286 PREHISTORIC EUROPE. 



and ridges of gravel and sand which appear almost everywhere 

 over the low grounds of Northern Europe. They belong to the 

 time when the ice-sheet was melting away. The erratics were 

 doubtless transported upon the back of the mer de glace, and 

 carried southward to its terminal front. They are found covered 

 by, embedded in, or perched upon, the surface of diluvial and 

 morainic sand and gravel. Sometimes they rest directly upon 

 boulder-clay, or, when that is absent upon some of the older 

 rocks. I have described the mode in which the loss appears to 

 have been formed. Its deposition doubtless continued so long 

 as muddy water overflowed the low grounds of Europe, and 

 hence it belongs to all stages of the Glacial Period ; and its 

 enormous development in Southern Eussia, as compared with its 

 sparser appearance upon the plains of Northern Germany, I have 

 already endeavoured to account for. The great accumulations 

 of " Decksand " and " Geschiebesand," which cover such broad 

 areas in Germany, Denmark, Southern Sweden, and Eussia, are 

 due partly to the action of diluvial waters derived from melting 

 snow and ice which swept across the low grounds as these were 

 vacated by the mer de glace. But much of the materials may 

 have been derived from the surface of the mer de glace, upon 

 which it is most probable that considerable quantities of water- 

 worn morainic detritus would tend to accumulate, and to be 

 shot over the terminal front of the ice-sheet. 



Erom this short review of the evidence we gather that the 

 Glacial Period of Germany and Denmark was characterised by 

 several great changes of climate. Dr. Penck has shown us that 

 there are at least three boulder-clays, and there may be more, 

 lying deeply buried under the drift accumulations of Middle 

 Germany. English geologists are especially fortunate in having 

 the secrets of their glacial formations laid bare in magnificent 

 sea-coast sections. Their brethren in Middle Germany, where 

 the drifts probably attain their greatest thickness, must be con- 

 tent with what is revealed to view in river-banks and artificial 

 exposures. The succession made out by them, however, is suf- 



