268 J. E. Marr— Glacial Deposits. 



contortion was actually produced in the Sudbury area, and con- 

 sequently we are led to ask the second question, why are not the 

 incoherent beds below also disturbed ? 



Four suggestions have been made to account for the contortion of 

 drift deposits : 



(i.) The pressure of material from an adjoining elevation, either 

 rock or ice. 



(ii.) The removal of soluble material, as chalk or ice, from the drift. 



(iii.) The grounding of true or false icebergs. 



(iv.) The pressure of superincumbent land-ice. 



The first explanation is inapplicable here. Not only do contortions 

 occur in the drifts which occupy the valley bottoms, but they are 

 also found in the accumulations which lie on the summits of ridges, 

 as at Mr. Green's pit, and the pits near the Cemetery. As the 

 summits of these ridges are covered with these drifts, it is clear that 

 no higher land occurred here in Glacial times. 



The second cause seems inadequate to produce the intense con- 

 tortion observed in many of the pits. The amount of chalk removed 

 cannot be very great, for the drifts are still very calcareous. A great 

 quantity of interstratified ice may have disappeared, but would this 

 produce such violent folds as that seen at the southern end of Mr. 

 Green's pit ? Overfolds of this nature are produced by lateral 

 pressure, but it is difficult to see how the removal of interstratified 

 material could cause the folding of one deposit nearly horizontally 

 over another for distances of several yards. 



In both of these cases, we have no explanation of the undoubted 

 fact that the drifts are largely derived from the underlying rocks. 

 The third explanation requires fuller consideration. Local rocks 

 might undoubtedly become churned up and included in the drifts by 

 the grounding of icebergs, and no doubt contortion can be caused 

 thereby, though we should also expect partial obliteration of the 

 lamination planes. Several difficulties confront us if we adopt this 

 explanation. Firstly, the derivation of a great part of the material 

 from local sources. If this material was denuded by icebergs, 

 whence comes its lamination ? If, on the other hand, the deposits 

 are ordinary marine accumulations, how are we to account for the 

 absence of fossils ? The loam, which is admirably adapted for 

 the preservation of organisms, contains patches of lignitic material, 

 which are apparently boulders derived from some earlier forest bed, 

 but no marine organisms. Of course, the sea might be too cold 

 for the existence of Molluscs, but the bones of Seals ought to occur. 

 The Greenlanders thaw out the seals which have been frozen in 

 winter whilst diving below the ice, and other seals are taken in 

 nets below the ice (K. J. V. Steenstrup, " Meddelelser om Grouland," 

 part iv. p. 99). The great difficulty in the way of accepting the 

 view that the drifts of this area were contorted by icebergs is the 

 fact that the incoherent Tertiary rochs immediately underneath the 

 violently folded Glacial deposits are never contorted even when standing 

 up in ridges (such as that in Mr. Green's pit), around which the drifts 

 have been folded. Ifc seems absolutely impossible that they should 



