POST-PLIOCENE PERIOD. 559 



II. GLACIAL DEPOSITS. Under this head is included a 

 great series of deposits which are widely spread over both 

 Europe and America, and which were formed at a time when 

 the climate of these countries was very much colder than it is at 

 present, and approached more or less closely to what we see at 

 the present day in the Arctic regions. These deposits are 

 known by the general name of the Glacial deposits, or by the 

 more specialised names of the Drift, the Northern Drift, the 

 Boulder-clay, the Till, &c. 



These glacial deposits are found in Britain as far south as 

 the Thames, over the whole of Northern Europe, in all the 

 more elevated portions of Southern and Central Europe, and 

 over the whole of North America, as far south as the 39th par- 

 allel. They generally occur as sands, clays, and gravels, 

 spread in widely-extended sheets over all the geological forma- 

 tions alike, except the most recent, and are commonly spoken 

 of under the general term of " Glacial drift." They vary much 

 in their exact nature in different districts, but they universally 

 consist of one, or all, of the following members : 



1. Unstratified clays, or loams, containing numerous angular 

 or sub-angular blocks of stone, which have often been trans- 

 ported for a greater or less distance from their parent rock, 

 and which often exhibit polished, grooved, or striated surfaces. 

 These beds are what is called Boulder-clay, or Till. 



2. Sands, gravels, and clays, often more or less regularly 

 stratified, but containing erratic blocks, often of large size, and 

 with their edges unworn, derived from considerable distances 

 from the place where they are now found. In these beds it is 

 not at all uncommon to find fossil shells ; and these, though of 

 existing species, are mostly of an Arctic character, comprising 

 a majority of forms which are now exclusively found in the icy 

 waters of the Arctic seas. These beds are often spoken of as 

 "Stratified Drift." 



3. Stratified sands and gravels, in which the pebbles are 

 worn and rounded, and which have been produced by a re- 

 arrangement of ordinary glacial beds by the sea. These beds 

 are commonly known as " Drift-gravels," or " Regenerated 

 Drift." 



The fossils of the Glacial deposits are almost exclusively 

 shells, which belong to existing species, but are referable to 

 species now inhabiting cold regions. The most important 

 Glacial shells are the following : 



