320 Notices of Memoirs — W. 0. Croshj — 



as the cold increased, deploj'ing upon the plains and forming sluggish 

 or stagnant Piedmont glaciers. With progressive refrigeration the 

 annual snowfall finally exceeds the annual melting of the ice. This 

 cumulative snowfall, which mantled alike hills and valleys, and 

 changed slowly through neve to glacial ice, must have tended in 

 some measure to check or arrest the motion of the ice which had 

 flowed outward from the mountains. Owing, however, to the 

 forward motion of the Piedmont glaciers, as well as to their 

 termination on tracts where a short time before ablation (or melting) 

 had been in excess of snowfall, they must have terminated some- 

 what abruptly or with high marginal gradients ; and the conditions, 

 therefore, were extremely favourable for their overriding the new 

 and still stationary ice-fields by which they were invested. The 

 overridden tract of ice must slowly acquire the motion of the 

 overriding sheet, and thus in its turn come to override other 

 tracts. In fact, it seems very probable that this process of over- 

 riding and absorption would continue almost indefinitely, extending, 

 possibly, over a large part of the glaciated area. 



Assuming a uniform annual snowfall over the area of the sedentary 

 ice-sheet, it is obvious that since its area is gradually extended, 

 southward by the progressive climatic refrigeration, while the 

 annual ablation as gradually diminishes northward, its thickness 

 must increase backward from the margin. If, however, as Upham 

 holds, and as certainly seems most probable, the precipitation of snow 

 over the growing ice- sheet was not uniform, but greatest from the 

 first one hundred to two hundred miles inward from the margin, 

 the surface-gradient must have culminated on these peripheral tracts, 

 diminishing gradually backward. This condition would, obviously, 

 favour an early beginning of outward movement or flow in the 

 marginal zone, and tend in an equal degree to retard motion in 

 the central area. 



We should hardly be justified in supposing that the great crustal 

 movement which gave us the Ice Age, was steadily progressive, 

 without interruption or reversal, until its final culmination. No 

 doubt the period of the growth and culmination of the ice-sheet, 

 equally with that of its waning and disappearance, was attended 

 with marked climatic oscillation. During these changes the pre- 

 cipitation and ablation would co-operate in accentuating the frontal 

 slope of the ice-sheet. The chief factors in determining movement 

 of the ice-sheet were its thickness and marginal gradient. 



The absence from a large part of the glaciated area of mountains 

 or dominant heights of land, requires us to assume that over 

 considerable tracts the sedentary ice-sheet did eventually begin to 

 flow without having experienced the overriding or shearing thrust 

 of the Piedmont ice-fields. The outward movement thus originating 

 in the peripheral tracts during a period of excessive ablation must 

 have extended backward, perhaps until it met the forward thrust 

 of the Piedmont glaciers. 



Assuming therefore that the movement of the ice-sheet was 

 inaugurated during a warm period, and that the southern margin 



