Iviii. WEST PURBECK MEETING. 



original construction of this monticle. The second point is 

 perhaps more important still. You remember what I said about 

 our proximity to the great thrust-plane which forms the back- 

 bone of the whole of the Isle of Purbeck. It is represented in 

 the geological map as passing along the junction of the Chalk 

 and the Tertiaries about 300 yards to the south of our present 

 position. It is quite possible that, instead of sticking to that 

 route, the line was deflected somewhat, so that the thrust- 

 plane has come in among the Tertiary clays, and muddled them 

 up with the sand and everything else in a most extraordinary 

 manner. The great thrust from the north has pressed these 

 clays against the hard chalk and doubled them up, making them 

 harder and fitter to withstand the forces of denudation. 

 Certainly, the clay workings give evidence which justifies that 

 view. The late Mr. Lawrence Pike, whose premature death we 

 all lament, told me that the valuable clay beds which he worked 

 for so many years often stand on end quite vertical and some- 

 times overthrust, so that a number of "S" curves can be 

 detected. That could not have been found out without the 

 workings in the clay, for it is impossible in these soft beds to 

 judge from the mere surface features what is underneath. 

 Therefore, these workings have given us most valuable insight 

 into the internal structure of this hill and its immediate 

 surroundings. And, as giving you some idea of how beds are 

 disturbed, we have already seen that the chalk dips on the 

 north at an angle of 80 degrees ; whilst the bed of pipeclay 

 in the Creech Woods beneath us is at present being mined on a 

 dip of 30 degrees to the south. That shows what change of dip 

 there has been in this neighbourhood. Of the third point there 

 is less evidence, though it is the most important of all. That is 

 the point where my enemy will probably attack me. All the 

 dwellers in the Bagshot district, whether in Dorset, Hampshire, 

 or Surrey, are well aware that one of the chief causes of high 

 ground in these areas is a good capping of plateau gravel. It 

 withstands most successfully the action of denudation, so that 

 what was originally a valley, like the great plateau at Binnegar, 



