ALLUVIAL DEPOSITS. 521 



from it. This tract of level ground forms what is called Alluvium, 

 and it is evidently composed of material deposited by the river. 



If we examine the river-banks we generally find a loamy or silty 

 deposit with seams of gravel : sometimes the entire bank is com- 

 posed of gravel, at others nothing but loam, very sandy clay or 

 peaty mud, may be seen. In fact we see many kinds of material 

 which may have been formed and deposited contemporaneously. 

 In the finer sediments we may often find remains of the common 

 river shells and perhaps a few land shells and bones of recent 

 Mammalia, as well as Neolithic and later works of Man. 



In clayey countries the Alluvium is often broad, and not always 

 to be readily distinguished, as the ground may rise almost im- 

 perceptibly away from it. In mountainous regions the Alluvium 

 is more marked, but the material composing it is generally very 

 coarse, and in such tracts it often forms an irregular surface, 

 especially as the streams are liable to become torrents. The level 

 of the Alluvium naturally falls with the descent of the river, but it 

 may generally be defined as that tract which would be flooded, 

 should the river overflow its banks. Hence the term 'Level' is 

 often applied to the meadows, moors, or marshes that form the 

 Alluvial flats. 



As each river (unless artificially checked) is continually changing 

 its course in its valley, so is it easy to understand the great breadth 

 of many valleys when compared with the size of their rivers or 

 streams, for it does not follow that any river existed of sufficient 

 bulk to fill its entire valley, but that the river has constantly 

 changed its position, and worn away cliff's so as to form and 

 occupy different portions of the valley at different times. If the 

 land be stationary the tendency of the river is rather to widen than 

 to deepen its valley, accumulations of gravel and coarser material 

 being formed opposite to the cliffs. A considerable thickness of 

 Alluvium is, however, suggestive of depression. Prof. Prestwich has 

 pointed out that in all rivers subject to floods and carrying down 

 much sediment, as for example the Severn in its lower course, three 

 forms of sediment will be deposited : ist, coarse gravel and shingle 

 in the more direct channel through which the waters flow with the 

 greatest velocity ; 2nd, sand and fine gravel in those portions of 

 the more direct channel where the velocity of the stream is checked 

 from any cause ; and 3rd, fine silt and sediment in those parts 

 where the flood-waters out of the direct channel remain for a time 

 in a state of comparative repose ; such places are the lee-side of 

 the hills, lateral valleys and plains, with any local depressions 

 or hollows. None or little would accumulate in the main channel, as 

 the scour of the retiring waters would there prevent its deposition.^ 

 In tidal channels like the Severn, near Aust, the mud occupies 

 a conspicuously inclined bank at low-tide ; and Prof. W. J. SoUas 



^ On the subject of Floods, see C. Walford, Joiini. Statist. Soc. xli. 433 ; 

 Prestwich, on the Holmfirth Flood, Q. J. viii. 225 ; W. Molyneux, Old River 

 Courses and Floods of the Trent Valley, 1876. 



