LLANO ESTACADO.—ITS MEAN ALTITUDE. -EXTENT AND BOUNDARIES. 9 
profile, is not caused by erosion alone, but that a gentle curvature of the strata exists in con- 
sequence of the presence of a line of elevation in the older rocks parallel with the Guadalupe 
range. Such a line of elevation, although far beneath the surface, is very probable; and the 
extended and gradual, but decided, elevation shown in the surface of the Llano, if it be not 
due entirely to the denudation and erosion of the Rio Pecos, favors the supposition. Such a 
line of elevation would produce a basin-shaped or trough-like disposition of the upper strata, 
even if they rested undisturbed, as originally deposited; and thus the conditions necessary to 
the success of artesian wells would be obtained. 
The mean altitude or general elevation of the great plateau of the Llano has never yet been 
given. The numerous measurements of the survey, however, furnish the means of determining 
this elevation, at least for that portion along the route near the parallel of 329. If we take 
the mean of the altitudes of 22 stations, from two to seven miles apart, between the Pecos, at 
the mouth of Delaware creek, and the Sulphur springs of the Colorado, we obtain, in round 
numbers, 4,500 feet as the mean altitude of the Llano along the line explored. A consideration 
of the courses of the rivers—the Pecos, Colorado, and Brazos—and the observations concerning 
the slope of the Llano,lead to the conclusion that the surface rises towards the north and 
northwest; or, in other words, that the Llano is highest in the northwestern portions, and 
descends gently, not only east, but southeasterly, towards the Gulf of Mexico. "The sources of 
the Pecos river, at Hurrah creek, according to the observations of Lieutenant Whipple, who 
passed along the Canadian river, near the parallel of 35°, are at an altitude of about 5,000 
feet; and this is over 200 miles north of the point at which Captain Pope crossed the stream. 
His observations upon the altitude of a portion of the Llano, between Amarillo and Rocky Dell 
creek, (longitude 102° 30',) range from 4,128 to 4,400 feet; while the observations of the 
altitude of the summit, on the 32d parallel, almost directly south of that point, shows a height 
of 4,700 feet. These results are not consistent with the conclusion of the descent of the surface 
of the plain from the north to the south, 
The formation of the Llano Estacado is one of the most marked physical features of the Ameri- 
can continent. Its surface, rising over a broad area to an altitude, in almost every part, of over 
4,000 feet, at the lowest estimate, and but little broken or traversed by river valleys, constitutes 
one of the most perfect examples of an elevated plateau, or mesa, that is found. The Great Basin, 
although generally supposed to be the broadest and most extensive plateau of the country, is 
entirely different in its character, being formed, not of extended and horizontal layers of rock, 
but of the débris and wash from the thousand mountains and ranges that rise at intervals from its 
surface, and only by their intersecting slopes produce that general elevation of the surface which 
has been regarded as a plateau. The Llano, on the contrary, is not broken by a single peak; 
and there is nothing to break the monotonous desert character of its surface, except an occa- 
sional river gorge or cañon, invisible from a distance, and often apparent only when the traveller 
stands on its brink. A 
According to the observations of Lieutenant Whipple, this great plateau continues beyond 
the valley of the Canadian far to the northward, and, although much cut and denuded by rivers, 
the table-like hills, with mural faces seen on all sides, show its former continuity. The borders 
of the plateau at the northward, along the Canadian and Red rivers, are well defined by a long 
line of vertical bluffs, rising like walls above the general level of the bordering country or river 
valleys. "These are described by Captain Marcy in his report," and were seen by Lieutenant 
Whipple as he approached the Llano from the east. Captain Pope has, however, ascertained 
that along the line of his survey this characteristic of the plain at the north is absent, and that, 
instead of a mural face on the west and east, the ascent from the level of the Pecos is very 
gentle, and **the summit-level was attained at a distance of thirty-five miles, without an abrupt 
ascent at any point, and without the appearance of any of the marked characteristics which had 
1 Exploration of the Red river of Louisiana, by Captains Marcy and McClellan, U. 8. A. [Ex. Doc., 33d Cong., lst sess.: 
Washington, 1854.] 
