1921] PALMER, THE FOREST FLORA OF THE OZARK REGION 229 



climatic and other changes that made it possible for the forests of the Gulf 

 coastal plain to begin a great northward movement in the central Miss- 

 issippi Valley and finally to overrun with many of its species the rocky 

 uplands of the Ozark region; what was the character of the Ozark flora 

 previous to the forest invasion; was it exterminated or absorbed by the 

 incoming races and what traces of the older floras are still to be found 



in the region? 



There is some uncertainty as to the exact period when the movement 

 began that resulted in the present elevation of the Ozark region, but 

 geological evidence indicates that it m.ust have been late in the Tertiary 

 period or toward its close. Prior to that time the ancient land surface 

 of the region had been worn down neariy to base level through the long 

 ages of erosion to which it had been subjected. The floor of this old 

 peneplain can now be seen in the nearly level surface of the uplands of 

 the Ozark plateau and the uniform height of the hilltops in the dissected 

 portions. We may then conceive of it prior to its elevation as a low 

 swampy plain, at least in its eastern portion, through which broad slug- 

 gish streams meandered towards the gradually receding Tertiary seas 

 on its southern border. On the southeastern side there persisted to a 

 much later date than marked the retreat of the coast line farther west 

 a great northward extension of the Gulf, known as the ^Mississippi Embay- 

 ment. With the warm currents of the southern seas thus brought to its 

 borders the climate must have been much milder and probably more humid 

 than that which prevails at present. Such a region would be in many 

 ways favorable for a dense forest growth; and it is reasonable to suppose 

 that an earlier phase of the great southern forest, with many species 

 closely allied to those of today, and with other semi-tropical forms long 

 since extinct, then occupied it. With the beginning of the movement 

 that obliterated the gulf embayment and elevated the Ozark region, with 

 its consequent rejuvenation of the streams and rapid erosion, carrying 

 off the mantle of soil and exposing the underlying rocks, the swamp and 

 moisture loving flora was gradually driven back and in time all but ex- 

 terminated. A more severe climate probably associated with the advance 

 of the ice sheets of the Glacial period, the terminal morains of which 

 barely failed to reach the northern border of the Ozarks, may have hastened 



surviv 



the bordering swamps and lowlands occupying the site of the ancient 

 embayment, the influence of which is still evident^ in the swamp flora as 

 far north as southern Illinois and southwestern Indiana. 



More or less directly associated with the colder chmatic cycle ushered 

 in with Glacial period, though probably some time subsequent, was the 

 incursion of a northeastern flora, of which abundant traces are still to 

 be found in cool, protected situations throughout the region and far be- 

 yond its borders. 



With the final disappearance of the ice sheet from the nortliern part 



of the continent came wide-spread climatic changes; but in the meantime 



