CHAPTER XXVII 



THE MIOCENE PERIOD 1 



FORMATIONS AND PHYSICAL HISTORY 



The geography of the North American continent during the 

 Miocene period was similar to that of the Eocene. The slight 

 emergence of the coastal borders after the Eocene (or early Oligo- 

 cene) was followed by a slight submergence of the same regions 

 during the Miocene. In the western interior, terrestrial aggrada- 

 tion of all phases continued, but the sites of principal deposition 

 differed somewhat from those of the preceding period. 



The Atlantic coast. In its surface distribution, the Miocene 

 sustains the same relation to the Eocene that the latter does to 

 the Cretaceous (Fig. 446), though in places the Miocene overlaps 

 the Eocene, completely concealing it. There is generally a slight 

 unconformity at the base of the Miocene. Like the other forma- 

 tions of the Coastal Plain, the beds dip seaward and are concealed 

 by younger beds some distance to landward from the present shore. 

 The system originally extended inland far beyond its present border, 

 as shown by numerous outliers. 



The Miocene of the Atlantic coast is composed chiefly of un- 

 consolidated sand and clay. In places there is shell marl, and local- 

 ly beds of diatomaceous earth of such thickness (30 or 40 feet) as to 

 be valuable commercially. At the north, the Miocene has a thick- 

 ness of 700 feet, but it thins southward. The Miocene of this coast 

 is generally called the Chesapeake formation. It was formerly 

 n yarded as Upper Miocene, the former Lower Miocene being now 

 classed as Oligocene. The fauna of the Chesapeake series has been 

 interpreted to indicate a climate somewhat cooler than that which 

 had preceded. 



The Gulf coast. The Miocene of the Gulf coast is rather thin, 

 and sustains the same general relations to older formations as that 



1 Dall imcl Harris, Bull. 84, U.S. Geol. Surv., and Dall, iSthAnn. Rept., U.S. 

 Geol. Surv., Pt. 1 1 . 



