32 Reviews — Late Pleistocene Oscillations 



and numerous maps and diagrams. The Scottish Survey geologists 

 are to be congratulated on the memorable results of a long, patient, 

 and intricate piece of work. 



G. W. T. 



III. — Late Pleistocene Oscillations of Sea-level in the Ottawa 

 Vallet. By W. A. Johnston. Geological Survey of Canada, 

 Museum Bulletin No. 24; 1916. 



ALTHOUGH the attention of British geologists is at the present 

 _ time mainly directed to economic problems, it is nevertheless 

 very desirable that a paper of such scientific importance as that 

 about to be discussed should be brought to their notice. The author 

 has made a notable contribution to the study of late-glacial changes 

 of sea-level, and the facts he has put on record might almost be said 

 to constitute a complete demonstration of the applicability of the 

 theory of isostasy to these changes. 



It will be recalled that this theory ascribes the raised or tilted 

 shorelines which are found around the centres of glacial dispersal to 

 the sinking in of the earth's crust beneath the pressure of the ice- 

 sheets, and its subsequent recovery when the ice has melted away. 

 The depression and recovery were greatest at the centres of dispersal 

 where the ice was thickest, with the consequence that the shorelines 

 are highest near these centres and descend gradually towards the 

 margins of the glaciated districts. Before they reach these margins, 

 however, they invariably pass beneath the present sea-level. There 

 are no late-glacial raised beaches in the peripheral parts of the 

 glaciated districts, the shorelines which were formed during the 

 retreat of the ice from these areas being all beneath the present sea- 

 level. This relation indicates very clearly that the general sea-level 

 must have been considerably lower during the earlier stages of retreat 

 than at the present day, and the same conclusion can be arrived at 

 on a priori grounds by considering the effect on the ocean-level of 

 the binding up of enormous quantities of water in the ice-sheets. 



We have, therefore, in seeking for an explanation of the late-glacial 

 changes in the relative level of land and sea, two factors to deal with. 

 The first is the isostatic recovery of the earth's crust, the second is 

 the general raising of level of the ocean due to the melting of 

 the ice-sheets. According as the first or second of these factors 

 predominated there occurred either emergence or submergence in the 

 isostatically affected areas. 



This appears to be the explanation of the curious fact established 

 by Brogger in the Christiania region, that the first change of level 

 after the retreat of the ice was one of submergence, which, at 

 a somewhat later stage of retreat, gave place to emergence. That 

 this is the course of events to be expected from the interplay of the 

 two factors mentioned is apparent from the following considerations. 



1. At the period of deposition of the earlier late-glacial marine 

 deposits from which Brogger drew his conclusions, about one-third to 

 one-half of the total retreat of the ice-margin had been accomplished; 

 and it is roughly at this stage of retreat, when the climate had 

 already considerably ameliorated, and there was at the same time 



