176 5. W. McCALLIE 



miles. It is divided into an upper and a lower series, the latter 

 being further divided into two formations, namely the Ripley and 

 the Eutaw (Fig. 4). 



The Lower Cretaceous, so far undifferentiated, lies immediately 

 upon the crystalline rocks below and is unconformably overlain 

 by the Eutaw. The basal unconformity represents an enormous 

 time interval, including all Paleozoic time and the Triassic^ and 

 Jurassic periods of the Mesozoic time. The formation is made up 

 predominantly of coarse-grained cross-bedded, arkosic sand, with 

 a subordinate amount of white clays in the form of lenses. It forms 

 an irregular belt varying from two to ten miles in width and extend- 

 ing entirely across the state. 



The Eutaw and Ripley formations , each divided into two or more 

 members, occupy an elongated area immediately south of the 

 Lower Cretaceous between the Ocmulgee and Chattahoochee 

 rivers. Both formations are of marine origin and consist mainly 

 of calcareous, micaceous sand and dark-gray sandy clay, with some 

 thin-bedded impure limestone. The upper member of the Ripley 

 formation is unconformably overlain by the Midway formation, 

 the basal member of the Eocene series. 



TERTIARY SYSTEM 



The Tertiary system of rocks has a far greater areal extent 

 than any other system in the state. With the exception of a 

 narrow belt of Quaternary along the Atlantic seaboard and narrow 

 irregular belts of Cretaceous immediately south of the ancient 

 Crystallines, it covers the entire Coastal Plain which comprises 

 more than one-half the area of the state. The system is repre- 

 sented by the following series: the Eocene, the Oligocene, the 

 Miocene, and the Pliocene. The first named is divided into five 

 formations, the Midway, the Wilcox, the McBean, the Ocala, and 

 the Keg Creek. The Keg Creek and the Ocala limestone forma- 

 tions both belong to the Jackson group, while the McBean formation 

 belongs to Claiborne group. 



I The Triassic rocks in Georgia are represented by diabase dikes which are widely 

 distributed over the Piedmont Plateau. 



