'S94-] 259 [Dod^e. 
lastly, the waste is in part removed in siicii a iiiaimer as to leave 
remnants of the filling, as terraces, on tlie sides of the valley. 
A river valley begins to he filled up, or aggraded, whenever the 
load furnished to the stream for transportation becomes greater 
than the carrying power of the river. If at any time afterwards 
the relation of these two is so changed that the carrying power 
is more than sufficient to transport the waste furnished it, the 
stream will begin to remove the filling previously deposited in 
the valley. 
The amount of waste furnished to a stream depends either on 
the size of the area undergoing destruction, or on the rate of 
wasting, or on l>oth. The carrying power of the stream 
depends on the grade of the stream bed, upon the volume of 
the stream, and upon the pressure under which the stream is 
running ; and increases or decreases with corresponding changes 
in anj^ of the factors. The effect of decrease of pressure is 
particularly noticeable in subglacial streams, where it verv 
frequently happens that a load tliat is easily carried while the 
stream has its channel hemmed in beneath the ice, l)ecomes too 
heavy for transportation as soon as the stream reaches the open 
air and ceases to be confined by the ice. 
In any aggrading stream the waste forms an alluvial plain 
which becomes a terrace plain when, on ceasing to aggiade, the 
river again begins to degrade its valley. We know that all 
alluvial plains customarily slope gently with the grade of the 
stream and away from the bed, as a consecpience of the method 
of their formation. Hence we can easily see why a terrace 
plain, which is formed from an alluvial plain, should also nor- 
mally slope in these two directions. The escarpment left by the 
stream, as it cuts down into its valley filling, forms the terrace 
escarpment, and the terrace plain ^])lus the escarpment completes 
the terrace. Alluvial terraces are thus seen to be the result of 
both constructive and destructive processes, and indicate a 
period of aggradation followed by one of degradation, on the 
part of the stream with which they are associated. 
Having now noted in a general way the conditions under 
which alluvial terraces are formed, I will explain more particu- 
larly the relation of such terraces to the rivers that have formed 
them. 
