PHYSIOGRAPHIC INFLUENCES IN TENNESSEE. 57 



thin and the surface is glady. A portion of the limestones contain a con- 

 siderable percentage of phosphatic matter, and their solution has left a 

 soil unusually rich in the form of plant food. The soils of this phosphatic 

 type attain their best development in the region of Columbia and Mount 

 Pleasant, and this is the secret of the high, natural fertility of that and 

 some of the other portions of the Centra! Basin. 



The bounding margin of the Central Plain or Basin is not a simple and 

 smooth slope, but on the contrary is usually very irregular, since it has 

 been formed by the headwater erosion of streams that are eating into the 

 surrounding Highland Plain and enlarging the area of the Central Basin 

 at the expense of this upper plain. The result has been an irregular, 



FIG. 5. Central Plain, with Highland Rim in background. 



crenulated margin such as may be seen at almost any point around the 

 basin. 



Conditions almost everywhere in the Central Plain have been good for 

 settlement and development. It may be looked upon indeed as a region 

 of physiographic liberty where man mig'ht settle practically anywhere and 

 develop according to his own inclination. There has been comparatively 

 little in the way of natural restraint or compulsion to determine and direct 

 man's activities in the development of the Central Plain. A good farm 

 or a village might be established almost anywhere. Perhaps the nearest 

 approach to guidance in the region has come from the existence at a few 

 places of geological faults or breaks in the rocks up along which salt 

 waters have for ages found their way to the surface, producing salt 

 springs or licks. These springs became the meeting place for the wild 



