1 82 THE STORY OF THE EARTH. 



northern and arctic seas. Eventually the crag 

 land was covered with boulder clay, and the 

 whole country experienced glacial conditions. 

 The cold is attributed by some to change of 

 form in the earth's orbit, by which the winters 

 increased in length. Others attribute it to up- 

 heaval of land. Upheaval of Scandinavia and 

 the North Sea would displace the shells south- 

 ward, and lead to a condensation of vapour, from 

 which glaciers would result large enough to cross 

 the plain of the North Sea and reach Britain. 



CHAPTER XXIII. 



GLACIAL PERIOD AND GRAVELS. 



So manifest a break in the succession occurs 

 with the superposition of the Glacial deposits 

 upon the Crag, that some geologists regard 



them as beginning a fourth great division o( 



the strata which is named Quaternary or Post- 

 tertiary. Others place them in a division of the 



tiary period, which is named Pleist 



The singular feature o( the formation which 



justifies • name is the wide d of 



the glacial conditions over the Earth. In many 

 countries where ice now is only a passing inci- 

 dent of Winter, Clays are found, blue, purple, or 



brown, full of fragments of rocks which are 



mostly local, though many have travelled from 

 distant places, These lx.ul.lers, which cause the 

 deposit tO be named Boulder Clay, are often 



smoothed and grooved or scratched on one side 



