554 MR. A. STRAHAN ON OVERTHRUSTS OF [JNbv. I 895,. 



From this point northwards to the fault all the heds are turned over 

 20° to 30° hey on d vertically. The Isle of Purbeck fault cannot 

 be seen, but inverted and vertical Upper Chalk is succeeded north- 

 wards so abruptly by nearly horizontal Upper Chalk and Tertiary 

 sands that there must be a fracture of some kind. 



In approaching this promontory from the east, the observer passes 

 along the foot of a cliff of inverted Upper Chalk, a projection from 

 which, opposite Man-o'-~War Eock, shows the Corfe marl-bed ' and 

 just reaches the Upper Greensand. In the east side of this pro- 

 jection the Upper Chalk, almost at the horizon of the Chalk Rock, 

 is intersected by a nearly vertical crush-plane, bounded by sharp 

 but curving sides, and about 10 to 20 feet wide. This crush after 

 running along the foot of the cliff for about g mile, crosses the 

 Durdle promontory, and is then lost to view under the shingle, 

 keeping close to or at the horizon of the Chalk Eock for the whole 

 distance. The Chalk, everywhere more or less brecciated and 

 hardened, has been completely ground up and recemented within 

 this crush ; a black flint-nodule, for example, on the east of the 

 projection mentioned above, has been not only crushed to powder, 

 but drawn out into a black streak like coal-dust. Other flints 

 close by are broken but not disarranged, and some are not even 

 broken. The compression hereabouts has been so great that the 

 outcrop of the Portland Stone in the Man-o'-War Eock is only 

 about 270 feet from that of the Chalk Eock in the cliff, though 

 there should be about 1100 feet of strata between the two. It is 

 surprising under such circumstances to be able to recognize most 

 of the well-known subdivisions, even the Corfe marl-bed being 

 easily detected. 



The horizontal fractures to which I have alluded occur at fairly 

 regular intervals one above the other, on either side of the Durdle 

 promontory ; but the principal runs near the foot of the cliff on the 

 western side, its course being conspicuously marked out for upwards 

 of 3 50 yards by a line of caves. These slide-planes are not actually 

 horizontal, but rise northwards at a gentle angle, and in each one 

 the roof has been pushed up Over the floor for a distance varying from 

 4 to 12 feet or possibly more.' The result has been to increase the 

 inversion of the strata ; thus the Corfe marl-bed, in which three 

 of these sli de-planes can be detected on the western side of the pro- 

 montory, is carried some 20 yards farther by them than by inversion 

 alone. It will be seen by Section 3 (PI. XVIII.) that the upper 

 limb of the monocline was upheld on its northern side by little more 

 than soft Tertiary strata, whereas behind the lower limb lay the 

 whole thickness of the Chalk. The conditions would seem, there- 

 fore, to have been favourable for the production of an overthrust, as 

 soon as the inverted fold had come into existence. The small slide- 

 planes described may be regarded as the first stage in such an 

 overthrust, which might (if continued) have placed inverted Chalk 

 on the top of the Tertiary strata of Newlands Warren. 



1 A thick bed of marl, which occurs in the upper part of the Lower Chalk 

 in the Isle of Purbeck and for some miles westwards. 



