110 REPORT— 1900. 



Within the area thus selected are various sharp monoclinal folds, all 

 with an east and west axis, and with the strata so bent as to become 

 nearly vertical. In places the lateral pressure and folding have been so 

 -violent as to pass into overthrust faulting on a considerable scale. None 

 •of the Tertiary disturbances in this part of England is a normal drop- 

 fault ; the supposed north and south Tertiary fault in the Medina valley, 

 though "often shown in old maps and text books, having no existence. 



The date of most violent disturbance in the system of folds above 

 ^alluded to is clearly later than Middle Oligocene ; for in the Isle of 

 Wight ijhe Hamstead Beds, which belong to that period, and are the 

 newest Tertiary strata there preserved, are tilted at a high angle. From 

 various considerations, which need not here be recapitulated, it seems 

 probable that this set of disturbances commenced in Eocene times, became 

 most violent in the Miocene period, and died away in Pliocene times.' 

 Though in our south-eastern counties older Pliocene strata to some extent 

 have been tilted, tlie disturbance has not yet been shown to affect newer 

 ■deposits, or to be still in progress. This last is one of the principal points 

 which our apparatus should decide. 



Consideration No. 1 limits our choice to a small group of faults, not 

 more than half-a-dozen, and as the apparatus employed needs a fairly 

 clean-cut fracture, unless the pipes are to be of unreasonable length, it is 

 ■ only at a few points on these faults that the observations can be made. 

 We have thus so greatly reduced the number of possible points at which 

 the apparatus could be fixed, that it will now be simplest to describe the 

 faults one by one, and point out to what extent they do or do not fulfil 

 the rest of the requirements. 



Working from east to west, the first Tertiary fault met with is in the 

 main monocline of the Isle of Wight, which occasionally passes into a 

 thrust-fault of no great extent. In one place the basement bed of the 

 London Clay is brought against Bracklesham Beds ; but the strata are too 

 soft and full of water to yield satisfactory fixed points. In the others, 

 plastic Clays of the Reading Series have slid over Chalk, the bedding being 

 vertical and the surface slope very high. At no point in the Isle of 

 Wight could a satisfactory site be found. 



Following this disturbed belt westward, we again meet with a sharp 

 monoclinal fold, passing into a slide-fault, at Ballard Cliff in Dorset. 

 This is the well-known ' Isle of Purbeck Fault,' which thrusts Chalk with 

 flints with curved bedding over similar rock with the bedding vertical. 

 The fault itself is very conspicuous in the cliff-face, curving through about 

 a tenth of a circle in a height of 280 feet.^ This fault might be a good 

 one for observation ; but though it is of considerable magnitude, the 

 locality is by no means convenient of access. The disturbance is, however, 

 a valuable one to study, for its character is clearly shown in the section. 

 The other faults with which we are now dealing apparently are all of this 

 type. 



The next Tertiary fault met with is close to Corfe Castle, where in the 

 sharpest part of tlie monoclinal curve the London Clay has been thrust 

 over the Heading Beds and abuts against the Upper Chalk. This slide- 

 fault is of small magnitude, and as in similar slides in the Isle of Wight, 



' Keid and Strahan, ' Geology of the Tsle of Wight,' chapter xiv. Memoirs of the 

 Geological Survey, 1889; Reid, 'Pliocene Deposits of Britain,' chapter v. ibid. 1890. 

 • ^ See Strahan, ' Geology of the Isle of Purbeck,' chapter xv. 3Iem. Geol. Sv/rvey, 

 1898. 



