144 A. p. COLEMAN MARINE AND FRESHWATER BEACHES 



The processes of iee retreat and differential elevation must not be 

 thouglit of as uniform in rate or even as continuous at a var3'ing rate. 

 Probabl}^ the ice-front oscillated, as we know is the case with modern 

 glaciers, and one lobe may even have advanced while others retreated, 

 as some Alpine glaciers are shown to have done in the last century. 



As regards differential elevation less is known, but the numerous ma- 

 rine terraces of the lower Saint Lawrence may imply an intermittent 

 elevation, each terrace indicating a halt or a slowing up of the motion. 

 On the other hand, many of the terraces on the north shore of lake 

 Superior and on Michipicoten island show beach ridge after beach ridge 

 with no sharp line between, often running almost continuousl}^ upward 

 as one goes inhmd and covering a range of from 50 to 100 feet. In fact, 

 if one talnilates the various beaches one finds that at one point or 

 another there are shorelines at every level from the present lake to 475 

 feet above it, with no ga}) greater than 10 feet between the successive 

 steps.* Above this, however, the intervals become greater and more 

 irregular. 



We know little of these earth movements, ])ut it is possible that they 

 begin haltingly, grow more rapid and uniform, and then slow down 

 irregularly as the brakes are applied. Following the common opinion 

 that the solid crust or lithos{)here resting on the layer of plasticity or 

 tektosphere, to use Murray's terms, does not yield instantly to the change 

 of conditions, there ma}^ be comparatively sudden accommodations 

 when the strain goes beyond endurance, and then periods of relative 

 quiescence. 



If the theory held by Mr Warren Upham and others is correct that 

 the oscillations of level are produced b}' the loading down of the region 

 with ice, jn'oducing subsidence, and then the removal of the load by 

 the melting of the ice, allowing it to rise again, there should be a rough 

 correspondence between the thickness of the ice-sheet and the level of 

 the land, though we should expect the stage of elevation to be always 

 in arrears, perhaps even to the extent of thousands of years. Doctor 

 Gilbert's researches appear to show that the region is still being tilted 

 up toward the northeast, though the Great lakes are supposed to have 

 been free from ice ever since Niagara falls began its work. 



The mechanism by which these changes of level are produced is, of 

 course, obscure, but it ma}^ be supposed that as the region becomes 

 weighted down the ]^lastic la3'er some miles below the surface yields to 

 the pressure and moves sluggishly outward in all directions, and after 

 relief from pressure creeps back to the area that has been lightened. 



*()n(. Hiir. Mini's, 189!), ij}t. If."), I'jG. 



