574 PROCEEDINGS OF THE ALBUQUERQUE MEETING 



REPRESENTATION OF FOKMEE PLAINS-LEVELS Bi' THE PLATEAD PLAINS 



Mesas, or plateau plains, are one of tlie most characteristic features of the 

 New Mexican arid region, and they are usually due to protective caps of old 

 lavas vi^hich flovsred out on the plains when the latter were at a higher level 

 than at the present time. For the most part they may be, therefore, regarded 

 as representing former levels of either general or local planation. Thus wind 

 and water are very much alike in some of their general leveling effects (plate 

 41, figure 1) from photograph by U. S. Geological Survey. 



NORMAL TORRENTIAL ACTION OF WATER IN THE MOUNTAINS 



In the higher mountains of the arid region the torrential effects of water are 

 not very different from what they are in the more humid regions. Whatever 

 the effects of wind erosion there are they are largely obscured by the excep- 

 tionally heavy precipitation. The Tijeras creek and the Rio Galisteo are good 

 examples. 



FREQUENT OCCUPATION OF PLAINS Bi" LAKES 



Intermont plains often contain somewhere within their boundaries lakes, 

 playas, and salinas, the waters of which are quite ephemeral in character. 

 Many, and perhaps all, of these shallow lake-basins, are hollowed out of the 

 plains-floor "by the wind. Laguna del Perro, and other lakelets of the Estancia 

 plains (plate 41, figure 2), the gypsum sands district of the Hueco bolson 

 and the Playa de los Pinos are typical. 



MARKED ABSENCE OF EOCK-WEATHEKING 



The extent, rapidity, and character of rock-decomposition such as takes 

 place in the humid regions is in the arid country almost unknown. Rocks of 

 all kinds exhibit little or no weathering. Their surfaces are characterized by 

 their wonderful freshness. This feature is best shown in the huge igneous 

 masses of the Cerrillos hills. 



REMARKABLE THINNESS OP SURFACE MANTLE 



The loam and d§bris covering the plains is unexpectedly thin, usually occur- 

 ring only as a veneer. This is typically displayed along the Rio Galisteo and 

 the Arroyo San Pedro east of the Sandia range. (See plate 40, figure 2, from 

 photograph by D. W. Johnson.) The Sonoran plains, described by McGee,' 

 further attest the general prevalency of the phenomenon. 



WIK^' 



TRANSPORTED NATURE OP THE SURFACE MATERIALS 



All of the finer detritus is manifestly far removed from its original location. 

 It appears to be a very rare occurrence for the surface materials of the plains 

 to give any suggestion of the rock-composition beneath. The mesa above Albu- 

 querque illustrates the point. 



GRAVELLY CHARACTER OP SURFACE DEPOSITS LARGELY ONLY APPARENT 



Predominance of gravels on the surface of the plains is illusory. The man- 

 tle is mainly loamy. It is not generally recognized that the gi-eat abundance 



' Bulletin of the Geological Society of America, vol. 8, 1807, p. 9X, 



