Page:Popular Science Monthly Volume 81.djvu/235

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development. They appear as even-topped surfaces more or less well elevated above the general plains-surface about. The margins of these truncated mounds form the brow of a precipitous escarpment which is one of their most characteristic features. Not infrequently the upper part of the escarpment is a vertical wall 100, 200 or even 500 feet in height. Mesa de Maya (armored mesa) and Llano Estacado (walled plain) are Spanish descriptive terms referring especially to this feature. The talus-like slopes below are the steepest of any angle of repose; and their meeting with the general plains-surface is as sharp as the strandline.

Mesa profiles and proportions are mainly functions of the geologic structure and of age. Some of these plateau-plains are so small in area and so high that they stand boldly out of the plain as conspicuous cones, or buttes. The Camaleon and Wagon-mound are illustrations. Others, as the Tooth of Time, the Enchanted Mesa (Fig. 2), the Covero, and the Sunset Tanks buttes are only a few acres in areal extent. The famous Toyalané (Fig. 1) and some of its neighbors are somewhat larger. From these to the great Chupadera Mesa and the Mesa Jumanes, which are a dozen miles across and a score of miles in length, or the vast Mesa de Maya, which extends along the northern border of Xew Mexico a hundred miles, there is every size.

Of the mesas of this description the foundation is generally some rock-layer more indurated than the rest of the section. Structurally they may be made up of (1) remnants of former plains worn out on the bevelled edges of folded strata, as in the case of the Mesa Jumanes; (2) slightly inclined strata of hard limestone or sandstone usually, which are intercalated in extensive beds of less resistant materials, as in the Chaca Mesa and other platform plains of the great Mesa Verde region; (3) almost horizontally disposed hard beds from which the soft superposed layers have been stripped, as the Toyalané, El Moro, the Tooth of Time (Fig. 3), and the Tucumcari; (4) old lava-sheets which cover soft shales and sandstones of which the Mesa de Maya, Mesa del Datil and Acoma Mesa (Fig. 4) are conspicuous examples (Fig. 5); and (5) surface-wash deposits locally hardened through the evaporation of moisture in the soil, leaving cemented lime-salts near the surface of the ground (Fig. 6), well represented by the Galisteo Ceja, south of Santa Fe.

The origin of most flat-topped hills is commonly ascribed to circumdenudation effects on an upraised peneplain. All remnants of the old graded surface are on the same level. Throughout the arid region the mesas or plateau-plains, which rise above the general plains-surface, also appear to be the direct result of circumdenudation, but of a very different kind. In marked contrast to the humid-land effects the remnantal plains of the desert, whether their surfaces be formed of stratum-planes,