76 



GEOLOGY. 



the steep slope remote from the sea {ah, Fig. 56) ; but on the slope where 

 the valley is growing there will be headward lengthening, as in the gen- 

 eral case already considered. If the surface drainage does not dis- 

 appear at the base of the steep slope, the run-off will find its way over 

 the plain along the lowest accessible route to the sea {de, Fig. 56). In 

 this case the valley may be growing throughout its length at the same 

 time. 



The conditions represented by ab, Fig. 56, may be no more than tem- 

 porary. Sooner or later a valley developing headward across the plain 



(hi, Fig. 56) may provide a channel 

 for the water descending from the high- 

 er land beyond. In this case the valley 

 develops in sections, the one on the slope 

 above, the other on the plain below, and 

 their union (compare fghi, Fig. 56, with 

 Fig. 57) results from their growth. 



The principles here sketched have 

 been in operation wherever land areas 

 were so elevated as to give rise to un- 

 equal slopes, and this has perhaps been 

 the rule rather than the exception. 

 The results effected by the operation 

 of these principles would of course be 

 dependent on the varieties of slope, on 

 the abruptness with which a slope of one 

 gradient gave place to another, on the 

 texture of the rock, the amount and dis- 

 tribution of precipitation, etc., etc. 



In the preceding paragraphs the 

 lengthening of a valley at its upper end 

 by head erosion has been repeatedly 

 referred to. If all valleys began their 

 development at the sea and lengthened 

 headward, it might seem that their sea- 

 ward ends should be their oldest parts; but since the development of 

 valleys is begun somewhat promptly after the land appears above the 

 sea, and since the emergence is generally gradual, that part of a valley 

 which is at the seashore at one time may be far inland a little later, be- 

 cause the land has been extended seaward. On an emerging land area 



Fig. 57. — Diagram representing 

 the further development of 

 the valleys fg and hi in Fig. 

 56. The head of the latter 

 (Fig. 56) has worked back 

 until it has reached the lower 

 end of the former. 



