Deposits of all sorts fomaed by glaciers or their meltwaters 
-constitute what is known as glacial drift. (A land surface that has 
been glaciated is thus veneered d with an irregular mantle of-drift, 
«which includes moraines, outwash plains, kames and erratics, as well 
‘as other types of-glacial, ‘glacio-lacustrine and glecio-fluvial de- 
posits that. need not be mentioned here, inasmuch es they are not re- 
presented in the area under consideration. The drift at any one 
plece may, ‘therefore, consist of either till or glacio-fluvial ma- 
“terials or both; and at some ‘places~: the drift may include Geposits 
formed-during several ‘successive stages of glaciation. 
: _ . Retreat of the Glaciers 
‘Whenever the climate of the Pleistocene became warmer or drier, 
he margins. of thé melting glaciers’ ‘receded, even though the ice 
composing them may have continued to move. forward. This retreat was 
induced not. only by more rapid’melting and evaporation of the ice, 
which caused.a thinning and down-wasting of its border regions ,but 
also by a reduced supnly because of smaller accumulations of snow at 
the center. The glacier fronts receded, gradually uncovering the 
land and spreading vast quantities of outwash over ithere and there. 
lekes were formed where drainage was’ blocked by glacial deposits as 
weil as in hollows. on the irregular surface of the drift. Plants-and 
animals reropulated the glaciated terrains, following closely behind 
the . dwindling ice and‘spreading across the rocky, glacier-borne 
soils. : US wee 
The ice recession was by no meang continuous. There were spells 
of cooler weather within centuries: of warmer conditions so’ that the 
gleciers pushed forward again for short distances,plowing and piling 
“up debris.they had previously dumped’on the land, At times the ice 
~front stood at almost the same position for scores of ‘years because 
only as much new ice was pushed forward each year as was melted. 
Consequently, moraines were heaped up to mark places where the ice 
front pane cred practically stationary,. just as they had marked the 
‘limit of maximum advance. Because this process was repeated fre- 
quently during the thousands of years of recession, a great ‘series 
of these recessional moraines was strimg across the countryside In 
the central United States where: the lands are relatively flet, Pree e 
are easy to recognize ; but in the-moumtainous comtry of the eastern 
areas. and in.most of New England, no such extensive systems of mo- 
»rainic belts have been observed. - 
Several Episodes of Glaciation 
. Geologists who studies the glacial deposits of North America 
and hurope during the latter part of the last century and the first 
part of this were soon convinced. that.during Pleistocene time the 
ice Sheets formed and spread, melted and Sioa speared several times, 
-and that the climate of the world was. even warmer during some of the 
intervals between flacial episodes than it is today, They subdivided 
the Pleistocene, therefore, into stages of See eal advance and 
