S06 R. T. HILL — GEOLOGY OF RED RIVER. 



region and a kind of promontory, the escarpment of which extends south- 

 ward to Austin. 



The foregoing are all the deposits laid down at marine baselevel in the 

 vicinity of the Preston section. Fluviatile deposits are highly developed 

 in the valley of Red river, but more will be said of them later. 



Review of the Section topographically. — Let us now review the topo- 

 graphic expression of the members of this section. By reference to the 

 serrated profile it will be seen that it is broken north and south of Den- 

 ison by two faults running parallel with the strike — one, the Preston 

 fault, with a downthrow to the northward by which the Dakota is brought 

 down to the Trinity, and the other, the Cooks spring fault of Taff, with 

 a downthrow to the south by which the Eagle Ford is brought down to 

 the Dakota. These fault systems play a most important part in the 

 production of the topographic features of the Denison section, for by 

 them the normal outcrops of these rocks are doubled, each member in 

 the section from Red river to Sherman, but not including the Austin 

 chalk at Sherman, being duplicated on the downthrow of the fault north 

 of the river. It will also be seen that each of these rock sheets has its 

 peculiar topographic expression. The outcrop is to the northward or 

 toward the mountains and the general dip to the southward. Where 

 the strata are composed of indurated material — such as the Goodiand 

 limestone, the Duck creek chalk, the Fort Worth limestone and the Main 

 street limestone — escarpments of stratification are produced, the sum- 

 mits of which, to the next harder member, are usually composed of the 

 intervening softer, less indurated beds, such as the Walnut clays, Kiamitia 

 clays. Marietta claj^s, Paw Paw clays, Eagle Ford shales, and the Trinity 

 sands. The conspicuousness and extent of these east-and-west escarp- 

 ments is proportionate to the induration and resistance to weathering of 

 the limestone beds. The three sandy terranes — the Trinity, the North 

 Denison, and the Dakota — outcrop in undulating hilly areas. The 

 hardest and most persistent of the limestone scarp-making members is 

 the Goodiand limestone, and its escarpments form one of the most con- 

 spicuous features in southern Indian Territory, extending east and west 

 from North Marietta to the near eastern edge of the Choctaw nation. 

 The valley occupied by the Trinity sands and lying between it and the 

 mountains is coextensive with this escarpment. Southward through 

 Texas the Goodiand limestone difi'erentiates into the Comanche peak 

 facies ; that is, into harder upper beds of Caprina limestone and softer 

 basal members of Comanche peak chalk and Walnut clays — and its 

 scarp-making properties increase until they constitute most of the mural 

 buttes and drapiage divides of the great central Texas region, as shown 

 in my previous papers. 



