THE PLEISTOCENE OR GLACIAL PERIOD. 511 



ing loess and fluvial deposits of all sorts. It is to be noted that this 

 is a prevailing habit, and not an exceptional mode of action. 



While lateral action of this kind seems to have been the most com- 

 mon mode of burial of artefacs and other superficial material, other 

 systematic methods are recognized. 1 One of the more important arises 

 from the scour-and-fill of streams when they run on beds of gravel, 

 sand, silt, or other loose material. The irregularities of a stream's 

 flow, particularly the swirls and rolls developed by its meanders, give 

 rise to shallows and deeps, and constantly shift them so that in time 

 they cover nearly, or quite, the whole of the bottoms occupied by 

 the stream. Similar action is to be assigned to all stages in the past 

 history of the stream, and, hence, any article found in an abandoned 

 terrace may as well be assigned to scour-and-fill just before the stream 

 abandoned it, as to any earlier period. A valley train, heading at the 

 ice-edge and hence usually called glacial without hesitation, is sub- 

 ject to this re-working process as long as the stream flows over it. The 

 depth of re-working is readily measured by the depths of the deeper 

 parts of the streams below their flood-plains, for it is known that these 

 deeper parts are filled before the river bottoms become ■ flood-plains 

 or terraces. The depths thus re-worked very commonly range from 

 one to three score feet for small rivers, and up to five or six score for 

 large streams (Vol. I, p. 195), and in some cases, reach even three 

 and four hundred feet. In view of this, no relic found in fluvial mate- 

 rial can, with full safety, be referred to an age older than the last stages 

 at which the stream floiced over its surface. 



Almost none of the glacial gravel trains were at once abandoned 

 by their streams, except in certain portions immediately adjacent to 

 the ice-border; indeed most of the glacial gravel trains were built up 

 in their lower stretches for some time after the glacial feeding stopped. 

 This was done by the transfer of material from the high-gradient por- 

 tions near the ice-edge, to portions of lower gradient below, as an inevi- 

 table consequence of the substitution of clearer waters for the over- 

 burdened glacial waters. There is then very little assurance that 

 an implement, even if found deep in a glacial gravel train, was buried 

 while the ice was present, unless it is found in the unshifted portions 

 immediately at the ice-edge, and the topography and relations give 



1 Criteria requisite for the reference of relics to a glacial age. Jour, of Geol., Vol. XI, 

 1903, pp. 64-85. Some methods not mentioned in this work are there discussed. 



