190 TRAN8ACTI0NS OP SECTION C. 



WEDNESDAY, SEPT EMU MM 10. 

 The following Papers were read : 



1. The Meso<zoic Rochs of the Bou-riieiiiuuth District. 

 By Sir Aubrey Steahan, K.B.E., F.E.S. 



Th ' Isle ' of Purbeck includes a part of the healthy tract underlain 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 esi>ecially farming 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 pro- 

 longed 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, however, 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 sharply-defined 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. Every- 

 where along the line of the Isle of Purbeck Fault the chalk is greatly hardened, 

 while the flints are broken, pulverised, and even drawn out into streaks of 

 flint-powder. The Isle of Purbeck Fault dies out under Wevmouth Bay, but 

 is replaced a mile or two to the north by the parallel and still more energetic 

 Ridgeway overthrust. 



As regards the regions which it is proposed to visit, in the neighbourhood 

 of Swanage the whole sequence from the base of upper chalk to the Portland 

 Stone is open to examination, but time will not admit of more than a brief 

 inspection of the Purbeck arrd Portland cliff-sections. The Upper Purbeck with 

 Paludina limestones (or ' marble ' beds) and Unio beds form Peveril Point, and 

 the Middle and Lower Purbeck beds are shown more or less cont'nuously in 

 Durlston Bay, a band composed of shells of Ostrea disforta (the ' Cinder Bed ') 

 forming an easilv recognised horizon. About 30 feet below the ' Cinder Bed ' 

 lies the Mammal Bed, a thin, earthy layer which has yielded the remains of 

 sereral genera of marsupials. Below this again are the Lower Purbeck lime- 

 stones and marls, some with gvpsum, casts of crystals of rock-salt and insect 

 remains, others yielding a brackish water estnarine fauna. A double fault, 

 with a downthrow south of 100 feet, near the zig-zag path, throws the Cinder 



