7 %■ 



EEOSION OF THE GLACIAL DRIET. 103 



stream at a time when the sea was so far away as to exercise no 

 influence due to the rise and fall of the tide. On the coasts of 

 Lancashire and Cheshire the encroaching sea has washed out the 

 previously submerged Peat- and Forest-beds. These submarine 

 forests occur in situ, rooted in solid Boulder Clay ; like forests 

 cannot now, however, thrive so near the sea. See also the Eev. W. 

 B. Clarke's paper, Proc. Geol. Soc. vol. ii. (1838) p. 599. 



With regard to the Axmouth landslip, referred to by Mr. H. B. 

 Woodward in the discussion, it was attributed to the action of 

 *' numerous springs issuing from the loose sand" (Upper Green- 

 sand), which had " gradually removed portions of it." ^ According 

 to the principle of subterranean erosion, this landslip was caused by 

 the movement, particle by particle, of the whole mass of quick- 

 sand laterally towards the nearest points of escape. Such so-called 

 " springs " are an effect — not a cause — of subterranean erosion. — 

 January 14th, 1892.] 



DlSCTJSSIOJf. 



Prof. Hull was disposed to accept Mr. Shone's views regarding 

 the changes of level of the Upper Boulder Clay in the district 

 referred to, and observed tbat a similar view had long since been 

 entertained to account for the submergence of the peat deposits on 

 the coast of the Wirral promontory. 



The Bev. Edwiist Hill added that the sinking of surface-clay 

 from such erosion of sand beneath would accelerate denudation of 

 the surface of the clay. But in beds deposited on an uneven sub- 

 surface, simple shrinking would produce a similarly uneven position. 



Mr. H. B. WooDWAED mentioned that Conybeare had referred to 

 similar undermining of sandy strata as a chief cause of the Great 

 Landslip at Lyme Begis. He asked the Author if he had con- 

 sidered the fact that Boulder Clay rests on an irregular surface of 

 the underlying strata, whether sands or clays, and often plunges 

 abruptly into hollows excavated during its accumulation. 



Mr. Shone, in reply, stated that he thought the term " subterra- 

 nean erosion " was perhaps preferable to " subterranean denudation." 



He was glad the Society had received his conclusions so favour- 

 ably. He maintained, and had endeavoured to prove, that wherever 

 water percolated through such unconsolidated beds as clays, sands, 

 and gravels along an inclined plane, it was constantly carrying the 

 lighter materials of such strata towards the nearest point of escape. 

 The nearer the approach to the point of escape, the greater became 

 the power of subterranean erosion. This was more especially the 

 case where clay rested upon sand and gravel. He believed it was 

 the cause of that lateral subsidence so characteristic of submerged 

 post-Glacial Peat and Forest-beds. Landslips were the extreme 

 rather than the ordinary effects of subterranean erosion. He also 

 remarked that the subterranean removal of sands and gravels 

 frequently resulted in causing the appearance of contorted Drift. 



1 Lyell, ' Principles,' 12th ed. (1875) vol. i. p. 541. 

 Q. J. G. S. No. 190. I 



