88 PROCEEDINGS OF WASHINGTON MEETING. 



northern half of this plain is similar to that of the Tertiary plains of eastern Colo- 

 rado, Kansas and northward, but it differs from them in that, instead of extending 

 to the Rocky mountains on the west or imperceptibly grading into the level of the 

 eastern areas, it is surrounded on every side (except a few miles at the southeastern 

 corner) by a more or less marked and often precipitous escarpment of erosion which 

 completely insulates it from all other regions, except the Edwards plateau toward 

 the southeast, which is its direct coastward continuation and genetically a portion 

 of it. 



Within the past few years the new railroads of Texas and New Mexico have 

 made accessible to the geologist this greatest of all the plains, and perhaps areally 

 the largest and least studied plateau of our country. < reographically it includes the 

 quadrangular region south of the Canadian, east of the Pecos, and west of the one 

 hundred and first meridian. The scarps which surround it are very irregular and 

 least conspicuous upon the eastern edge, and are marked by many deep, vertically 

 incised canons, such as canyon Blanco, winch is nine hundred feel deep. Easterly 

 projections of these plains extend down the principal drainage divides, ami prob- 

 ably were once continuous across the present denuded region to the Grand prairie, 

 as is still the case with the divide of the Pecos and Colorado. The northern and 

 western escarpment valleys, i.e., those of the Canadian and Pecos,are more precipi- 

 tous, being over 1,200 feet deep, and receive none of the surface drainage of the 

 plain, owing to the diverse slope. The surface of this plain is nearly smooth and 

 unbroken except at its edges, and constitutes as a whole the largest area without 

 surface drainage in our country. It slopes eastward to the rate of 20 feet per mile, 

 and its greatest elevation, at the northwestern corner, is 5,500 feet. Hydrograph- 

 ically the whole surface is void of running streams, and the small amount of surface 

 water not imbibed by the soil is found in a few widely distributed ponds. Its east- 

 ern and northern edges are incised by deep, vertical canyons of tributaries of the 

 Red, Brazos and Colorado systems, which are cutting into it by retrogressive or 

 headwater erosion. Two streams have cut completely through the plains and into 

 the Red bed and Cretaceous floor; these are the Canadian and Pecos. But neither 

 of these receives any of the surface drainage of the plain and both are true moun- 

 tain streams. 



The residual soil of the plain is mostly composed of the transported sedimentary 

 debris of the Rocky mountains and the Las Vegas plateau. From its structure 

 and composition it is evident that the soil is a littoral or alluvial deposit laid down 

 in late Tertiary time. This soil differs from most others in Texas, and, notwith- 

 standing the deficient rainfall, the plains an> being rapidly settled by an industrial 

 population. 



The geologic structure of the Llano Estacado is as simple and uniform as its 

 topography, consisting of a surface sheet of unconsolidated porous sediments, com- 

 posed mostly of water-worn gravel, sand and silt occurring in horizontal layers. 

 averaging 200 feet in thickness throughout its extent, as ascertained by numerous 

 well borings and measurements of tin' escarpments, and deposited unconformably 

 upon the various older rocks which constitute its floor. The greatest thickness of 

 the formation is toward the northern margin of the plain, and it gradually thins 

 southeastward. 



The peculiar heterogeneous character of the unconsolidated formation has been 

 well described by Professor Robert Hay as grits, mortar beds and marls. Certain 

 layers are composed of hard siliceous pebbles, which are recognizable as the debris 



