246 The Realm of Nature CHAP. 



over the more gentle slopes that separate mountains from 

 plains, and the velocity of the stream rarely reaches 5 

 miles an hour, and is usually not more than 2 miles. The 

 work of a river in this part of its course is at the same time 

 destructive and constructive. A stream dashing along at 

 8 miles an hour can drag boulders 4 feet in diameter ; at 2 

 miles an hour stones as large as a hen's egg are rattled 

 along; at ij mile an hour the current can just roll pebbles 

 i inch in diameter ; when gliding at half a mile an hour 

 gravel as large as peas is swept forward ; while at a quarter 

 of a mile an hour a river cannot disturb fine sand. In the 

 slackening current of the valley track heavy stones brought 

 down by the torrent cannot be stirred, and the pebbles, 

 gravel, and sand are successively deposited as the slope 

 decreases ; and, since a river is retarded by friction with 

 the sides and bottom and flows slowest at the edges, the 

 deposit of stones and sand takes place chiefly at the sides, 

 where they form a shore or terrace. This is the constructive 

 work of a valley river, and the terraces built up are termed 

 alluvial deposits. The stones stranded in these terraces 

 gradually get weathered and crumble to pieces ; and during 

 floods the river sweeps away the fragments which are 

 readily broken by friction into sand or mud, and are 

 deposited in new terraces farther down stream. The 

 material swept along the bed of the river acts like coarse 

 sand-paper, scouring the hard clay or rock which forms the 

 river-bed ; and as the stream sinks in its deepening channel 

 it leaves its old terraces lining the valley at higher levels. 

 The river also attacks the banks, pressing now against one 

 side, now against the other, undermining cliffs and carrying 

 away the fallen fragments, thus widening the flat bottom 

 of the valley. Other conditions being the same, a valley cut 

 through horizontal strata is equally steep on both sides ; but 

 if the strata dip across the stream, the bank toward which 

 they dip becomes much less steep than the other on 

 account of the greater erosive action of springs and 

 percolating rain along the bedding planes. 



322. Plain Track. On the almost imperceptible slope 

 of its plain track the work of a river becomes entirely con- 



