THE FLUCTUATIONS OF THE OTERO SODA LAKE. 39 



Manifestly a basin such as that of Otero must contain a lake if the rainfall is sufficiently 

 heavy. Since the total precipitation of this region, however, is only 10 inches per year 

 on an average, and since the filUng of the basin with waste has made the bottom very flat, 

 no permanent lake can now exist. The water which runs down from the mountains during 

 the winter rainy season or during the sudden thunder-showers of summer spreads out in 

 shallow sheets and soon evaporates. The so-called Otero Soda Lake is really one of a series 

 of large playas having a length of about 40 iniles north and south, and a width of 6 miles 

 or more. On all sides except the west other smaller playas, several times in a year, are 

 similarly filled with water which soon evaporates, leaving white plains of soda and gypsum. 

 During the glacial period the area covered by all the playas appears to have been included 

 within a single great lake, which was probably 60 miles long and 30 wide. The evidence 

 for this is found in certain old strands, plainly visible at the base of the San Andreas Moun- 

 tains on the west side of the basin. These have never been studied with care, and my 

 visit was too brief to allow of more than a cursory examination. They are visible in many 

 places, however, and can be seen extending almost unbroken for 20 miles or more. The 

 lowest and most prominent Ues over 200 feet above the present level of the main playa. 

 Above it at intervals of from 40 to 80 feet three others can be seen. On the east side of 

 the basin none of the old strands are visible. Possibly they were small and insignificant 

 because of the extremely gentle slope of the plain on this side, and have been concealed by 

 the large amount of debris which, since their formation, has been washed down from the 

 high plateau. Even on the west side, where the mountains are far lower and less extensive 

 than the plateau to the east, great fans of gravel have been washed out from all the canyons, 

 burying the bottoms of the cUfTs along the old strands, and in many cases completely 

 concealing the cliffs themselves. This subject, together with that of a possible outlet to 

 the south at the time of the lake's greatest expansion, must be left for future study; so, too, 

 must the interesting question of the relation of the more recent phases of faulting and 

 uphft to the times of expansion of the ancient lake. All these, important as they are, do 

 not bear on our present problem. The one essential fact is that the old strands seem to 

 furnish strong evidence that in former times the basin was occupied more than once by 

 a lake whose extent was far greater than that of the present playas. Almost without 

 further proof we may, I think, assume that the epochs of expansion must have coincided 

 with the glacial epochs of more northern regions. The importance of this lies in its indica- 

 tion that during the glacial period the climate of North America changed almost as much 

 in the warmer, di'ier portions of the continent as in the colder, moister portions. We 

 can not yet say with assurance whether the strands represent the main epochs of the 

 glacial period, or whether, as is more likely, at least part represent the well-known post- 

 glacial stages which have been so much studied of late in the Alps, Scotland, and elsewhere. 

 Whatever be their exact age, it is evident that they belong to the most recent geological 

 times, and that any changes which have taken place since their formation are sufficiently 

 recent to fall within the period of man's probable presence in America. 



Unmistakable evidence that such changes have actually occurred in the most recent 

 post-glacial times is found in certain minor strands of the old lake and in a series of gypsum 

 dunes of various ages. When the playa or Soda Lake is at its greatest extent, its temporary 

 waters are deep enough to be raised by the wind into Uttle waves of sufficient strength to 

 cut a tiny bluff, which at the southeastern corner of the playa has a height of about 2 feet. 

 Above the httle bluff lies a terrace 100 to 200 feet wide. Like the bottom of the playa 

 itself, the terrace is covered with crystals of soda and gypsum, but these saline deposits 

 are not fresh hke those of the playa, and they are studded with vegetation, indicating a 

 considerable lapse of time since their deposition. At rare intervals the water may even 

 now come up over this terrace, but scarcely for long enough periods or to sufficient depth 

 to allow the waves to cut so steep and pronounced a bluff as that which rises to a height 



