296 
TOPOGRAPHIC TERMS OF SPANISH AMERICA 
lying between this and the mountain may become a bolson, and 
the highest crest of the escarpment of the cuesta may represent 
a sim})le monoclinal summit of the type defined by Russell and 
Gilbert as “ basin ranges.” The escarpment of a cuesta is often 
produced by a fault running parallel with it, and still another 
bolson may be developed in the trough thus formed at its foot. 
This process, many times rej)eated, produces alternations of 
bolson plains and of basin ranges of the cuesta type. 
Rolsons alwa}'-s lie in valleys between the mountains, mesas, 
or cuestas, and are of subsequent origin. 
.Mesas are remnants of ])lains, once more extensive, but now 
constantly diminishing in area by degradation. Plazas are plains 
cut out of mesas, representing areas from which the mesas have 
been removed, and, conversely to the mesa, are increasing in 
area. Bolsons are ancient valleys which have been and usually 
are still being filled up by degradation of the surrounding moun- 
tains, mesas, and cuestas. The mesa plains in general constitute 
the plateau regions bordering the lateral and terminal portions 
of cordilleras, and occur chiefly as the platform surrounding the 
eastern line of the Rocky mountain cordilleras. 
The j)lazas lie mostly east of the true mountains, principally 
along the Pecos and Canadian valleys of New Mexico, but are 
es{)ccially develoj)ed in the plateau countries wherever the forma- 
tion known as the Red Beds enters into the substructure. 
The bolsons generally lie interiorward of the plateau (mesa) 
regions bordering the interior side of the easternmost ranges of 
the cordilleran front and usually increase in area westward. 
The chief plaza countries of the Southwest are from 2,000 to 
4,000 feet in altitude. The altitude of the mesa country varies 
with the continental slope, but around the Rocky mountain re- 
gion has an average of more than 5,000 feet. The bojsons usually 
lie l)etween 4,000 and 5,000 feet, although some of them are below 
sea level. 
NAMES APPLIED TO DECLIVITIES 
The terms applied to declivities are : 
Ceja, Puerto, Escabrodura, 
Cejita, Bajada, Balcones. 
Ceja . — The late General Albert Pike wrote the first descrij^tion of which 
I find mention of the great escarpment constituting the eastern breaks of 
that })ortion of the mesa (plateau) of the plains known as the Llano 
Estacado. I have been unable to find his book, but George Wilkins 
