i8o THE BOOK OF NATURE STUDY 



spicuous in comparison to the volume of the stream; and here 

 more and more of the transported material is dropped. The valley 

 at last opens out into a plain, and here the deposited debris so 

 chokes and clogs the stream that it readily divides into distributaries 

 which are constantly changing their courses and ever finding new 

 exits to the sea. This last portion is the delta of the river which 

 it has built up to and above sea-level and is constantly pushing 

 out into the sea. 



The extent to which a river conforms to this type is dependent 

 upon the factors just mentioned, and especially on its age as a 

 whole or in part. Its main function in its upper part, so long as it 

 has sufficient fall, water, velocity, and excavating material, is to 

 incise or saw a trench along its water-way. But the fact that it 

 makes thus a slope down to its own bed gives power to numerous 

 lateral agencies capable of opening out the sides of its gorge-like 

 valley. Rain, springs, frost, wind, and the numerous other 

 denuding agencies will tend to disintegrate the rock of the flanks 

 of the trench, and rain, wind, and streamlets will wash some of 

 the material into the main stream. The effect of this is to cut 

 back the sides of the trench and to reduce them gradually to a 

 slope on which soil can form and rest. Soil-making agents 

 will then work up the rock surface into subsoil and soil which, 

 resting on an unstable slope, will inevitably travel steadily 

 downward towards the river, allowing new subsoil and soil to 

 form and travel down behind it. The river will continue to 

 scavenge away the material travelling down to it, thus keeping 

 up the movement and the action of the lateral agencies. So, if 

 time is allowed, the sides of all valleys will be opened out, at a rate 

 depending on the hardness or softness of the rock of the valley sides. 

 When the average slopes have become softened down by these 

 means, the shape and outline of the valley are spoken of as mature 

 (Fig. 68). When, on the other hand, the river's downward 

 erosion has outstripped the work of the lateral agencies, and 

 the valley sides are abrupt, and all slopes sharp and angular, 

 the topography is spoken of as youthful or immature. Counter- 

 dip streams are usually less mature than others, because they 

 necessarily originate after the main relief of the land has been 

 sketched out (Fig. 69). Not only are they usually steep sided, 



