634 STRATIGRAPHICAL GEOLOGY 



of Dublin, and they attain a great thickness near Carlow, where 

 a colliery shaft traversed 89 feet of stiff red clay with stones of 

 various kinds overlying 33 feet of Limestone Drift. 



Drifts with marine shells occur again in Wicklow and Wexford, 

 chiefly below 600 feet, but on Three Rock Mountain in Wexford 

 they occur at a height of 1300 feet, and include some more southern 

 forms, as Dosinia lincta, Venus striatula, and Venus casina. 



No notice of Irish drifts would be complete without a reference 

 to the long winding banks or ridges of gravel which are known as 

 eskers. These ridges often rest on the surface of the other Glacial 

 deposits, and from these the materials composing them have been 

 chiefly derived. They often extend in sinuous lines for many 

 miles, but are confined to the plains and low grounds, not extending 

 above the 400 feet contour. They may therefore be regarded as 

 among the latest of the Pleistocene deposits, but the occurrence of 

 large boulders on these banks shows that ice was still present. 



Morainic mounds occur in some of the valleys of the more 

 mountainous regions, and testify to the existence of small valley 

 glaciers during the last stage, of the Glacial period. 



D. THE GLACIAL DEPOSITS OF EUROPE 



Ice-extension. The principal centre of ice -dispersion in 

 Europe was the Scandinavian region. The striated rock-surfaces 

 of Norway and Sweden show that a great snow-field accumulated 

 on the highlands of those countries, forming ice -sheets which 

 radiated, outwards in all directions from the central axis of the 

 Scandinavian chain. It has also been shown that the ice passed 

 over mountain ridges which are 2700 feet high, so that the ice- 

 sheet in such places must have been at least 3000 feet thick, and 

 some authorities think 5000 feet a probable estimate of its 

 maximum thickness. 



To the west and north-west this ice-sheet must have advanced 

 far into the Atlantic before terminating in ice-clift's like those of 

 the Antarctic ice -sheet of the present day. South-westward it 

 passed into the basin of the North Sea, and is believed to have 

 crossed it so as to impinge upon the eastern coast of England. 

 Southward it certainly crossed the Baltic, which is a shallow sea, 

 and invaded the plains of Holland, Germany, and Russia. Eastward 

 it not only crossed the Gulf of Bothnia, biit passed up the Gulf of 

 Finland and over Northern Russia to the Ural Mountains, and 

 south-east into the very centre of the Russian plain (see Fig. 208). 



In the North Sea it was met by the independent and smaller 

 ice -sheet formed by the eastern part of the Scottish ice; and the 



