74 Dr. James GeiJcie — Preservation of Dejoosits under 'Till.'' 



How could such, loose stuff as gravel, sand, and silt withstand the 

 grinding action of a superincumbent ice-sheet? And how, if these 

 deposits have been thus traversed by ice, can we reconcile that with 

 our belief in the excavating power of an ice-sheet ? This, indeed, is 

 the principal objection urged by Mr, S. V. Wood and others who 

 deny altogether that Till or Boulder-clay has been amassed below 

 ice. They deem it quite impossible that an ice-sheet could pass over 

 such beds as I have referred to without obliterating them ; and having 

 assumed so much (for, of course, it is solely upon their preconceived 

 ideas as to how a great ice-sheet would behave, that their objection 

 is based), they find no difficulty in concluding that Till could not 

 have been accumulated under ice, and that the occurrence of Till 

 resting upon beds of gravel, sand, and clay, is no proof whatever 

 of the former passage of an ice-sheet. In this short paper, however, 

 I do not mean to enter upon the question of the mode of formation 

 of Till. Quite enough ink has already been shed upon the subject. 

 Every day supplies me with fresh evidence in favour of the view 

 which I sup]Dort — namely, that Till has not only been formed but 

 also accumulated under an ice-sheet ; and I am not aware of any 

 phenomena which this theory of the origin of Till fails to explain, 

 although very many facts are known to me which are quite irrecon- 

 cilable with any other view. My object at present, however, is not 

 to point out the inadequacy of other theories and hypotheses — in 

 doing which there would necessarily be much repetition of many 

 unanswered arguments — but simply to show that a strong and well- 

 grounded belief in the erosive power of ice is quite consistent with 

 an equally firm conviction that in many cases thick masses of ice 

 have crept over beds of gravel, sand, silt, clay, and peat. The one 

 persuasion is no more contradictory to the other, than a serene confi- 

 dence in the denuding powers of running water is opposed to the 

 belief that the same water also deposits and accumulates detritus 

 while it flows. One may surely hold that a certain deep ravine or 

 glen, in the upper part of a river- valley, has been excavated in the 

 course of ages by the stream one sees at the bottom, and at the same 

 time assert, without the fear of being considered self-contradictory, 

 that the broad alluvia (overlying, it may be, incoherent marine 

 strata), in the lower and more open reaches of the valley, have been 

 deposited by the very same river that dug out the deep ravine above. 

 What I and others, who hold with me, maintain is siniply this : first, 

 that, in regions where the erosive action of the ice-sheet was great, 

 little or no Till was allowed to gather, and rock-basins were scooped 

 out, when the nature of the ground was favourable to that end ; and 

 secondly, that, in places where the grinding-power exerted was less, 

 thick Till frequently accumulated, and subglacial and interglacial 

 beds were often preserved. 



Our ice-sheet flowed, we cannot doubt, with a differential motion : 

 it must have moved faster in some places than in others. In steep 

 valleys and over a hilly country its course would often be compara- 

 tively rapid, but very irregular — lagging here, flowing quickly there 

 — while in wide, open valleys that sloped gently to the sea, such for 



