66 



PHYSIOGRAPHY 



valleys are, in some cases, no more than grown-up ravines, whose 

 heads are advancing after the manner of hillside gullies. As it 

 grows in length, the head of one valley may reach the lower end of 



Fig. 54. Fig. 55. 



Fig. 54. Two young valleys heading toward each other. 

 Fig. 55. Valleys of Fig. 54 developed headward until their respective heads 

 have met, and the divide has been lowered a little at the point of meeting. 



Fig. 56. Fig. 57. 



Fig. 56. Two small valleys, a and 6, have developed, the one on the steeper 



slope above, and the other on the gentler slope below. 

 Fig. 57. Represents the two valleys of Fig. 56, further developed, b has 



grown until its head has reached the lower end of a, and the two have 



become one. The two figures represent one method by which valleys 



grow longer. 



another, when the two become one (Figs. 56 and 57). Not all 

 valleys, however, are lengthened at their heads after the manner 

 outlined. Thus the head of the St. Lawrence River is at the foot 

 of Lake Ontario, and will remain there as long as the lake shore 

 remains where it now is. 



If one valley reaches another under the conditions shown by 



