340 



HISTORICAL PALAEONTOLOGY. 



fluviatile deposits, but are of a coarser nature, consisting of 

 sands and gravels. Every river gives origin to deposits of 

 this kind at different points along the course of its valley ; 

 and it is not uncommon to find that there exist in the valley 

 of a single river two or more sets of these gravel-beds, formed 

 by the river itself, but formed at times when the river ran 

 at different levels, and therefore formed at different periods. 

 These different accumulations are known as the " high-level " 

 and " low-level " gravels ; and a reference to the accompany- 

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

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



Fig. 255. Recent and Post-Pliocene Alluvial Deposits, i, 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 

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



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 beside its new 

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

 " low-level " gravels. We find, for instance, 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 

 we may find gravels, sand, and loam (fig. 255, 2 2') 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, 3 3') 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 



