126 Dr. BucKLAND on the Formation of Valleys hy Elevation, 



these basins, whilst its surface is diversified by undulations of valleys and hills, 

 the latter varying from 600 to 1000 feet above the level of the sea. 



Had the tertiary strata of our two basins been ever united together in one, it 

 were to be expected that some traces of such union would remain in the space 

 that now divides them ; and such traces we actually find on those portions of this 

 district that constitute even the highest summits of the chalk-formation that 

 exist in England, viz. on the top of Combe Hill, a little east of Inkpen Hill near 

 Highclere ; on Chidbury Hill and Beacon Hill, that form two of the highest 

 summits of Salisbury Plain between Amesbury and Everleigh ; on the top of 

 Clay -pit Hill on Chiltern Down, near Heytesbury ; and at still more distant 

 points, on the lofty summits of Blackdown Hill between Dorchester and Ab- 

 botsbury in Dorset, and of Nettlebed and Stokenchurch in Oxfordshire. 



At all these elevated points, the identity of character which the strata on 

 these detached hills and ridges uniformly possess with that of the plastic clay 

 formation in the basins of London and the New Forest, affords abundant 

 ground for concluding that these two basins were originally united together in 

 one continuous deposit across the now intervening chalk of Salisbury Plain 

 in Wilts, and the plains of Andover and Basingstoke in Hants, and that the 

 greater integrity in which the tertiary strata are preserved within the basins, 

 has resulted from the protection which their comparatively low position has 

 afforded them from the ravages of diluvial denudation. 



The tender and destructible nature of the sand, clay and gravel composing 

 this formation, would render them peculiarly liable to be swept away by 

 the transit of violent currents of water ; and the wreck of the harder por- 

 tions of sandy strata thus destroyed, is sufficiently evident in the enormous 

 blocks of sandstone which not only occur in Wilts, in numbers so great as to 

 lie like a flock of sheep in the valleys near Hungerford, and thence derive the 

 local name of gray wethers, but are in more or less abundance co-extensive 

 with the entire surface of the chalk, from the Wolds of Yorkshire to the Hills 

 of Sidmouth and Blackdown in Devonshire. 



Their abundance at Clatford Bottom near Marlborough, between the Dru- 

 idical temples of Abury and Stonehenge, whose materials they have supplied, is 

 mentioned in my "Reliquiae Diluvianae." Windsor Castle is built of a similar 

 stone, found in insulated blocks on Bagshot Heath. Another example of their 

 occurrence in vast numbers on the naked surface of the chalk, at Ashdown 

 Park on the south of Swindon, has been described by Dr. Kidd. A similar 

 case occurs on the slope and summit of the chalk hill which overhangs the 

 village of Portisham near Abbotsbury in Dorset : blocks of the same kind 

 abound also near Sidmouth. In all these cases many of the blocks still remain 



