486 Whitaker — On Subaerial Denudation. 



This is an exceptional case of quick fall in the level of a Chalk 

 escarpment, and I cannot see how such a ridge can have been formed 

 as a sea-cliff, which has of course a level base. To explain away 

 the difficulty of the rise of the base-line by supposing that there 

 have been local sinkings or upheavals, is a groundless and un- 

 warrantable assumption until such changes have been proved, not 

 simply imagined.] 



Fig. 2. — Rough, outline of the form of part of the Chalk ridge in the Isle of Purbeck. 



E. 

 Nine Barrow Down. 



^siw^^;;;;,;;;;;;^??''''"^ 



XX. Sea level. ab. Top of Chalk ridge. 



1, 2. Foot of the ridge. 3. Level of highest part of the foot of the ridge. 



5. — Tertiary Escarpments. 



The escarpment of the Lower Tertiary beds is neither so high nor 

 so steep as that of the Chalk ; nevertheless it often forms a well- 

 marked ridge with a somewhat winding course, as on the north and 

 north-west of London, from Eickmansworth to beyond Hatfield, 

 along which line the Colne flows south-westward and the Lea east- 

 ward at the foot of the hills, receiving on their way streamlets that 

 run down the slopes and carry off the sand and clay of which those 

 slopes consist. Some of these streams are simply the result of the 

 drainage of a clay-country, others start as springs from the Drift 

 gravel which caps the London Clay on the high grounds, and some 

 end their course in swallow-holes in the Chalk. 



The thickly wooded hills of " the Blean," between Canterbury 

 and Faversham, show many examples of swallow-holes, the largest 

 of which have been described by Mr. Prestwich.^ When near the 

 top one sees springs, thrown out from the gravel by the London 

 Clay, and down the slopes there are small water-courses ; but outside 

 the close woods, which end mostly at the foot of the hills, the ground 

 is generally dry, the water having sunk into holes at the junction 

 of the Tertiary beds and the Chalk, which may commonly be seen 

 at the re-entering angles of the line of outcrop of the latter forma- 

 tion. From the southern point of these hills to Grove Ferry and 

 the Eeculvers, the London Clay, which foiTQS by far the greater part 

 of that district, is wholly cut off by the Stour and the Wantsume 

 channel, not a particle I believe existing on the right side of the 

 river, and the Oldhaven and Woolwich Beds occur only as outliers ; 

 in other words, the left bank of the Stour is an escarpment of 

 London Clay, etc. 



In many places the outcrop of the Chalk, and of the beds between 

 it and the London Clay is masked by a loam, which is nothing but 



1 Quart. Journ. Gaol. Soc, vol. x. p. 222 (1854). 



