﻿26 



S M 1 r 1 1 SON 1 A X M ISCF.I.LAX li( )US C( )LI .ECIK >N S 



74 



ancient shore lines l)ecause it is composed of beach worn fragments 

 of shells and other fossils. Above this comes the liigby limestone, the 

 source of much of the Tennessee brown phosphate and which also is 

 made up almost entirely of the comminuted remains of fossils. Next 

 is the Dove limestone, an almost pure, dove-colored, lithographic- 

 like limestone which shows its shallow water origin in the worm 

 tubes penetrating it and its sun-cracked upper surface. A slab 

 of this limestone a foot thick, as shown in figure 31 and now on 



Fig. 31. — Stratum of dove limestone showing sun-cracked upper surface 

 and penetrating worm tuljes. indicative of shallow water origin. (Photo- 

 graph by Bassler. ) 



exhibition in the National Museum, well illustrates the polygonal 

 upper surface and the penetrating worm tubes, both featui"es indica- 

 tive of the origin of the rock on old mud flats which were periodically 

 above water and thus became sun cracked. The sticceeding Ward 

 limestone is of the more typical blue variety but here the rock is filled 

 with millions of fossil shells which under the influence of weathering 

 are changed to silica and are left free in great numbers in the soil. 

 This section is only a ]:)ortion of the entire geological sequence at 

 Nashville but it well illustrates the various types of limestone out- 

 cropping throughout the Central Basin. 



