884 STRATIGRAPHICAL GEOLOGY. [Boox VI. 
vast snow-fields, from which enormous glaciers descended into the — 
plains, overriding ranges of minor hills on their way. The greater. 
portion of Britain was similarly ice-covered. In North America 
also, Canada and the eastern States of the American Union down to 
about the 39th parallel of north latitude, lay under the northern ice- 
sheet. The effect of the movement of the ice was necessarily to 
remove the soils and superficial deposits of the land surface. Hence 
in the areas of country so affected, the ground having been scraped 
and smoothed, the glacial accumulations laid down upon it usually 
rest abruptly, and without any connection, on older rocks,  Con- 
siderable local differences may be observed in the nature and suc- 
cession of the different deposits of the glacial period, as they are 
traced from district to district. It is hardly possible to determine, 
in some cases, whether certain portions of the series are coeval or 
belong to different epochs. But the following leading facts have 
been established. First, there was a gradual increase of the cold, 
though with warm intervals, until the conditions of modern North 
Greenland extended as far south as Middlesex, Wales, the south-west 
of Ireland, and 50° N. lat. in central Europe, and about 39° N. lat. in 
Kastern America. This was the culmination of the Ice Age,—the 
first or chief period of glaciation. Then followed a long interval 
marked probably by a succession of warmer interglacial periods, and 
during some part of its continuance by a partial depression of the land 
and the spread of cold Arctic water over the submerged tracts, with 
abundant floating ice. The subsidence was succeeded by a re-eleva- 
tion, with renewed augmentation of the snow-fields and glaciers,—a 
second period of glaciation. Very gradually, and after intervals of 
increase and diminution, the ice retired towards the north, and with 
it the Arctic flora and fauna that had peopled the plains of Hurope, 
Canada, and New England. ‘The existing snow-fields and glaciers 
of the Pyrenees, Switzerland, and Norway are remnants of the great 
ice-sheets of the glacial period, while the Arctic plants of the moun- 
tains are relics of the northern vegetation that covered the lowlands 
of Kurope from Norway to Spain. 
The general succession of events has been the same throughout 
all the European region north of the Alps, and in Canada, Labrador, 
and the north-eastern States, though of course with local modifica- 
tions. The followmg summary embodies the main facts in the 
history of the Ice Age. Some local details are given in subsequent 
ages, 
Pre-glacial Land-surfaces,—Here and there fragments of 
the land over which the ice-sheets of the glacial period settled have _ 
escaped the general extensive ice-abrasion of that ancient terrestrial 
surface, and have even retained portions of the forest growth that 
covered them. One of the best known of these fragments is the 
“ Torest bed,” already referred to (p. 874). Above that deposit there 
is seen here and there on the Norfolk coast a local or intermittent 
bed of clay containing remains of Arctic plants (Sala polaris, Betula 
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