FOKEST MIGRATION IN PREHISTORIC PERIOD 79 



by numerous early observers, is so unlike our modern experience with 

 floodplain vegetation in the Middle West that it at once stimulates 

 inquiry into the circumstances of more recent forest development. 



In a forest advance along the bluffs of a river flowing from north to 

 south through a region of prevailingly westerly winds, the greatest pro- 

 tection against atmospheric conditions will be found on the western lee 

 side of the river, rather than on the windward eastern side. Actual 

 conditions throughout the Middle West, however, show an unexpected 

 distribution, in that by far the largest areas of forest were originally 

 found on the eastern side of such streams. This has been discussed in 

 an earlier article, 50 in which the explanation was offered that the irregu- 

 lar distribution is due to prairie fires, which, driven before a westerly 

 wind, have gradually destroyed much of the forests on the western side 

 but have had little effect on those to the east of the stream. The period 

 of great forest migration must therefore have closed with the advent of 

 the Indian and the prairie fire. 



The extent of forest advance before that time can only be approxi- 

 mated by a study of the present distribution of morainal forests and of 

 the relative amounts on different sides of streams. Planimeter measure- 

 ments have been made of maps showing forest distribution in two parts 

 of Iowa and in three counties in Illinois and have given results of 

 remarkable but not necessarily significant uniformity. In Shimek's 51 

 map of the Lake Okoboji region, Iowa, 26.5% only of the forests are 

 on the west side of the lakes, aud 73.5% on the east side. McGee's 29 

 map of northeastern Iowa shows for the Wapsipinicon Kiver 27.9% on 

 the west and 72.1% on the east, and for the Cedar River 29.7% on the 

 west and 70,3% on the east. Tracings of the original land surveys 

 of Champaign, Piatt, and Moultrie Counties, Illinois, show forests on 

 the west side of the streams amounting to 31%, 31.6%, and 29.8%, and 

 on the east side 67.3%, 67.4%, and 70.2% respectively. In Champaign 

 County 1.7% and in Piatt County 1% of the forest area is in isolated 

 groves apart from any river system. If we may assume that forest was 

 originally developed equally on both sides of the streams, the total area 

 would be increased by amounts ranging from 36% of the present area 

 in Piatt County to 44% along the Wapsipinicon River. The lakes of 

 the Okoboji region would have additional forest on the west side amount- 

 ing to 47% of the present total, and if they were carried completely 

 around the lakes the increase would be still greater. 



soGleason, H. A. The relation of forest distribution and prairie fires in the 

 Middle West. Torreya 13: 173-181. 1913. 



si Shimek, B. The plant geography of the Lake Okoboji region. Bull. State Univ. 

 Iowa. Bulletins from the laboratories of natural history 72. 1915. 



