42 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



siderable amount of solution has removed much of the gypsum from the outer surface, 

 leaving only the impurities in the form of soil. On the gent'e slopes thus formed, and in 

 the small but important supply of soil, vegetation of a grassy type has established itself 

 and is able to persist even in a dry time like the present. 



Underneath the Intermediate Gypsum lies the Tularosa Gypsum, as it has been named 

 by Mr. Free. In this the typical dune topography has completely disappeared, as has 

 the cross-bedding. Nevertheless the eohan origin of the deposit is quite well established 

 by the sun-cracks, rounded grains, and other characteristics which Mr. Free has detected 

 under the microscope. The Tularosa Gypsum is much more extensive than the others. 

 On the east it extends nearly to the foot of the mountains and is often found buried under 

 a thin layer of alluvium recently brought down from the mountains. A similar deposit, 

 but probably of considerably greater age, is also found high on the flanks of the fault scarp 

 which bounds the Otero Basin on the east. It forms part of the old basin deposit of pied- 

 mont gravel, silt, and clay which was uplifted 1,000 to 2,000 feet at the time when the 

 last great fault occurred. 



Two possibilities suggest themselves in regard to the origin of the Intermediate and 

 Tularosa beds of gypsum: The first is obviously that they indicate periods of aridity like 

 the present, and that their relation to one another and to the later gypsum is the same 

 as that of the free and the fixed portions of the White Sands. The other, suggested by 

 Mr. Free, is that the older dunes were formed as a narrow strip on the immediate edges 

 of a lake wMch was in gradual process of desiccation. As the lake retreated it was always 

 bordered by a strip a mile or two wide where it had laid down gypsum. From this a 

 narrow band of dunes was formed. The dunes quickly became fixed by vegetation, since 

 the climate, as indicated by the size of the lake, was moister than now. Thereafter they 

 remained unchanged except for the normal processes of weathering. On the lakeward 

 side new dunes were continually in process of formation, followed by fixation, thus con- 

 tinually broadening the dune area in proportion to the retreat of the lake. Inasmuch as 

 we can not follow the Intermediate and Tularosa gypsums under the White Sands we 

 can not tell whether they extend as far as the borders of the modern playa, nor can we 

 ascertain whether in the interval between their formation the lake expanded. Hence we 

 can not choose between the two theories. The fact that the two divisions of the White 

 Sands apparently represent distinct pulsations rather than pauses in climatic change affords 

 a presumption that the same is true in the other cases. 



CONCLUSION. 



The general conclusion of the whole may be summed up in the words of IVIr. Free in 

 the report already referred to: 



"The whole history of Lake Otero and of the period since its disappearance is a record of 

 great and continuous climatic changes, with major fluctuations indicated by the variations of 

 the great ancient lake and its deposits. On these fluctuations are superposed many series of 

 minor pulsations, the greater of which can be read in the triple record of changing topography 

 in lake, dunes, and arroyos. To assign a time scale to these various changes and to date them 

 in years or centuries is not easy, but it is probable that something can be done by careful com- 

 parative study of various lines of evidence and of various regions. In general it can be said that 

 the Otero Basin shows the kind of climatic fluctuations which Huntington's work has shown to 

 be typical, namely, large, long-period pulsations, upon which are superposed series after series 

 of smaller pulsations of less and less amplitude and shorter and shorter period." 



