108 F. L. Kitcliin and J. Pringle — 



We may suggest that the coherence of the clays and sands would 

 be increased by freezing, and that the transported sheet of strata 

 may have been moved very slowly into its new position while held 

 firmly in the ice-stream, and deposited upon snow or snow-covered 

 ice. The subsequent melting would |)roceed so gradually as to 

 let down the mass on to its new substratum with a minimum of 

 disturbance. It may have been at that time that the limestone- 

 lenticles of the inverted upper surface of the boulder became forced 

 down by pressure into the yielding material of the floor. 



While no known agency except the movement of an ice-sheet 

 is adequate to have produced the result now seen, a consideration 

 of the exact modus operatidi may be left to those who have had more 

 experience in the study of glacial phenomena. Sir Archibald Geikie 

 has commented on the difficulty of conceiving how extensive thin 

 sheets of soft strata can have been ploughed out of their place and 

 pushed forward.^ 



This striking example of glacial transportation falls into line- 

 with the similar instances along the tract of country to the north- 

 east. There is the well-known mass of Cretaceous strata at Roslyn 

 Hill Pit, Ely, discussed in papers by the late Osmond Fisher and by 

 Professor T. G. Bonney ; ^ while at Biggleswade, near Bedford, 

 a mass of Ampthill Clay nearly 70 feet thick has been found incor- 

 jDorated in the Boulder-clay .° Numerous other examples of the 

 transportation of large masses or " cakes " of Jurassic and 

 Cretaceous rocks in the Midland area have been recorded by various 

 writers.^ We may also refer to the boulder of Cretaceous strata 

 240 yards in length at Leavad, in Caithness.® 



It is impossible to say how far glacial erosion, subsequently to 

 the removal of the inverted strata to their present position, may have 

 reduced the thickness of the mass. It may be remembered, however, 

 that the fauna found at the present top of the reversed beds of Gault 

 shows that the whole of the Upper Gault in situ at Shenley Hilt 

 can have been little more than 40 feet in thickness. Hence it appears- 

 certain that in relation to its area, the whole mass must have been 

 thin from the time of its detachment. Again, we prefer to leave- 

 a discussion of this purely glacial aspect of the problem to those who 

 are better qualified to offer an opinion. 



' In a discussion on Mr. Home's paper, cited below, p. 381. 



2 0. Fisher, "On Roslyn or Roswell Hill Clay-pit near Ely " : Geol. Mag.,. 

 1868, p. 407. T. G. Boimey, " Notes on the Roslyn Hill Clay-pit " : Geol. 

 Mag., 1872, p. 403. Also Cambridgeshire Geology, 1875, p. 69. 



^ H. Home, " On a transported mass of Ampthill Clay in the Boulder- clay 

 at Biggleswade, Bedfordshire " : Quart. Journ. Geol. Soc, vol. lix, 1903, p. 375. 



* For references to some striking cases see H. B. Woodward, op. cit., 1897, 

 p. 495. 



5 R. G. Carruthers, " On the occurrence of a Cretaceous Boulder of unusual 

 size at Leavad, in Caithness " : Summary of Progress for 1910 (Mem. Geol. 

 Surv.), 1911, p. 80. D. Tait, " On a large glacially transported mass of Lower 

 Cretaceous Rock at Leavad, in the Coimty of Caithness " : Trans. Edin. 

 Geol. Soc, vol. X, pt. i, 1911. 



