UPPER CRETACEOUS. 653 



The Aimona chalk is succeeded by a series of blue, chalky, somewhat glauconitic marls, 

 in places impure chalk [the Marlbrook formation]. * * * About 200 to 300 feet above the 

 base of this formation is a very chalky layer 20 to 50 feet thick, which has been called the 

 Saratoga chalk marl or the Saratoga formation.'' * * * The thickness of the Marlbrook 

 formation ranges from 750 feet at Texarkana to 50 feet or less at Arkadelphia. 



Above the Marlbrook marl is a series of sandy beds which are of vast economic importance 

 to a strip of country along the Iron Mountain Railway between Arkadelphia and Texarkana, 

 since they are the source of the main water supply of that region. Like the other sandy beds 

 of the Cretaceous, at the outcrop they are distinguished with difficulty from the surficial sands 

 that mantle the region. * * * 



The outcrop at Nacatoch Bluff, on Little Missouri River, in Clark County, is one of the 

 most complete exposures occurring along this belt and shows the calcareous and quartzitic 

 rocks which, when encountered in wells, are called "water rocks." 



Section of Nacatoch Bluff on Little Missouri River, ClarTc County, ArTc. 



Feet. 



1. Slope not well exposed; seems to be composed entirely of chert and quartz gravel 20 



2. Ledge o£ foseiliferous arenaceous limestone, containing many large fossil shells: Exogyra 



costata, Pectunculus sp., and Ostrea subspatulata 1 



3. Sand 10 



4. Very impure sandy limestone or calcareous sandstone, with many imperfect fossils 1 



5. Sand 4 



6. Hard fossUiferous calcareous sandttone 5 



7 . Sand 5 



8. Slope on which landslip masses render positive determination difficult; seems to be com- 



posed of sand 25 



Level of Little Missouri River. 



The dark laminated clays which overhe the Nacatoch sand and form the "blue dirt" of 

 the well driUers along the Une of the Iron Mountain Railway from Arkadelphia to Texarkana 

 were named by Hill in 1888 the Arkadelphia shales, from the outcrops at Arkadelphia, Clark 

 County.'' These beds contain uppermost Cretaceous fossils for 100 to 200 feet above the 

 Nacatoch sands, the fossil-bearing beds being well developed on Yellow Creek 3 to 4 miles 

 northwest of Fulton, 5 to 6 miles north of Hope, north and northwest of Emmet, and at Arka- 

 delphia. Thus far no fossils have been found in the upper portion of this formation, which 

 extends without any apparent break to the Eocene sand beds forming the sandy hiUs (the crest 

 of the Sulphur Wold) south of the Iron Mountain Railway. This absence of fossils, together 

 with the fact that the Midway (Eocene) formation, though commonly characterized by lime- 

 stones, contains dark-colored clays, makes the exact determination of the top of the Cretaceous 



in this section particularly difficult. 



********* 



The total thickness of the Arkadelphia clay, excluding the beds which appear to be strati- 

 graphicaUy Eocene, is from 200 to 300 feet at Arkadelphia, 500 feet at Lanesburg, 500 to 600 

 feet at Hope and Spring Hill, and 500 feet at Texarkana and Shreveport. 



I 16. WESTERN TENNESSEE. 



The Cretaceous of western Tennessee comprises the Coffee sand member of the 

 Eutaw formation, the Selma chalk ("Rotten limestone ")j and the Ripley forma- 

 tion. The deposits are described by Safford®^" and Glenn.^^^ The section does 

 not differ materially from that in Mississippi.^"* (See p. 654.) According to 

 recent field studies by L. W. Stephenson the Coffee sand member represents all of 

 the Eutaw formation that is present in western Tennessee. 



« Branner, J. C, Trans. Am. Inst. Min. Eng., 1897, pp. 42-63; Taff, J. A., Twenty-second Ann. Kept. U. S. Geol. 

 Survey, pt. 3, 1902, pp. 714-723. 



6 Ann. Kept. Geol. Survey Arkansas for 1888, vol. 2, 1888, pp. 53-56. 



