138 



CONTORTED STRATA AT ST. ACHEUL. 



CHAP. VIII, 



illustration in one of his published sections, deserves notice. 

 It consists in flexures and contortions of the strata of sand, 



FiK. 21 A. 





j--.^^ .. . — ^^^^^ ■■■,■1 



Contorted fluviatile strata at St. Acheul (Prestwich, Phil. Trans. 1861, p. 299). 



1 Surface soil. 



2 Brown loam as in fig. 21, p. 135, — thickness, six feet. 



3 White sand with bent and folded layers of marl, — thickness, six feet. 



4 Gravel, as in fig. 21, p. 135, with bones of mammalia and flint implements. 



A Graves filled with made ground and human hones. 



6 and c Seams of laminated marl often bent round upon themselves. 



d Beds of gravel with sharp curves. 



marl, and gravel (as seen at b, c, and d, fig. 21 a), "which 

 they have evidently undergone since their original deposition, 

 and from which both the underlying chalk and part of the 

 overlying beds of sand No. 3 are usually exempt. 



In my former writings I have attributed this kind of 

 derangement to two causes ; first, the pressure of ice running 

 aground on yielding banks of mud and sand; and, secondly, 

 the melting of masses of ice and snow of unequal thickness, 

 on which horizontal layers of mud, sand, and other fine and 

 coarse materials had accumulated. The late Mr. Trimmer 

 first pointed out in what manner the unequal failure of sup- 

 port caused by the liquefaction of underlying or intercalated 

 snow and ice might give rise to such complicated foldings.* 



* See Chapter XII. 



