82 STUDIES FOR STUDENTS 



tary and, by acceleration, aids their excavating action. It is at 

 this stage pre-eminently that the tributaries cut down their val- 

 leys into adjustment with the main stream bed. On the other 

 hand, when the active impinging bend of the river has shifted 

 elsewhere, and in its stead a flood-plain is being built up across 

 the mouth of the tributary, the drainage of the latter is checked, 

 and if the tributary be small and its waters incompetent in com- 

 parison with the flood-plain aggradation of the river, the valley 

 mouth will be filled to a height corresponding to that of the 

 highest flood-plain. 



Further, if the mouth of the tributary be blocked by the upper 

 flood-plain beyond the time of the latter's growth, the wash from 

 the tributary will build a delta or fan upon it, and this further 

 growth will continue until the waters from the tributary valley 

 have built up a suitable gradient for themselves across the flood- 

 plain to the river. This only holds good in valleys of incompe- 

 tent drainage which cannot cut and maintain a trench for them- 

 selves. If the tributary valley has a large competent stream, it 

 will maintain a channel-way across the flood-plain to the river, 

 and less aggradation will result from the shifting of the 

 meanders. 



A series of aggradation deposits of the' recomposition type are 

 to be looked for on the lower slopes and in the mouths of all 

 tributary valleys, caused by the shifting action of the primary 

 stream, and these may reach depths of twenty or thirty feet, and 

 theoretically much more. As these deposits are subject to geo- 

 logically rapid construction and removal, they are peculiarly well 

 adapted to bury whatever may occupy the surface in the erosion 

 stage. But such burial must be referred to the date of the 

 recomposition and aggregation of the material, and not to that 

 of the chief parent formation. 



Beside this systematic, constantly recurrent case there may 

 be others assignable to the causes named and to more special 

 agencies. 



PalcBontological criteria. — The previous discussion has been 

 confined to formations within the glaciated area, or immediately 

 connected therewith. Formations not physically connected with 



