POST-TERTIARY PERIOD. 



555 



Oscillations of level have taken place in many regions at later 

 times, and these throw some doubt over the exact amount of 

 change in particular regions during the Terrace epoch. Yet its 

 characteristics stand out in bold relief : — 



(1.) The continental extent of the area that was elevated. 



(2.) The fact that there was an increase in the amount of eleva- 

 tion from the south to the north. 



The elevated sea-beaches of the Arctic are higher than those of 

 the St. Lawrence, and those of the St. Lawrence higher than those 

 of southern New England ; and the upper terraces of the Great 

 Lakes higher than those south of the Ohio. The change of level 

 was eminently northern, like that introducing the Champlain epoch, 

 only different in direction, — upward instead of downward. 



The formation of the river-terraces (fig. 830, p. 548) has been 

 stated to be a consequence of the elevation of the land. This is 

 illustrated in the following cuts. The condition of a river and its 



Fi£. 831. 



■ection of a valley in the Champlain epoch, with dotted lines showing the terraces of the 



Terrace epoch. 



river-flat in the Champlain epoch is shown in fig. 831, in which R is 

 the river-channel, in the broad river-flat ff / . Eivers in an open 

 country have always both these two elements, a channel and a river- 

 flat or flood-plain. The stream occupies the former during ordinary 

 low water, but spreads over the latter during freshets. The sweeping 

 violence of the flood determines the limits, other things being 

 equal, and the flat surface of the flood-plain or river-flat. 



If now the interior of a continent be raised, say 100 feet, while 

 along the sea-coast it is little changed, the river will have an in- 

 creased angle of slope, a quicker flow, and greater power of erosion; 

 and it will gradually wear down its channel, if there are no rocks 

 to prevent, until the old slope is again attained. The flood-plain 

 will also sink at the same rate, although with more or less changed 

 limits, owing to many causes of variation. The line dd / Avould then 

 be the flood-plain or river-flat with its channel (below R). After 

 another similar elevation, b b' might be the flood-plain and chan- 

 nel. In fig. 832 a section of a valley, thus terraced, is represented. 



