752 T. C. CHAMBERLIN 



forced to unite and flow along the ice border. At first the newly 

 assembled streams flowed either in the valleys of the smaller 

 streams that entered into the combination, or in a new trench 

 cut by the new stream across the cols between the united valleys. 

 Thus at first it would not as a rule come into possession of a 

 valley bottom of capacity adequate to the united floods, and in 

 normal adjustment to them, and hence found little opportunity 

 to make deposits. To the limited extent provided, its burden of 

 glacial detritus was thrown down in these new and inadequate 

 valleys, and as a natural consequence, it has been removed in the 

 later process of working out an adequate valley and a suitable 

 adjustment. The river is still engaged in making this adjustment. 

 4. The significance of valley adjustment. — If a great change is 

 brought about in the drainage system of a region, such as the 

 creation of the Missouri river by the junction of numerous ante- 

 cedent rivers, and a new channel is developed to fit the new river, 

 there at once arises the question whether the existing features of 

 the valleys tributary to the new channel belong to the old or 

 the new regime. In part they usually belong to both, and it 

 becomes necessary to discriminate between these parts. This 

 may be done by the study of their adjustments, a method espe- 

 cially applicable to small tributaries that have no permanent 

 streams, as in the present case. The tributaries of the old 

 system were adjusted to the old channel and cannot be presumed 

 to be adjusted to the new channel, except in the rare case of exact 

 coincidence of the old and the new channels. In relation to the 

 new system, inherited tributaries usually present either the buried 

 or the hanging type, or else they have become refashioned into 

 adjustment to the new system. Such refashioning affects espe- 

 cially the mouths of tributaries. It often so happens therefore 

 that refashioned configuration in conformity to the new system 

 may dominate the mouth of a tributary, while its upper portions 

 retain almost wholly the old configuration. These facts warn us 

 of the danger of assigning great antiquity to fluvial deposits in 

 the immediate months of tributary valleys if these valleys are 

 adjusted to the present river or the present bottoms ; especially 

 is this true if the tributary is scarcely more than a ravine, and 



