THE WORK OF RUNNING WATER. 



205 



surface of the hard layer marks the lower limit of the terrace, which 

 commonly has a distinct slope toward the stream. Except where 

 interrupted by tributary valleys, such terraces are likely to be con- 

 tinuous in a valley so long as the structure remains the same and the 

 stream sustains the same relation to it. Such terraces would first 

 show themselves in the older part of the valley. The effect of inclina- 

 tion of the hard stratum on the development of such terraces will be 

 readily inferred. Terraces and benches of this sort are not equally 

 distinct at all stages of a valley's history. For great distinctness, the 



Fig. 193. — Diagram illustrating a distinct terrace and a "second bottom (6)." which 

 may be regarded as a low terrace. 



hard layer should have been exposed long enough to allow the general 

 processes of erosion to have effected considerable differential wear, but 

 not long enough to allow the topographic effects of unequal resistance 

 to be obhterated. 



Normal flood-plain terraces. — It has been seen that deposition 

 in a river vahey stands in more or less definite relationship to the stage 

 of its development, and that the deposition which leads to the develop- 

 ment of an alluvial plain is likely to take place where the higher gradient 

 of the upper course gives place to the gentler gradient of the lower. 

 It has also been seen that as a stream's history advances, the stretch 

 where the gradient is high recedes up-stream, and that the point which 

 marks the head of active deposition follows. It follows that a river 

 flat or flood-plain normally begins in the lower part of a valley, and 

 works progressively headward, its upper end following, at some con- 

 siderable distance, the head of the valley itself. 



The commoner river terraces are remnants of former flood-plains, 

 below which the streams which made them have cut their channels. 

 It has already been pointed out (p. 184) that processes of erosion and 

 deposition work together in the development of flood-plains, and that 

 some flood-plains have but little afluvium (Fig. 174), while others owe 

 their origin wholly to stream deposits (Fig. 175). It follows that ter- 

 races developed from flood-plains may be of rock, of alluvium, or of 

 rock covered with alluvium. 



