84 BOTANICAL GAZETTE [august 



Steep and high, but pass back into a complex of gently rounded, 

 semi-detached hills, which present a sinuate or undulating sky line 

 of jumbled peaks; it is a vast mountain system in miniature. Back a 

 few miles from the bluffs these hills pass imperceptibly into the low, 

 rolling prairie hills, which extend with scarcely a variation in either 

 direction. So perfectly is this tributary drainage system established 

 that upon the prairie, swamps and "sloughs" rarely occur. The 

 small streams which have threaded the upland are characterized by 

 ravines of depth and precipitousness, especially in the loess, where 

 they usually end abruptly in a bluff, again dividing the upland into a 

 series of ridges, intricately related, which pass back into the low, 

 rolling hills of gentle profile. 



Origin of the prairie 



The uplift of the Rocky Mountains, which terminated the Cre- 

 taceous, introduced a modifying element which exercised an ever- 

 increasing influence upon the chmate of the Great Plains region. 

 Intercepting the eastward-moving moisture-laden winds from the 

 Pacific, a decrease in the annual precipitation to the east of the range 

 must of necessity have followed. The greatest reduction would have 

 been nearest the mountains, decreasing to the eastward. When this 

 interior continental land was finally left by the interior sea and opened 

 to migration, invasion must have been in large degree controlled by 

 this graduated distribution of rainfall. The subsequent origin of 

 the Cascades could have served only to accentuate this distributional 

 difference and reduction in precipitation. Under such conditions the 

 Tertiary phytogeographical distribution of this central region must 

 have been adjusted. Whatever the source, it would seem highly 

 probable that while the entire Mississippi valley was occupied by the 

 rich Tertiary forest, the region lying to the west and bounded by the 

 Rocky Mountains and extending northward into Assinaboia was, on 

 account of the low precipitation, denied to tree invasion and came 

 to be occupied by a prairie formation, increasing in its xerophytism 

 westward just as it does today. 



Toward the close of the Tertiary (late Pliocene) fossil evidence 

 points conclusively to climatic change. A retrogressive succession of 

 floral waves swept southward under the influence of the gradually 



