522 K. T. HILL — THE COMANCHE SERIES. 



The firm persistent limestones and harder chalks outcrop as escarpments of 

 stratification, producing landmarks which can be traced for immense dis- 

 tances. Thus, the outcrop of the Goodland limestone in Indian territory 

 forms an escarpment some 200 miles long, overlooking the valley of the 

 Trinity and Paluxy sands. The Duck Creek, Denison, Fort Worth and 

 Caprina limestones produce similar landmarks. The softer disintegrated 

 chalks nearly always occur on the slopes or faces of these escarpments ; 

 while the clays and sandy terranes, such as the Walnut clays and Trinity 

 and Paluxy sands, weather into extensive plains or semi-valleys extending 

 interiorward from the escarpments. Where the headwater erosion of the 

 superimposed drainage above mentioned encroaches upon the shorter and 

 more precipitous drainage slopes of the older and deeper incised drainage, 

 buttes and mesas are evolved. When the Comanche Peak beds, surmounted 

 by the Caprina limestone, constitute the divide, these buttes are invariably 

 of the following types: (1) Flat-topped mesas surrounded by precipitous 

 escarpments ; (2) Slopes of 45° composed of the Comanche Peak chalk ; 

 (3) Basal plains or pediments composed of the Exogyra texana clays and the 

 Gryphcea beds. If the divide is composed of the Glen Rose beds the result- 

 ing buttes are usually conical, encircled by benches resulting from the 

 alternating soft and hard layers. The great difference of induration in the 

 respective terranes is also productive, especially in the eastern half of the 

 area, of extensive plains coincident in slope with the dip, and terminating 

 eastward against an escarpment of the overlying beds, which invariably 

 deflects the drainage parallel to the strike, and on the west by a jump-off or 

 escarpment of its own foundation strata. These dip planes are beautifully 

 shown in northern Texas and southern Indian territory, where they consti- 

 tute the prevalent topography and extend over vast areas. So extensive 

 have been this planing-ofF in central Texas from higher to lower dip planes 

 and the successive pauses at harder strata in the process of base-leveling 

 that in the Burnet-Llano district the old plains can be traced where the 

 drainage valleys have widened or narrowed and cut through from the 

 Caprina limestone to the Glen Rose beds, the Glen Rose beds to the Car- 

 boniferous limestones, the Carboniferous limestones to the upper Cambrian, 

 until finally the Archean and granite rocks are reached in which the 

 Colorado is now cutting some 700 feet below the base of the Trinity and 

 4,000 feet below the former level of the upper Cretaceous. 



It has been denied* by those who have not studied the Mesozoic and 

 Cenozoic history of the Texas region that this erosion has taken place, and 

 that this central Paleozoic district was ever covered ; but he who restores 

 the denuded strata or studies the topography so beautifully recorded in the 



♦Preliminary Report on the Geology of the Central Mineral Region of Texas, by Theo. B. Com- 

 stock: First (Second) Annual Report of the Geological Survey of Texas, Austin, 1889. pp. 314-316. 



