UPPER CRETACEOUS OF TENNESSEE 387 



at a stage when no coarse detritus was being washed in. Such a 

 sea was especially favorable for the incoming hundreds of marine 

 organisms wliich developed in great hordes among various forms, 

 especially of mollusca. Conditions favorable for marine life in the 

 extreme northern part of the embayment were not of long duration. 

 Orogenic changes brought in coarse sediments and this sea began 

 to recede, being filled in with sands which make up the Ripley 

 formation. This recession did not take place suddenly and with 

 uniformity but very gradually with some oscillations and sinuosities 

 of the strand line which brought about the various members of the 

 Ripley formation that are well developed in McNairy County and 

 in the northern part of Mississippi. 



The Selma formation is a chalky clay of more than three hundred 

 feet in thickness in the southern part of the state but near the 

 Kentucky line it is less than fifty feet in thickness and is made up 

 of sandy, micaceous clay. This formation is well exposed in the 

 eastern part of McNairy County, giving rise to barren, limy hills 

 known as ''bald knobs" or glades that are frequently covered with 

 species of Ostrea, Gryphaea, Exogyra, and Anomia. Toward the 

 north it becomes thinner and loses its chalky nature, yet it may 

 be readily recognized. It may be studied in the vicinity of Chester- 

 field in Henderson County; about two miles west of Holladay 

 in Benton County; two and one-half miles west of Camden on 

 the N. C. and St. L. R.R. in Benton County; four miles north of 

 Camden on the Big Sandy Road; and about two miles west of 

 Dulac in Henry County. Marine fossils are less abundant in the 

 northern part of the state. In Henry and Benton counties all the 

 exposures of this formation have not yet been examined. Among 

 the collections now at hand for study the most northerly locality 

 at which a collection has been made from this formation is at the 

 Dickinson place four miles north of Camden, Benton County, on 

 the Big Sandy Road. From this locaHty a fauna of about 

 twelve species has been obtained. Some of the same species with 

 a few additional ones have been collected in the new N. C. and 

 St. L. R.R. cut two and one-half miles west of Camden. 



Less than forty species of marine fossils are known from the 

 Selma formation in contrast to more than three hundred and 



