PREGLACIAL HISTOEY 



57 



2. Former migrations are indicated by the occurrence and distribu- 

 tion of relic colonies and species, by ecological and taxonomic evidence 

 as stated by Adams, by glacial history and topography, by fossils, and 

 by inferences as to former climate. 



3. Evidence from all available sources must be combined to build a 

 plausible and possible chain of events, leading on to and culminating 

 in the known present distribution of plants. 



4. Conclusions as to early stages of plant distribution and migra- 

 tion are reached in reverse order of their occurrence, that is, from 

 present to past. The accuracy of each conclusion depends on the 

 accuracy with which the conditions of the following stage have been 

 interpreted. Each earlier stage must therefore be discussed in less 

 detail and with greater probability of error. 



Part IV. The Development and History of Vegetation in the 



Middle West 



Preglacial. — From the close of the Carboniferous Period to the 

 Lower Cretaceous or Comanchean Period, the area now comprised 

 between the Missouri Valley and the Appalachian Mountain system 

 was continuously land. The pteridophytic and gymnospermous flora 

 which occupied it can be partly reconstructed from the fossils of those 

 ages, but its migrations are unknown. During the Lower Cretaceous 

 the first representatives of the angiosperms must have arrived in our 

 area, since the fossil remains of such plants are preserved in con- 

 temporary deposits still farther west, and through the Upper Cretaceous 

 angiosperms constituted the dominant vegetation. It is not probable 

 that herbs were prominently developed; angiosperms consisted chiefly 

 of woody plants of families and genera still in existence in the area 

 and of other groups now primarily or exclusively tropical. This flora 

 is undoubtedly continuous with the forest flora of the Tertiary, usually 

 designated the arctotertiary flora, and through it with the forests of 

 the present. 



While there may have been differentiation of floristic types in the 

 Cretaceous, correlated with local climatic conditions, the first of the 

 great vegetational segregations which is still of importance in our 

 region began at the close of the Cretaceous with the uplift of the 

 Cordilleran complex of mountains. 28 Intercepting the moisture-laden 

 winds from the Pacific and restricting the rainfall of the lands imme- 

 diately east of them to moisture derived from the Gulf of Mexico, 

 the elevation of these mountains led to the development of semiarid 

 conditions over the Great Plains, which soon had an effect on the 



28 Harvey, Leroy H., Floral succession in the prairie-grass formation in south- 

 eastern South Dakota. Bot. Gaz. 46: 81-108. 1908. 



