Whitaker — On Subaerial Denudation. 487 



the " rainwash " of the slopes of clay and sand, and is sometimes 

 thick enough to be worked for bricks. If so much has been left, 

 how much more must have been washed away altogether, — ^all, be it 

 remembered, being the product of mere surface-denudation, 



London Clay hills show many traces of landslips, as may be well 

 seen on the left side of the Lea, where some of the sharper slopes 

 are made quite irregular by the many falls. 



Whilst, therefore. Chalk is in great part carried away in chemical 

 solution, the clays and sands of the Tertiary beds are wasted by 

 mechanical means. 



Where the dip is at a high angle the Lower Tertiary formations 

 have no escarpments, or, at all events, give rise to but a slight 

 feature, as in the Isle of Purbeck, the Isle of Wight, and Surrey; 

 whilst where the beds are flat, or dip at a very small angle, they 

 have a good escarpment, as in Berkshire, Hertfordshire, and Kent. 

 The great difference which the amount of dip has had in causing 

 the denuding powers to form a flat or a slope may be well seen in 

 the Isle of Wight, where the vertical beds of Alum Bay are in a 

 valley between the Chalk ridge and the rising ground formed by the 

 gently inclined higher series of Headon Hill, 



West and north-west of London there is a peculiarity in the range 

 and outcrop of the Lower Tertiary beds worthy of notice here. 

 The escarpment trends nearly north-east and south-west along a 

 line through Twyford, Eickmansworth, and Hatfield, roughly parallel 

 to which, and a few miles from it outward, are a number of outliers 

 (like skirmishers thrown out from the main body) ranged along a 

 line from the hills near Wargrave and Beaconsfield, through Chal- 

 font St, Giles, Sarratt, Abbot's Langley, St, Alban's, Digswell, 

 Datchworth, and Bennington, Again, inwards from the escarpment, 

 but also parallel to it and a few miles from it, there are a few inliers 

 along a line through Windsor, Pinner, and Northaw, The outliers 

 I look on as the relics of a former escarpment, and the inliers as the 

 signs of a future one. The outliers mark a line where denudation 

 has been delayed (I do not say stopped) ; the escarpment perhaps 

 one where it is now delayed ; and the inliers one where it will be 

 delayed (of course on the supposition that no great physical change 

 takes place), when the part between them and the present escarp- 

 ment will be cut off as outliers. Each of these lines is in great 

 part, I believe, through points where a slight change of dip takes 

 place, which may have in some measure enabled the beds better to 

 withstand denudation in the case of the outliers, or may have made 

 them fall an easier prey to it in the case of the inliers, there being 

 an inward dip in the former and an outward dip in the latter. 

 Further out in the Chalk district there are traces of another line of 

 outliers, better marked westward, along a line through Lane End 

 (near Wycombe), Turville Common, Nettlebed, and Woodcot Com- 

 mon (east of Goring). The inner line merges into that of the 

 escarpment near Reading, and further westward the outer line does 

 so too. I have noticed like arrangements in line in Kent, but none 

 so marked as the above, perhaps because the dip is generally less on 



