CHAPTER VI 



THE GLACIAL OR PLEISTOCENE PERIOD 



The warm Pliocene Period was followed by an interval of intense cold 

 during which time nearly the whole of British America and the United States 

 as far south as northern Pennsylvania, southern Ohio, Indiana and Illinois, 

 central Missouri, eastern Nebraska, central South Dakota, nearly the 

 whole of North Dakota and northern Wyoming, Idaho, and Washington, was 

 covered by immense ice fields, embracing altogether an area of approximately 

 4,000,000 square miles and attaining a thickness of over 5,000 feet 1 (Plate LVI). 

 Formerly the Glacial Period was thot to consist of but a single ice invasion, 

 but it is now known that as many as five well characterized invasions occurred, 

 each separated by an interglacial period of considerable duration. 



1. EFFECT OF THE ICE ON THE TOPOGRAPHY 



This thick mass of moving ice planed off the hills carrying with it the 

 residuary soil which had formed during the long Mesozoic and Cenozoic 

 interval, ground up and removed much of the underlying rocks and scratched 

 and grooved the surface of the more resistant rocks. The first effect of the 

 presence of this ice sheet was probably the ponding of the many northward 

 flowing streams, the valleys forming vast lakes, the drainage from which 

 caused the formation of new river valleys, which in their turn cut canyons 

 and gorges. 



2. EFFECT OF THE ICE ON THE BIOTA 



The effect on the life of the glaciated area was marked and to a certain 

 degree cataclysmal. As the ice advanced the winters became long and the 

 summers short, the seasons being marked by fogs and violent storms. The 

 luxuriant vegetation was overridden, the trees being broken and their remains 

 incorporated in the drift with the other debris — soil and rocks. In many 

 places the newly formed ponds covered the forests and thus killed the trees. 

 The biota was not, however, all killed at once. The process was slow, occupy- 

 ing many years. It is probable that the Arctic plants and such north tem- 

 perate trees as some conifers, oaks, ash, and a few others, kept possession of 

 the territory in front of the ice, much as do the same genera at the present time 

 in high latitudes. The temperate species migrated, by seed, to points far 



1 The thickness of the ice has been variously estimated at from 3 to 8 miles. See Cham- 

 berlin and Salisbury, Geology, III, p. 357. 



