426 TERTIARY. 



in the Hampshire district, which includes the Tertiary beds of 

 Dorset, Hants, the Isle of Wight, and Sussex. In the London 

 Basin the seaward extension of the strata on the east is unknown, 

 but wherever observed the beds rest upon a foundation of 

 Chalk. The formation of these so-called Basins is due to dis- 

 turbance and denudation subsequent to the Eocene period, whereby 

 the former continuity of the strata has been destroyed. The chief 

 anticlinals affecting the area are that of Weymouth, the Isle of 

 Purbeck and Isle of Wight; that of Portsdown and Highdown, 

 near Chichester; that of the Vale of Wardour and Winchester; 

 the Wealden anticlinal, and that of Pewsey and Kingsclere. 

 These disturbances were produced at an epoch during which the 

 north-western parts of the British area were subject to great 

 volcanic activity.^ 



Although there is nowhere any marked unconformity in stratifi- 

 cation between the Chalk and overlying Eocene strata, yet the 

 change from deep-sea conditions to those of comparatively 

 shallow-water is abrupt ; and when we consider that in Denmark 

 and Holland there are passage-beds between the Cretaceous and 

 Tertiary formations which are not represented in England, the 

 natural conclusion is that in our country these two great groups 

 of rocks are unconformable. The level surface of the Chalk 

 beneath the Tertiary beds may, as Dr. John Evans has suggested, 

 be a plain of marine denudation ; ^ and no doubt the Chalk was 

 gradually upheaved in places above the sea-level, so as to form 

 clifis, for the earlier Tertiary shingles are almost entirely made up 

 of flint. At the same time in certain areas the Eocene sediments 

 were laid down on an undisturbed foundation; and thus, as Mr. 

 Whitaker has remarked, in the Isle of Thanet the Tertiary beds 

 appear to be conformable to the Chalk. ^ 



The irregular hollows or "pipes," due to the dissolution of the 

 Chalk by subaerial agents, are features that have in many instances 

 been produced on the surface of the Chalk, after the deposition of 

 the newer strata. These "pipes" occur where the surface of the 

 Chalk is easily reached by water, and they are absent or rare where 

 the Chalk is covered by impervious strata; hence " pipes" beneath 

 a clayey stratum of considerable thickness indicate unconformity. 



The former extent of the Eocene strata is not known ; the lowest 

 member (Thanet Sands) is overlapped towards the west by the 

 Woolwich and Reading Beds, and although there is no evidence 

 that the London Clay ever was deposited immediately on the 

 Chalk, yet Mr. John Gunn thinks it highly probable that the 

 London Clay did overlap the older Eocene strata in Norfolk, and 

 Mr. J. S. Gardner has suggested a similar overlap in Dorsetshire. 



The Tertiary strata were mapped for the Geological Survey in the southern 

 counties chiefly by H. W. Bristow, Joshua Trimmer, W. Whitaker, and T. R. 

 Polwhele ; and in the eastern counties by W. Whitaker, W. H. Penning, F. J. 

 Bennett, W. H. Dalton, C. Reid, and the writer. 



^ See J. S. Gardner, Proc. Royal Soc. xxxviii. 14; and Prestwich, Q.J. ii. 252. 

 - P. Geol. Assoc, v. 499. ^ Q. J. xxi. 397, 405. 



