218 the plant life of maryland 



Conclusions. 



The Lower Midland District is geologically old, which is to say 

 that it has undergone but little recent change of elevation due to 

 crnstal movements, and has been altered in its surface features only 

 by the prolonged erosion to which it has been subjected. The soil 

 conditions were doubtless somewhat similar to those of the present 

 day throughout Mesozoic time, excepting for the more recent Cre- 

 taceous and Pleistocene deposits which constitute the Gravel soils 

 of the "Fall-line" border of the District. The climatic conditions 

 underwent profound oscillations during late Pliocene and early 

 Pleistocene time, owing to the successive epochs of glaciation to 

 the northward, and this gave occasion for the northward migration 

 of southern forms alternately with the pushing southward of north- 

 ern forms. We thus see that while the topographic conditions gov- 

 erning the local distribution of vegetation have been only slowly 

 changing through long periods of geological time the character of 

 the flora has been changed several times in a fashion which has 

 resulted in an admixture of northern and southern species. The 

 more recent Coastal Plain has been seen to have been a highway 

 along which southern species have migrated northward. Very many 

 of these have not spread beyond the "Fall-line," apparently on ac- 

 count of unsuitable soil conditions, a few others show by their 

 abundance on the Coastal Plain and their occurrence in favorable 

 habitats on the Lower Midland District that they are migrating 

 into this District from the favorable highway. 



The fact of the dependence of the nature and distribution of 

 vegetation upon the character of the topography causes a yoimg and 

 diversified topography to present a greater differentiation of veg- 

 etation than does a mature one. We have such a difference well 

 exemplified in the Coastal Zone, where both Upland and shore-line 

 changes are going on with relative rapidity, and in the Lower Mid- 

 land, where the topography is well matured. In theory the climax 

 of topographic development is the reaching of base level, but in 

 so far as vegetation is concerned, the reaching of maturity is a 

 climax which is so nearly stable as to present uniform conditions 



