466 PB0CEEDINQ3 OP THE GEOLOGICAL SOCIETY. [May 24j 



gravel between Boxley and Maidstone. It is clear from the position 

 of the gravel at a that the Gault valley (6) could not have existed at 

 the time of the deposition of the gravel ; for when the bed of the 

 Medway was at a this must have been the lowest ground of the 

 neighbourhood. Since the deposition of the gravel at a, the Gault 

 valley has been eaten out to a depth of 120 feet, and breadth of 

 l-L mile, and the main Medway valley to a depth of 200 feet, 

 and breadth of two miles. But this is not all; for when gravel 

 was deposited at a, the sides of the valley very likely began to rise 

 a little to the east of a, as shown by the dotted line ; the place of 

 the hill is now occupied by a valley, and what was the bottom of a 

 valley now caps the top of a hill. 



The gravel at Marden is another interesting case. This gravel 

 lies about 50 feet above the level of the Teise, and is surrounded on 

 nearly all sides, in some directions for miles, by lower ground. As 

 before, this gravel, which once occupied the bottom of a valley, now 

 forms the tops of hiUs. These cases (though on a much smaller scale) 

 are exactly similar to that of the basalt-capped hills of the neigh- 

 bourhood of Clermont, so well described by Mr. Scrope*. Both the lava 

 and the gravel were once in the very bottom of the valleys, whilst 

 now they cap the hill-tops. The denudation implied by this fact is 

 very great ; for not only must everything below the level of the pre- 

 sent gravel-plateau have been denuded since the deposition of the 

 gravel, but also the very walls of the valley which confined the 

 river at the time of the deposition of the gravel must themselves 

 have been washed away. 



y. On the Mode of Deposition of Beds of Ch^avel and Loam, and 

 on the Action of Streams and Rivers in modifying their Channels. — 

 Before proceeding to the discussion of the origin of escarpments, it 

 may be well to say a few words on the mode of deposition of beds 

 of gravel and loam, and on the action of streams and rivers in mo- 

 difying their channels. 



Gravel occurs and is now being formed in the bed of the present 

 river Medway. It probably underHes the modern alluvium in most 

 places, usually rising from beneath it to join the old river-gravels at 

 the edge of the modern alluvium. Gravel is being constantly brought 

 down by the river, but chiefly of course when the rush of water is 

 greatest ; and, as a rule, it is deposited only in the river-bed. No 

 doubt during floods there will be exceptions ; but even then only the 

 finer gravel will be swept over the banks, and that will quickly 

 come to rest, while the finer loam will remain much longer in sus- 

 pension. 



During dry weather, or such times as the river is confined within 

 its banks, no permanent deposit of loam will be formed. At times 

 the river may rim comparatively clear, the matter held in suspen- 

 sion being small. Of the little it contains, the larger portion "will 

 be carried out to sea ; some may settle down in sheltered portions 



* Mr. Scrope (Volcanos of Central France, 2nd edit. 1858, p. 203) speaks of 

 basaltic lava occurring 1500 feet " above the water-channels of the proximate 

 valleys." 



