EOLIAN GEOLOGY 
171 
pendent of stream action. These plateau plains are vestiges of 
former general plains-levels. General letting down of a plains- 
surface is not so very different from the linear incision of a stream 
channel. In place of a single attack-line the whole country goes 
down. 
Of the mesas of the Southwestern country the foundation is 
invariably some hard rock-layer. Structurally they are made up 
of (1) remnants of former plains worn out on the beveled edges 
of flexed strata, as in the cases of the Mesa Jumanes and the 
Chupadera Mesa; (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 El Moro 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 are conspicuous exam¬ 
ples; and (5) surface-wash deposits locally hardened through the 
evaporation of soil moisture leaving near the surface of the 
ground, the lime salts to firmly cement the soils. This is well 
represented by the Galisteo Ceja, south of Santa Fe. 
The origin of most flat-topped hills is ascribed to circumdenuda- 
tion effects on an upraised peneplain; but all remnants of an old 
graded surface are on the same general level. Throughout the 
arid region the mesas or plateau-plains, which rise above the sur¬ 
face of the general plains-level, also appear to be the direct result 
of circumdenudation; but as will be seen later on, of a very differ¬ 
ent kind. In marked contrast to the humid-land effects the rem- 
nantal plains of the desert, whether their surfaces be on stratum- 
planes, beveled tables of flexed strata, lava-sheets, or cemented 
regolith, are of quite different elevations even in the same district. 
In New Mexico, for example, these plains attain all altitudes 
above the general plains-surface, from a few feet in the instance 
of the very recently formed Malagro malpais, in the Hueco bolson, 
northeast of El Paso, to the broad Mesa de Maya, which is 3,5(X) 
feet above the general plains-surface, and 9,000 feet above sea- 
level. The Sierra del Datil, in western New Mexico, has a mag¬ 
nificent northward-facing escarpment 1,000 feet high; and in 
sight of it is the Acoma Mesa 500 feet above the plains-floor. 
