Kev. J. Brodie on Level Terraces. 
337 
waves is exerted in forming these level terraces. With this 
purpose in view, let us trace the course of a pebble lying on 
the beach. When the wave strikes against the shore, the 
pebble is hurried upwards towards the land ; but when the 
billow retreats, it is carried back. This process is repeated 
again and again. If the retreating waves prove the stronger, 
the stone is carried farther and farther back, till at last it 
reaches a depth where the agitation of the surface can no 
longer disturb its repose. Other pieces of stone or shell are 
in like manner brought down and laid beside the first, and 
others, again, are carried still farther into the deep, rolling 
over the former ones, and resting behind them. In this 
manner a terrace or level bed is formed beneath the water. 
This deposition of sedimentary matter under the water is 
not, however, the only level terrace that is produced by the 
action of the waves. Every observer who has walked along 
the shore must have remarked that, while many substances 
swept backwards and forwards by the wave are gradually 
carried into the deep, others are throw^n up on the beach, 
and there left dry. The material thus cast up, of whatever 
kind it may be, does not find a resting-place till it has been 
carried up beyond the reach of the highest flood. When the 
land, to use a common expression, is gaining on the sea, and 
the quantity of debris brought along by the tide is large, the 
gravel, sea-weed, and shells thus drawn up form a terrace, 
which varies in height according to the strength of the waves. 
A similar agency produces level terraces on the sides of lakes, 
and, on a smaller scale, on the sides of artificial ponds and 
reservoirs. When the banks are steep and of friable mate- 
rial, the billows raised by the wind wash down a portion of 
the bank, and form a general deposit under the water, like 
the lower terrace produced by the retreating waves of the 
sea. On the other hand, when the banks slope gently down 
to the water's edge, the floating mud, carried along by the 
waves, settles down among the reeds and rushes that usually 
grow in such situations, and, mingling with the decaying 
vegetation, forms a level deposit which corresponds in some 
measure to the terraces thrown up by the advancing billows 
VOL. Jll. 2 X 
