TOPOGRAPHIC TERMS OF SPANISH AMERICA 
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mesas, and malpais mesas. A bolson mesa is a bench mesa form- 
ing the outer escarpment of a drainage valley v’hicb has l>een 
cut through a bolson. Example, the El Paso mesa. Stream- 
terrace, talus-fan, and malpais mesas are self-explanator}^ terms. 
A cuesta is a structural plain, so tilted that it has a perceptibly 
sloping surface. The cuesta is, in a manner, a transitional fea- 
ture between mesa and mountain. The Cuesta del Burro of tlie 
Marfa, Texas, sheet of the United States Geological Survey is an 
example. 
Bolson .* — The third type of plain is the bolson. Bolsons are 
basin valleys which have not, or originally had not, an}^ out- 
flowing drainage, and are lined with sedimentary debris derived 
from the surrounding countiy. 
A 'plaza t may be defined as the sublevel floor of an extensive, 
wide, flat valley lying between the cejas of mesas. In conception 
it resembles a canon in being limited by cliffs, but differs from 
a canon in the element of narrowness, the floor of a plaza being 
an exceptionally wide valley plain. Examide, Plaza Larga, the 
flat valley of a southern tributar}’- of the Canadian, in New 
Mexico, near the Texas line. The valleys of the Pecos and 
Canadian rivers in eastern New Mexico are plazas of great mag- 
nitude. 
^lesas and cuestas are structural plains, representing surfaces 
resulting from the survival of hard layers of rock. 
The plaza is a degradational plain, lying between steep escarp- 
ments, and formed by the cutting away of the hard, rock floor 
of the mesas through the underlying unconsolidated beds, to still 
lower strata of harder rock beneath it. 
The bolson is an aggradational plain, formed by the filling 
up of ancient structural and erosion valleys by the debris of tbe 
marginal country. 
The edges of the rock-sheets composing mesas in some in- 
stances upturn into mountain structure. The mountain struc- 
ture sometimes flattens out into mesa structure. 
The cuesta is a transitional feature, and connecting step be- 
tween the mountain, mesa, and bolson. When a cuesta slopes 
toward a mountain and has its ceja or escarpment on tlie side 
farthest from and subparallel to tbe mountain range, tbe valley 
•Literally a large purse. Example, Holson ile Mapimi. Lake Honnevillo is a 
liolson. 
+ Literally an open, level area, sueh as a public square, a market place, or a drill 
ground. Applied in topography to local stretches of level, soarp-hordoreil valh-ys. in 
a generally hilly region. 
