WIND-GRAVED MESAS 229 



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. Xot 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 strand- 

 line. 



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 Toyalane (Fig. 1) and some of its neighbors are somewhat 

 larger. From these to the great Chupadera Mesa and the Mesa Ju- 

 manes, 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 Toyalane, 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 circum- 

 denudation 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 rem- 

 nantal plains of the desert, whether their surfaces be formed of stratum- 



