VALLEYS, AND HOW THEY ARE FORMED 409 



sinuous, ribbon-like notch where the 

 banks converge, but covers the floor of 

 the valley and rasps the sides as well. 

 This results in a form which is U-shaped 

 in section. 



But the glacial period was only an 

 episode in the history of the valleys. 



DIAGRAMMATIC SECTION OF A GLACIER- 

 FORMED VALLEY. 



Long before the ice gathered on the 

 hills rivers had been at work and had 

 left the distinctive marks of fluviatile 

 action. 



In man}^ instances the ice has not 

 obliterated the earlier forms, but has 

 carved out a U-shaped valley in the 

 lower part of an older river vaUey. 



Thus we have presented to us a valley 

 within a valley — the higher slopes gentle 

 and converging to a V if continued, 

 the lower part precipitous and in section 

 like a U. At the junction of the two 

 slopes there is a sudden steepening, and 

 streams entering from lateral x'aUe^^s 

 usually enter the main river as waterfalls. 

 In unglaciated 

 regions the tribu- 

 taries invariably 

 join the trunk 

 rivers at an 

 accordant level. 



The inosculat- 

 ing spurs of tj^pi- 

 cal river valleys 

 are also modified ; 

 they become 

 blunted and steep 

 instead of grading 

 themselves to the 

 floor of the valle}-, 

 and sometimes 

 they are entirely 

 removed by 

 glacial action. 



Now let us con- 

 sider the case of 

 a well-established 

 river, wdth lateral 

 valleys through 

 which tributary 



streams flow in a normal and well-graded 

 course. 



The overdeepening caused by glacial 

 action would disturb the accordance at 

 the point of confluence, and removing 

 the lower part of the banks, the lateral 

 valleys would be left hanging on the 

 slopes. Hanging valleys having this 

 origin are very common in Switzerland, 

 and many examples can be seen in North 

 Wales and in Cumberland. 



Thus U-shaped valleys, blunted spurs 

 and hanging valleys are regarded by 

 many geologists as the work of glaciers, 

 and this view is strengthened by the 

 fact that they only occur in places where 

 signs of glaciation are apparent. Another 

 type of \-alley of a very exceptional and 

 puzzling character is met with in the 

 glaciated parts of our islands. 



They show evident signs of having 

 been excavated by running water, and 

 yet they are streamless. They are so 

 situated as to preclude the possibility 

 of the present system of drainage having 

 taken any part in their formation. We 

 know from recent investigation that 

 they belong to the period when ice 

 advanced over the land and obstructed 

 the pre-glacial drainage. Finding their 

 natural outlets to the sea blocked by 



'THUS "«^E HAVE A VALLEY WITHIN A 

 Lakes in Val Piora. above Airolo. 



VALLEY.' 



