ANCIENT EARTHWORKS 



ture of this pipe, it was withdrawn and another substituted. This was driven down to the 

 depth of 84 ft. 6 in., when the point was stopped by some hard mass which could not be pene- 

 trated, and the pipe had to be withdrawn, a task which was effected only with the greatest 

 difficulty, though the first pipe had been withdrawn with ease, and the second had been driven 

 down, as nearly as possible, in the track of the first. Then, as the money spent had somewhat 

 exceeded the amount subscribed, it was resolved to withdraw as much of the timber as might 

 be practicable, and to fill up the hole. 



The Exploration Committee, as a whole, were not able to accept any view as to the causes 

 of this subsidence as conclusive. The present writer, who was a member of the Committee, 

 was permitted, however, to append to the Report some observations pointing out that the 

 clayey beds of the Woolwich series (which were the source of the main difficulty during the 

 exploration), by concentrating the water falling as rain on the surface of Blackheath towards 

 the base of the pebble beds, would prevent it from acting on the chalk beneath. This fact 

 would consequently make any explanation of the subsidence as the result of the natural action 

 of water on the chalk untenable. But that, granting the existence of a shaft ending in a 

 chamber in the chalk, under Blackheath, similar to that at Eltham Park, the result of the 

 action of the water at the base of the Blackheath Pebble Beds on a neglected and disused shaft 

 would be to cause an enlargement there which would ultimately produce a subsidence result- 

 ing in a hole at the surface such as had appeared (see figs. A,B, C, fig. 10). And (to quote the 

 Report) ' Professor Prestwich pronounces, with confidence, that so far as he can judge, the 

 cause of the subsidence is not geological : Mr. Whitaker leans to the same opinion.' ' 



BlaeKhtorti\i 

 ftttle Seatty 



m 



Sand^Z 



riyc. 



Fig. 10. Diagram Sections showing necessary Results of Long Disuse on a Shaft and 

 Chamber at Blackheath. 



During the present year (1906) a tunnel, in connexion with the Main Drainage works 

 of the London County Council, has been in process of formation in the chalk under Black- 

 heath, and shafts have been sunk at intervals along its course. I noticed that two of these 

 shafts, one north of the road from the south-western corner of Greenwich Park to Morden 

 College, the other on the southern side of the Shooters Hill Road, were in a line with the 

 subsidence which appeared at the surface on 12 April, 1878. Much interesting information 

 was kindly given me by Mr. B. C. Cass, a member of the firm of Messrs. S. Pearson and Sons, the 

 contractors, about the workings under Blackheath. I learned from him that the water found 

 at the base of the pebble beds forming the surface caused no serious difficulty when the shafts 

 were being sunk ; and that, under the spot at which the above-mentioned subsidence occurred, 

 the chalk, at a depth of about 120 ft. from the surface, was found to be broken up and mixed 

 with soil and other material from beds nearer the surface. Though this discovery is evidently 

 important as indicating at this spot a special artificial connexion between the chalk and 

 the surface, it seems at first somewhat disappointing to hear only of a confusion of rocks where 

 a distinct chamber in the chalk, like that at Eltham, might be expected. But it must be 

 remembered that the Eltham shaft and chamber were accidentally discovered after they had 



• Thus I may claim that the best geological authorities were against a merely geological explanation, 

 and so far, at least, in favour of my view. On the other hand, up to 1881, deneholes had been ignored 

 by everybody but Mr. Spurrell. I feel accordingly that it would not be justifiable, in an account of 

 the deneholes of Kent, to omit the subsidences at Blackheath, and their probable origin. 



453 



