568 Reviews — Dr. G. F. Wright — Ice Age in North America. 



The extreme glacial margin is often, but not always, marked by 

 a distinct terminal moraine, with a certain amount of intra-morainic 

 and extra-inorainic drifts. The last-named drifts include what is 

 termed the " fringe" or " attenuated border" of sporadic or modified 

 glacial deposits, which may extend about 20 or 30 miles south 

 of the glacial margin; and they comprise coarse 'over-wash' gravel 

 and boulders formed by marginal drainage. As the author remarks, 

 " it has been questioned by some whether the larger part of the grist 

 of the glacier has not been thus transported far beyond the extreme 

 limits reached by the ice itself," and by subglacial streams " surcharged 

 and milky-white with sediment". The terminal moraines form 

 marked features, rising in places from 150 to 300 feet above the 

 general level of the country, accompanied by huge boulders and 

 by kettle-holes and the small lakelets enclosed hy them. Other 

 moraines have been attributed by some geologists to a " second Glacial 

 epoch", but they are more appropriately termed "moraines of 

 retrocession" ; and there are also minor local moraines. 



It is estimated that not less than one million square miles of 

 territory in North America " is covered with an average depth of fifty 

 feet of glacial debris, forming the most permanently productive part 

 of the continent ". 



Various stages in the Glacial Period, indicating advance and retreat 

 of the ice, have been described more or less locally under the names 

 Sub-Aftonian, Aftonian, Kansan, Iowan, Illinoisan, and Wisconsin 

 (the latest). In the earlier three stages, the deposits are more 

 oxidized — a feature attributed to their antiquity ; the materials 

 having been the first taken up by the advancing ice from the already 

 decomposed and oxidized soils and subsoils. 



Certain ' forest-beds ' and peaty deposits have been taken to mark 

 a second Glacial epoch or one or more Interglacial periods. The 

 author, however, remarks that while in places these accumulations 

 have glacial drift both under and over them, thejr may belong to 

 various times of recession and advance of the ice, and may in fact have 

 originated " in front of the margin of the slowly retreating ice if only 

 there were comparatively brief periods of readvance". Moreover, the 

 buried vegetable deposits " do not mark a warm climate, but a climate 

 much colder than the present", and "it may Avell be questioned 

 whether an interval of two or three centuries would not suffice for the 

 accumulation of the peat described". Most of the facts relating to 

 the Ice Age support "the theory of but one epoch with the natural 

 oscillations accompanying the retreat of so vast an ice-front". 



The influence on drainage systems is not the least fascinating of 

 subjects connected with the Glacial Period. The infilling of pre- 

 Glacial valleys so diverted the lines of superficial drainage that many 

 streams took courses over rocky beds at levels higher than they 

 formerly occupied, and the glaciated region became one of waterfalls. 

 The falls of Niagara are an example. Before the Ice Age nearly all 

 the streams in the eastern United States occupied deeper channels than 

 they do now. " There were then probably no Great Lakes, and few, 

 if any, waterfalls, as there are now no lakes and waterfalls south of 

 the glaciated region. All the rivers had cut their channels down so 



