CRITERIA REQUISITE TO A GLACIAL AGE 7 1 



the deposits of this class are likely to be the uninterrupted suc- 

 cessors of those of the previous class, and hence to overlie them 

 conformably. There is therefore the most extreme difficulty in 

 distinguishing the two under most conditions. In the case of 

 outwash aprons and certain abandoned valleys, and in other par- 

 ticular cases closely adjacent to the ice edge, the discrimination 

 can be made, but in most cases at a distance from the ice edge 

 I know of no certain means by which it can be done. The post- 

 glacial modified drift should theoretically be somewhat more 

 weathered than the true glacio-fluvial drift, but the amount of 

 weathering which took place after the ice retreated and before 

 the material was carried away and deposited would not in most 

 cases be great, and what there was would largely be worn off in 

 the transportation preliminary to the deposit of the material as 

 modified drift. It is therefore an unavailable criterion. 



3. The third class consists of the first and perhaps the second 

 class reworked by the postglacial streams after they exchanged 

 their aggrading habits to degrading ont.'-,. In this class the mate- 

 rial is identical, or essentially identical, with that of the preceding 

 classes, since it is the same material reworked, with some addi- 

 tion from the same general source, the drift of the region, and 

 its structure is so closely similar that a careful glacialist would 

 hardly venture to discriminate between them. The topography, 

 especially the relations of the deposits to the terrace systems, 

 where such exist, often aids in forming an impression of the 

 probabilities, but in most cases it cannot go much beyond that. 



Scour- and- fill. — The process which is most concerned in intro- 

 ducing the confusion of this third class is scoiir-a?id-fill, one of 

 the most obvious processes in all geology, but one of the most 

 neglected. It is a very familiar observation, if we but recall 

 it, that every active stream has shallows and deeps, and that 

 the differences in depth of water are very considerable. It is 

 also certain that these are constantly changing, and that with 

 the shifting of the course of the river they undergo complete 

 obliteration and are replaced by flood plains. The whole bottom 

 of the valley, so far as involved in the range of the river's 

 meanders and other shiftings, is subject to this alternate erosion 



