364 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 



following its withdrawal there occurred the uplift of the Rocky 

 Mountains on the west. These mountains, according to authorities, 

 intercepted the moisture-laden winds from the Pacific Ocean and re- 

 stricted the rainfall on the lands immediately east of them to moisture 

 derived from the Gulf of JSIexico. Gradual decrease in precipitation 

 resulted ultimately in a grassy type of vegetation in this area. It is 

 believed that this grassland type of vegetation has occupied parts of 

 the Great Plains continuously for millions of years, and that vast 

 armlike projections of grassland have pushed out many times in 

 several directions and withdrawn again when changes in climate 

 occurred. 



Millions of years after the formation of the mountains on the west, 

 there occurred a series of events that exerted a significant influence 

 upon the vegetation of the Great Plains. During the later Tertiary, 

 gradual cooling of the climate in higher latitudes caused significant 

 changes in the environment, which resulted in the disappearance of 

 subtropical species of plants from north and west America. Appar- 

 ently a distinct separation developed between the northern flora, 

 predominantly gymnosperms, and the southern flora which was con- 

 trolled by angiosperms. These two primarily aborescent types (in 

 addition to the grasslands) have maintained their identity in North 

 America since preglacial times. 



As cooling of the higher latitudes continued, the Tertiary period 

 came to a close and it was followed by the period of glaciers. It is 

 not the purpose of this paper to describe in any detail the cause or 

 the extent of glacial periods, but rather to consider briefly their 

 effect upon the vegetation in the wake of their advance. As the ice 

 moved down from the north there was started a migration southward 

 of all living forms. Belts of vegetative types such as tundra, bog 

 scrub, coniferous forest, and deciduous forest were usually main- 

 tained through the east and middle west as they moved southward. 

 The width of each belt of vegetation, however, varied with topog- 

 raphy. Farther west the treeless plains region was covered by prairie 

 vegetation. This vast area of level land probably was bordered on 

 the north by a broad belt of tundra. 



With retreat of the ice, the new bare glacial soil was naturally first 

 invaded by the mosses and lichens of the tundra. After further 

 retreat of the ice the climate became more suitable for plant growth, 

 and as a consequence the belts of vegetation proceeded northward 

 from the position they occupied at the southernmost advance of the 

 glaciers. In the east the succession northward was in the order of 

 tundra, bog scrub, and conifers. The prairie grasses from the plains 

 region, however, not only invaded the immediate adjoining tundra to 

 the north but also succeeded in penetrating the glaciated regions of 



