714 C. R. KEYES TOYALANE AND LUCERO 



manner of bold-coast isles out of a glassy sea. Mesas, or "tables," the 

 Spanish-speaking people aptly denominate them. The margin of a mesa 

 forms the brow of a precipitous escarpment, which is one of its most 

 characteristic features. Not infrequently the upper part of the escarp- 

 ment is a vertical wall 100, 200, or even 500 feet in height. Mesa de 

 Maya (armored -inesa) and Llano Estacado (walled plain) are Spanish 

 descriptive terms referring especially to this feature. The talus-like 

 slopes below are steep, and their meeting with the general plains surface 

 is sharp. 



Profile and proportion are mainly functions of the geologic substruc- 

 ture. Some plateau plains are so small in area and so high that they 

 stand boldly out of the general plain as conspicuous cones, or buttes. 

 The Camaleon and Wagon Mound in eastern Xew Mexico are illustra- 

 tions. Others, as the Enchanted Mesa and the Covero, in western Xew 

 Mexico, and the Sunset Tanks buttes in Arizona, are only a few acres in 

 areal extent. Toyalane and its neighbors are somewhat larger. From 

 these, to. the . great Chupadera Mesa and 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 New Mexico a hun- 

 dred miles, there is every size. 



Structure of the Plateau Plain 



The foundation of the plateau plain is generally some rock layer more 

 indurated than the rest of the section. Structurally it may be made up 

 of ( 1 ) remnants of former plains worn out on the beveled edges of folded 

 strata, as in the cases of the Mesa Jumanes and the Chupadera Mesa; 



(2) slightly inclined strata of hard limestone or sandstone, which usually 

 are intercalated in extensive beds of less resistant materials, as in the 

 Chaca Mesa and other platform plains of the great Mesa Yerde region ; 



(3) almost horizontally disposed hard beds, from which the soft super- 

 posed layers have been stripped, as the Toyalane, El Moro, and the Tu- 

 cumcari; (4) old lava-sheets, which cover soft shales and sandstones of 

 which the Mesa de Maya, Mesa del Datil, and Acoma Mesa are conspicu- 

 ous examples, and (5) surface-wash deposits, locally hardened through 

 the evaporation of soil moisture, leaving the lime salts near the surface of 

 the ground, well represented by the Galisteo Ceja south of Santa Fe. 



Most flat-topped hills are now commonly ascribed to circumdenudation 

 effects on an upraised peneplain. All remnants of the old graded surface 

 are on the same general level. Tliroughout the arid region the plateau 

 plains wliicli rise above the surface of the general plains level also appear 



