00 PROCEEDINGS OF WASHINGTON MEETING. 



and the upper and lower Cretaceous sheets, as well as the rocks of the mountains 

 proper, were degraded and redeposited. Especially is this true of the great rock 

 sheets of the Comanche series, so fully developed to the eastward, and the absence 

 of which to the westward in the Rocky mountain region has so long been a subject 

 of perplexity, They had already suffered much degradation in the haseleveling 

 which took place during the Dakota epoch, and the degradation of the Llano Esta- 

 cado epoch still further reduced, almost obliterated, the remainder. 



The great canyon of the Canadian lying between the northern escarpment of the 

 Llano and the southern escarpment of the Raton-Las Vegas plateau averages 40 

 miles in width, and is 200 miles in length and 1,200 feet in depth. This is an un- 

 doubted valley of erosion, which has removed 8,000 square miles of the plains and 

 2,000 cubic miles of the earth. The valley of the Pecos from the mountains to the 

 Texas line has removed a similar amount. On the eastern margin, over the vast 

 central denuded region, the erosion is just as plain to a geologist. The eastward- 

 projecting tongues forming the divides of every stream from the Platte to the Llano 

 all testify that they are but the rapidly decaying remnants of the greater areas that 

 have been destroyed, and these divides extend as far east as the 98th meridian. 



There is still further evidence of this eastward extension in two interesting areas, 

 the Edwards plateau and the Washington and Fayette prairies of the east. 



'I'm-: Edwards Plateau. 



The ( 'olorado river cuts a very deep canyon through the < irand prairie in Travis 

 and Burnet counties, separating the central or Fort Worth area from the southern 

 or Edwards plateau. The latter is that portion of the (irand prairie south of the 

 Colorado and east of the Pecos. Its width from east to west is greater than its 

 length from north to south, and as it lies mostly within the truly arid region it is 

 not well adapted to agriculture. Its surface is more uniform than that of the arid 

 Llano, being composed of hard limestone strata which terminate on all sides by 

 descending fault escarpments, instead of dipping beneath some newer formation as 

 do all the rock sheets of the northern divisions of the (irand prairie. This region 

 has hitherto had no specific name, being usually called " the mountains," from the 

 escarpments which surround it. It is now proposed to call it the Edwards plateau, 

 from Edwards county, where it is greatly developed. 



This plateau is one of the most extensive and unique topographic features of the 

 whole region. It consists of a vast rocky plain of hard Comanche limestone, 

 covered by a scrubby growth of oak, juniper, mesquite, nopal, and sophora (or 

 false laurel). It is a good grazing country, hut little adapted to agriculture, except 

 on patches of alluvial soil in the creek bottoms, owing to the intense dryness of 

 its rocky sub-structure. It, in conjunction with the Llano Estacado, is a typical 

 plateau of the mesa type, its eastern and southern margins being everywhere 

 marked by descending or step-oil' escarpments, the result of the great Balcones 

 fault by which the whole Black prairie region east of it has dropped down from 

 500 to 1,000 feet. 



The downthrow east of this great fault is conspicuous only south of the Colora- 

 do-Brazos divide, some ten miles north of Austin. From that point southwest- 

 ward to Del Lio, where it crosses into .Mexico, it becomes more and more con- 

 spicuous as a great escarpment line, visible to the westward of the International 

 railway as far south as San Antonio, and from that point westward, north of the 



