1921] PALMER, THE FOREST FLORA OF THE OZARK REGION 217 



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issippi, up which the abundant flora and fauna of the southern region 

 have pushed until checked by the pressure of species coming down the 

 great river or its eastern tributaries; northward beyond the valley of 

 the Missouri River lies a diversified region, in which the forest plants of 

 the northeastern states have occupied the valleys of numerous streams 

 dividing the high, rolling prairies; while on the west it merges gradually 

 into the great plains, where forest and hills give place to the vast treeless 

 prairies with their wealth of grasses and flowering plants and their depend- 

 ent faunas. Thus situated, and since nearly all of the region is more or 

 less densely forested, it is not surprising that the flora of the Ozarks 

 presents many interesting features and evidences of the checkered history 



through which it has passed. 



Two main divisions, rather distinct in structure and aspect from each 

 other, are found in the Ozark region: The northern portion comprises 

 the broad, flat-topped dome of the Ozark plateau, much of the interior 

 of which has a comparatively level, rocky surface, while the marginal 

 slopes and escarpments bordering the valleys of the larger streams have 

 been deeply dissected by long erosion into a topography of extreme rug- 

 gedness. The southern portion consists of the Boston Mountains, an 

 area of much greater diversity and characterized by sharply folded strata 

 forming well defined ridges and peaks, in places truly mountainous, al- 

 though the hills seldom rise more than a few hundred feet above the 

 dividing valleys. Most of the plateau division has an average altitude 

 above sea level of from 300 to 500 meters, while only a few points in the 



Boston Mountains exceed 600 meters. 



The valleys of the Mississippi and Missouri Rivers may roughly be 

 taken as the eastern and northern boundaries of the Ozark region; but 

 some distance below the junction of the two streams the highlands reach 

 the margin of the former and stand out as a line of bold bluffs along its 

 course. Between the towns of Chester and Thebes in Illinois a low, 

 outlying spur crosses the river and extends across the southern point of 

 the state to the Ohio River, The name of Shawnee Hills has been aj)plied 

 to at least the eastern part of this ridge; and in its more rocky portions 

 it has much in common with the topography and flora of the Trans- 

 Misssissippi Ozarks. The Boston Mountain division lies mainly between 



the White and Arkansas Rivers. To the southwest of the Arkansas 

 River valley is a detached area of closely folded beds of shales slates and 

 sandstones, known as the Ouichita Mountains; so far as the forest flora 

 is concerned it may well be considered as part of the Ozark region. The 

 entire region as thus defined occupies nearly the southern half of Missouri, 

 with a narrow spur crossing southern Illinois, the northwestern portions 

 of Arkansas, comi^rising about one third of the entire state, and a long 

 triangular strip in eastern Oklahoma, bounded roughly on the west by a 

 line extending from the town of Wagoner to the northeast corner of the 

 state. 



