58 



CHAPTER V. 



THE POST-PLIOCENE PERIOD. 



UNSTRATIFIED DRIFT TRAVELLED BOULDERS STRIATED ROCK SUR- 

 FACES PEAT UNDER BOULDER CLAY — ORIGIN OF DRIFT STRATIFIED 



GRAVELS REMAINS OF MASTODON. 



The deposits last described are found in the bed or on the margin of 

 the existing waters, and they rest on the ordinary upland soils, which 

 are consequently older than they. These soils and subsoils, which are 

 often of great depth, and which over a great part of the region under 

 consideration completely hide the rocks which lie beneath, belong to 

 the formations which we are now to describe. The soils and subsoils of 

 any country, so far at least as they consist of mineral matter, are 

 derived from the waste of the rocks of which that country is composed. 

 Hence we are in no way surprised to find the soil overlying sandstone 

 rocks to be sandy, that over shales and slates to consist in great part 

 of clay, or that overlying limestone to be calcareous ; and we may 

 attribute such appearances to the mere waste or decay of the under- 

 lying rock, by the action of the air, the water, and the frost. This 

 waste may have been proceeding ever since the country emerged from 

 beneath the deep, and need not necessarily belong to one geological 

 period more than to another. But the case becomes very different 

 where we find the soil to consist of or to contain materials for whose 

 presence we cannot account by any causes now in operation in the 

 locality ; and this we shall find to be the case with the formations of 

 that time which immediately preceded our Modern epoch, and which 

 we name the Post- Pliocene ; but which, from the nature of its deposits, 

 and the conditions which they imply, has also received such names as 

 the drift, the boulder formation, and the glacial period. 



If we examine the materials exposed in ordinary excavations, or on 

 the coasts and river banks, and which extend from the surface down 

 to the solid rocks, we find them to consist of clay or sand intermixed 

 with large stones, or occasionally of large stones with their interstices 

 filled with soil, or possibly in a few localities of rolled gravel, like that 



