National Geographic : 1896 Sep
TOPOGRAPHIC TERMS OF SPANISH AMERICA mesas, and malpais mesas. A bolson mesa is a bench mesa form ing the outer escarpment of a drainage valley which .has been cut through a bolson. Example, the El Paso mesa. Stream terrace, talus-fan, and malpais mesas are self-explanatory 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 the 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, any out flowing drainage, and are lined with sedimentary debris derived from the surrounding country. 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 cation in being limited by cliffs, but differs from a cation in the element of narrowness, the floor of a plaza being an exceptionally wide valley plain. Example, Plaza Larga, the flat valley of a southern tributary 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. Mesas 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 the 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 the side farthest from and subparallel to the mountain range, the valley *Literally a large purse. Example, Bolson de Mapimi. Lake Bonneville is a holson. f Literally an open, level area, such as a public square, a market place, or a drill ground. Applied in topography to local stretches of level, scarp-bordered valleys, in a generally hilly region.