LOWER CRETACEOUS. 605 



The Trinity sand, the oldest and lowest formation of the Coastal Plain series in this region, 

 outcrops in a narrow band 6 to 12 miles wide, which extends westward along the base of the 

 Ouachita and Arbuckle mountains from Wolf Creek in Pike County, Ark., into Indian Terri- 

 tory to a point somewhat west of Ardmore, where it turns south toward Austin. Because of 

 its extensive development about the headquarters of Trinity River in Wise, Tarrant, and 

 Parker counties, Tex., it has been called the Trinity sand or Trinity formation. 



South of Tishomingo and Atoka, in the Choctaw Nation, this formation consists of a fine 

 yellowish pack sand, with irregular conglomerate beds at the base, and contains occasional 

 lenticular masses of clays, the whole having an aggregate thickness of 200 to 400 feet." In 

 this region it contains no fossils except occasional tree trunks, thought by Taff to be driftwood. 

 Eastward these beds become more clayey in character and in southern Arkansas contain fre- 

 quent thin layers of yellowish bjnestone, composed largely of the small oyster Ostrea franlclini 

 Coquand, together with other near-shore or brackish-water forms. Occasional thin beds of 

 gypsum, hke that at the Plaster or Gypsum Bluff on Little Missouri River near Murfreesboro 

 occur. * * * 



Above the Trinity formation is a thin bed of chalky limestone 15 to 25 feet thick, which, 

 because of its whiteness and firmness, contrasts sharply both with the Trinity sand and with 

 the series of marls with thin limestone layers that overlie it. Because of these features it is 

 easily traced frora its type locality, 2 miles north of Goodland, on the St. Louis & San Fran- 

 cisco Railroad, in the Choctaw Nation, Indian Territory, both eastward to Cerro Gordo, on 

 Little River, in Arkansas, and westward past Oakland and ]\Iarietta, in the Chickasaw Nation. 

 It is everywhere in this region characterized by pecuhar fossils. South of Red River in Texas 

 it increases gradually in thickness until it reaches a total of 700 feet on the Rio Grande,* 

 where it is composed of two members which are called the Comanche Peak and Edwards 

 formations. * * * 



Above the Goodland limestone, near Cerro Gordo, Ark., is a series of calcareous clays, 

 containing thin beds of limestone and having a total thickness of over 250 feet." These beds 

 have not yet been carefully studied, but Hill's reconnaissance in 1888 has shown that they may 

 be regarded as the representatives of the Preston, Fort Worth, and Denison formations of the 

 Texas section and of their equivalents in Indian Territory — the Kiamichi, Caddo, Bokchito, 

 and Bennington formations of Taff [Washita group]. * * * 



At the close of the lower Cretaceous there was evidently a considerable change in the depth 

 of the ocean waters in the Texas and Arkansas areas, since overlying the marine deposits, of 

 the Washita series are sands and clays containing lignite and other remains of land plants of 

 species very similar indeed to those of to-day, whose ancestors they probably were. In many 

 places there is some evidence of slight erosion of the lower beds before these httoral deposits 

 were laid down, and some portions of this area may have been dry land for a limited time. 



I 16-17. ALABAMA, GEORGIA, AND SOUTH CAROLINA. 



L. W. Stephenson has contributed the following notes: 



. The basal portion of the Cretaceous deposits in the region included between the Roanoke 

 Valley ia North Carolina and the Alabama Valley in Alabama is composed of highly cross- 

 bedded arkosic sands, in general of coarse texture, with subordinate interbedded layers and 

 lenses of light-colored clays of greater or lesser purity, reaching an estimated maximum thick- 

 ness of 500 or 600 feet. These have been designated the "Cape Fear" formation in North 

 Carolina by the writer <* and the "Hamburg beds" in South Carohna by Earle Sloan " and have 



o Taff, J. A., Atoka folio (No. 79), 1902, and Tishomingo folio (No. 98), 1903, Geol. Atlas U. S., U. S. Geol. Survey. 



6 Hill, R. T., Twenty-first Ann. Rept. V. S. Geol. Survey, pt. 7, 1901, p. 214. 



« Ann. Rept. Geol. Survey Arkansas for 1888, vol. 2, 1888, pp. 110-111. 



<2 Some facts relating to the Mesozoic deposits of the Coastal Plain of North Carolina: Johns Hopkins Univ. Circ, 

 new ser,. No. 7, July, 1907, pp. 93-99. 



« Clays of South Carolina: Bull. South Carolina Geol. Survey, 4th ser., No. 1, 1904, pp. 72-75; Handbook of South 

 Carolina, State Dept. Agr., Commerce, and Immigration, Columbia, S. C, 1907, pp. 85-88. 



