Sir Aubrey Strahan — Geology of the Isle of Purbeck. 533 



II. — The Geology of the Isle of Purbeck. 



Abstract of a lecture delivered to Section C, British Association, Bournemouth, 

 by Sir Aubrey Strahan, K.B.E., Sc.D., F.K.S. 



rpHE " Isle" of Purbeck includes part of a heathy tract underlain 

 jL by the Tertiary beds of the Hampshire Basin, a central ridge 

 formed by the Chalk which rises abruptly from beneath those beds, 

 and, in its southern part, a hilly region underlain by Wealden, 

 Purbeck, Portland, and Kimmeridge strata, and terminated by bold 

 cliffs. Each formation gives rise to characteristic features in the 

 landscape, the Portland Stone especially forming a dominant 

 escarpment and vertical sea-cliffs. 



The emergence of the Chalk and underlying formations from 

 beneath the Tertiary beds is due to an extremely sharp fold 

 accompanied by overthrusting. The age of the movement is proved 

 in the Isle of Wight to have been post-Oligocene, inasmuch as 

 the Oligocene strata are there involved in it. On the other hand it 

 was accomplished, and the uplifted strata were exposed to prolonged 

 denudation, in pre-Pliocene times. The sagging of the strata which 

 led to the formation of the Hampshire and London Basins and the 

 arching-up of the intervening Wealden anticline are attributable to 

 the same period and to the same earth-movement. So energetic 

 a movement, coming into activity at so late a geological age, had 

 a profound influence upon the physical geography of the south-east 

 of England. The principal rivers, the Thames and Frome, each 

 followed a syncline eastwards. On either side they received 

 tributaries which rose upon the anticlines. The anticlines, how- 

 ever, have suffered severe denudation and no longer maintain their 

 dominance of elevation, but the rivers have kept their courses, and 

 now cross in narrow defiles the Chalk ridges which formed the 

 foundations of the once continuous Chalk arch. Admirable examples 

 of such defiles are shown at Corfe Castle. 



The curve of the strata in the Isle of Purbeck may be compared 

 to the figure 2. The lower limb of the 2 represents the horizontal 

 beds of the Hampshire Basin, the middle limb shows the strata in 

 a vertical or inverted position, while the upper limb illustrates the 

 gentle curve by which they regain a more normal position. The 

 strain, however, was too great to be relieved by folding alone, and 

 overthrusting on a considerable scale came into play. The cliff- 

 section of Ballard Down shows curving strata which belonged to the 

 lower limb of the 2 resting upon the edges of vertical strata which 

 belonged to the middle limb, a shai'ply denned slide-plane (the 

 Isle of Purbeck fault) separating the two. Westwards from 

 Lulworth Cove innumerable subsidiary thrust-planes can be detected 

 in the Chalk, and less easily in the Wealden and Purbeck beds. 

 Everywhere along the line of the Isle of Purbeck fault the Chalk is 

 greatly hardened, while the flints are broken, pulverized, and even 

 drawn out into streaks of flint powder. The Isle of Purbeck fault 

 dies out under Weymouth Bay, but is replaced a mile or two to the 

 north by the parallel and still more energetic Bidgeway overthrust. 



As regards the regions which it was proposed to visit, in the 



