THE QUATERNARY PERIOD. 353 



ing diagram will explain the origin and nature of these de- 

 posits (fig. 255). When a river begins to occupy a particular 

 line of drainage, and to form its own channel, it will deposit 

 fluviatile sands and gravels along its sides. As it goes on 

 deepening the bed or valley through which it flows, it will 

 deposit other fluviatile strata at a lower level besides its new 

 bed. In this way have arisen the terms " high-level " and 

 " low-level " gravels. We find, for instances, a modern river 

 flowing through a valley which it has to a great extent or 

 entirely formed itself ; by the side of its immediate channel 



Fig. 255 . Recent and Post-Pliocene Alluvial Deposits. 1, Peat of the recent period ; 

 2, Gravel of the modern river ; 2', Loam of the modern river ; 3, Lower-level valley- 

 gravel with bones of extinct Mammals (Post-Pliocene) ; 3', Loam of the same age as 3; 

 4, Higher-level valley-gravel (Post-Pliocene) ; 4'. Loam of the same age as 4 ; 5, Upland 

 gravel of various kinds (often glacial drift) ; 6, Older rocks. (After Sir Charles Lyell.) 



we may find gravels, sand, and loam (fig. 255, 22') deposited 

 by the river flowing in its present bed. These are recent 

 fluviatile or alluvial deposits. At some distance from the 

 present bed of the river, and at a higher level, we may find 

 other sands and gravels, quite like the recent ones in charac- 

 ter and origin, but formed at a time when the stream flowed 

 at a higher level, and before it had excavated its valley to its 

 present depth. These (fig. 255, 33') are the so-called "low- 

 level gravels " of a river. At a still higher level, and still 

 farther removed from the present bed of the river, we may 

 find another terrace, composed of just the same materials as 

 the lower one, but formed at a still earlier period, when the 

 excavation of the valley had proceeded to a much less extent. 

 These (fig. 255, 4 4') are the so-called "high-level gravels" 

 of a river, and there may be one or more terraces of these. 



The important fact to remember about these fluviatile de- 

 posits is this that here the ordinary geological rule is reversed. 

 The high-level gravels are, of course, the highest, so far as 

 their actual elevation above the sea is concerned ; but geo- 

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