THE 



GEOLOGICAL MAGAZINE 



NEW SERIES. DECADE III. VOL. Vlil. 



No. VIII.— AUGUST, 1891. 



oiaica-xisTJ^L ^AuieTiaXiES. 



I. — On the Sands and Gkavels Intercalated in the 

 Boulder-clay. 



By G. W. BuLMAN, M.A., B.Sc, Corbridge-on-Tyne. 



THE interpretation of the sands and gravels intercalated in the 

 glacial drift is one of the most interesting problems in glacial 

 geology. Do they, on the one hand, represent the deposits of one 

 or more mild intervals alternating with periods of intense cold ; or 

 were they, on the other, laid down during one continuous cold, 

 period marked by such slight oscillations as can be shown to occur 

 in connection with existing glaciers and ice-sheets ? 



The former view has been ably advocated by geologists of repute 

 — notably by Prof. J. Geikie — and implies the belief in one or more 

 intervals of mild climate, each comparable in duration to the cold, 

 period which it followed and. preceded. It is founded partly on 

 the idea that the sands and gravels are due to such aqueous action 

 as would indicate a permanent retreat of the ice ; and partly on 

 some few mammalian and other remains found in them. 



Prof. Geikie expresses his opinion as follows : — 



1. " The till itself is a truly glacial deposit, due to the grinding 

 action over the surface of the country of immense masses of glacier 

 ice. But no one will doubt that its intercalated and subjacent beds of 

 silt, sand, and gravel have had a very different origin. They occur 

 in such layers as could only have been spread out by the action of 

 running water." ^ 



2. " We have found that there is abundant proof to show that the 

 accumulation of a moraine profonde by one great ice-sheet was inter- 

 rupted several times ; that the ice-sheet vanished from the low 

 grounds, and even from many of the upland valleys, and that 

 rivers and lakes then appeared where before all had been ice and 

 snow. We have also learned that during such mild interglacial 

 periods, Oxen, Deer, Horses, Mammoths, Eeindeer, and no doubt 

 other animals besides these occupied the land."^ 



But, admitting such evidence, it is obvious that there must have 

 been not one but several such mild interglacial periods. And so 

 we find Prof. Geikie stating, in his address to the Geological Section 

 of the British Association at Newcastle in 1889, that, " In some 

 places three or more such boulder-clays have been observed over- 

 1 " Great Ice Age," p. 165. " Ditto, p. 204. 



DECADE III. — VOL. VIII. — ^NO. Till. 22 



