MESOZOIC AND CENOZOIC GEOGRAPHY 481 
vania, have been added to this river by the reversal of north- 
flowing streams. 
Rivers that emerged from the ice into valleys that sloped 
toward the south, poured into them, not only quantities of water, 
but also a great amount of sediment, which had been ground from 
the rocks during the passage of the ice over them. In this way 
many valleys were partly filled with sediment, which the streams 
could not carry far from the ice front. By this means terraces 
were constructed, and broad plains of stratified gravel were built. 
All of these features may be seen on the land 
occupied by the margin of this ancient glacier; but 
when the ice finally withdrew by melting, the line 
of action of these ice agents was gradually removed 
to more and more northerly points. This withdrawal 
of the ice was rapid in the main, but was accompanied 
by numerous halts, during which, for periods of time 
variable in length, the ice front stood at various points 
north of its southernmost limit. Whenever it stood 
for a sufficiently long time, the material brought to 
the front was gathered into marginal deposits, known 
as terminal moraines (Figs. 266 and 267). The southern- 
most of these is along the line marked upon the map 
(Fig. 265) as the southernmost limit of the ice advance; 
and north of this are numerous other and usually smaller 
moraines. 
The terminal moraine represents not merely the ac- 
cumulation of debris brought and dropped by the ice, 
but also certain deposits of this debris which have been 
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