1865.] FOSTER AND XOPLEY DENTDATIOK OF THE -WEALD. 467 



along its course, but these deposits will generally be swept away by 

 the next rush of water ; occasionally they are preseiwed, as proved 

 by lenticular beds of sand and loam interstratified with gravel. 



During floods much matter is carried down in suspension by the 

 water. This is deposited by the flood-waters, when, having over- 

 flowed their river-banks, their velocity is lost or diminished. 



Rivers are constantly changing their courses. This is accom- 

 plished by the undermining of one bank, accompanied by a gradual 

 silting -up of the channel on the opposite side. A river may in this 

 way, if the land continues stationary, travel many times across its 

 plain, rearranging and depositing gravel as it goes*. It is interest- 

 ing to notice that a river, in undermining its banks in the way just 

 described, lays bare gravel deposited long before, and now mixes 

 this with other gravel that it has just brought down. Thus fossils 

 of very different ages (as measured in years) may he found im- 

 bedded together. 



It is also important to notice that the width of an alluvial plain 

 does not depend entirely upon the size of its river, as is frequently 

 assumed in reasoning upon old river-alluvia. This is well shown in 

 our area ; and from the description already given (p. 446), it will be 

 seen that the alluvial flat, like the general valley, is broader where 

 passing over the softer beds. 



It is manifest too that when the river is not deepening its channel 

 the valley must be growing broader, because rain running down the 

 hiU-sides washes down material which, when it reaches the river, 

 is carried away. The river, too, often reaches the edges of the allu- 

 vial plain, and then undermines the rocks that bound it. Each 

 successive flood adds to the thickness of the alluvial deposits, and 

 these gradually creep up the sides of the valley. Thus, if no eleva- 

 tion occurs, the alluvial plain will gradually widen. This efifect will 

 be produced much more rapidly if a depression occurs; the river will 

 then raise its bed and thicken the deposit of gravel. 



The greatest floods occur now a little way within the Chalk 

 escarpment near Snodland, and just within the Lower Greensand 

 escarpment at Yalding, which has been called the " Sink of Kent." 

 At both these places the drainage of a considerable area is concen- 

 trated into a narrow gorge, and this is doubtless the cause of the 

 floods. It is probable that these cases are analogous to the former 

 condition of the country, tuhen the great deposits of briclc-earth at 

 Maidstone and Hadlow were formed. Thus, when the brick-earth, 

 now let into pipes on both sides of the river at Maidstone, was de- 

 posited, the Chalk escarpment was further south than at present, and 

 the gorge was much nearer the brick-earth beds. The proximity of 

 Maidstone to the then-existing gorge may very likely be the reason 

 why the old alluvium was subject to those often-repeated floods, 

 which have produced the thick deposits of brick-earth which now 

 remain. A similar explanation may be off'ered to account for the 



* See Fergusson " On the Delta of the Ganges," Quart. Joum. Geol. Soc. 

 vol. xix., 1863, p. 321. 



