40 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



of 15 or 16 feet back of the terrace. Therefore we infer that at some time not more than a 

 few hundred years ago the water must have stood higher than now, and at such a level as 

 to cover the terrace and cut a well-defined bluff at a level about 4 feet above that of the 

 floor of the playa. If the lake rose to this level it must have contained water most of the 

 time, for it would scarcely be possible every year for evaporation to remove 4 feet of water 

 in the few months which intervene between the end of one rainy season and the time 

 when the lake would be replenished by the rains of the succeeding season, whether it be 

 summer or winter. 



The top of the bluff overlooking the 4-foot strand forms another terrace, like the one 

 above the present strand, but much older and more covered with vegetation. Back of 

 this, and at an altitude of about 20 feet above the floor of the plaj'a, a third small bluff 

 rises about 10 feet. It clearty marks the strand along which the lake rested at some time 

 hundreds of years before the day of the 4-foot strand. At that time the lake was evidently 

 much larger than the present playa, and was so deep that it can not possibly have been 

 subject to complete desiccation. Back of the 20-foot strand there may possibly be still 

 another belonging to times much more recent than the main strands 200 feet or more 

 above the lake. This doubtful third member of the group of minor strands lies about 

 60 feet above the floor of the playa. I saw evidence of it only at the southeast corner of 

 the playa, where a low bluff and a line of gypsum dunes run parallel to the present shoreline 

 at a distance of from a third to a quarter of a mile from it; they seem to indicate another 

 old strand, but the evidence is not sufficiently clear to permit certainty. Nevertheless, 

 even if we omit the 60-foot strand, those at elevations of 20 and 4 feet are suflBcient to 

 show that in times long after the end of the glacial period the Otero Lake has varied in 

 size, apparently because of distinct climatic fluctuations. The strands, however, do not 

 suffice to show whether the fluctuations were merely pauses on the way toward aridity 

 or were distinct periods of increased moisture following times of aridity. 



The most pecuhar feature of the Otero Basin, and one of the most significant from a 

 climatic point of view, is the unique dunes of pure white gypsum, the "White Sands," 

 as they are called. Along the east side of the main playa thej^ form a large tract nearly 

 20 miles long and 10 wide in places. The dunes are like the ordinary tj'pe in shape and 

 movement. Their peculiarity consists in the fact that they are composed of almost pure 

 gjT^sum, which gives them a dazzling white appearance. When the plaj'as become dry 

 in the rainless foresummer or in the fall the strong southwest winds which then prevail 

 sweep across the smooth expanses and pick up clouds of gypsum crystals which have 

 been laid down by the diminishing water. These are swept beyond the limits of the 

 playas and are there heaped up into dunes ranging from 5 to 40 feet in height. In course 

 of time the dunes are gradually moved forward by the winds, while new ones form behind 

 them. At present the area of fresh dunes is constantlj' increasing by the blowing forward 

 of the gypsum sands. For example, at the plaster mill a few miles southeast of Alamogordo, 

 and at many other places along the eastern margin of the dune area, the sand can be seen 

 advancing like a great white wall. At some points it is overwhelming bushes and small 

 trees, which in many cases are completely buried, only to reappear at length behind the 

 dunes when the winds have swept the gypsum beyond them. At other points the White 

 Sands are encroaching upon old roads, some of which can be seen buried to a depth of 

 20 feet. An old stage-driver informed Dr. MacDougal that when he used to drive the 

 overland stage through the Otero Basin in the early eighties he was accustomed to water 

 his horses at a well on the eastern edge of the dune area. Now, however, the well is hidden 

 in the midst of the moving sand, a mile or more from the margin. 



This rapid movement of the dunes does not appear to have lasted for any great length 

 of time, probably for no more than a few score or a hundred years. There is no means, 

 however, of judging exactly how long it has lasted, but if there has been an advance of a 



