carrying away the mud 

 washed off the neighboring lands by the rains, or of undermining an 

 occasional bank. The process continuing, the valley may become 

 dozens of miles in width. With the subsequent rise in the land, a 

 lower base level is formed, and accordingly the process is repeated 

 from the canon-making stage to that of mature valleys, as illustrated 

 in Fig. 1. 



With the land rising at intermittent epochs, the valley becomes 

 a series of steps, as illustrated in Fig. 2. Each of the steps is being 

 cut slowly backward, but, owing to the character of the rocks, the 



Fig. 1. — Cross-section representing a 

 broad valley, abrba; with base 

 level lowered, the valley (bcrcb) 

 was produced. Another rise of the 

 district causes the stream to make 

 a new canon (d) too recently for 

 the valley to become mature. The 

 terraces at b b and c c show the posi- 

 tions of former base levels. 



Fro. 2. — Longitudinal section of a valley dissecting a plateau (represented by broken shading 

 along a line a </ x), showing three base levels, with the front of the platforms {b c and d e) 

 characterized by waterfalls. 



lower platforms may be worn away faster than the upper, so that 

 in time the steps will disappear as the valley becomes mature, 

 when its whole floor is as low as it is possible for the stream to 

 deepen it. 



If upon the completion of a portion of a valley the country 

 subsides, the lower reaches may become submerged, as shown in 



