This province, a fairly large one and important from 

 an ecological point of view, is the least well defined on the 

 continent. Yet it forms a distinct unit. It is basically a desert 

 province and is duly ringed with scrublands; however, 

 these are of the South Scrub Belt only, for the following reason. 

 The northern boundary of the province is formed by the steep 

 southern-facing slopes of the South Montane Block's uplands, 

 but the northeastern frontier consists of a thin belt of high 

 mountains. These reach from the Estancia Valley to the 

 escarpment that forms the southern rim of the Prairie Belt on 

 the Stockton and Edwards Plateaus. We passed through that 

 narrow belt on our way northwest from the East Chaparral to 



the South Montane Block (see Chapter 15, where we described 

 this as a meeting place of three provinces and somewhat 

 distinct from all of them). The narrow eastern boundary of this 

 province is the eastern edge of the Big Bend district of the 

 Rio Grande valley. 



The equally narrow western boundary of this province is 

 drawn arbitrarily across the South Scrub Belt between the 

 Arizona block of the South Montane Province and the north- 

 western tip of the Sierra Occidental. The rest of the province 

 forms a long tongue extending to the southeast into the central 

 highlands of Mexico between the Sierras Occidental and 

 Oriental. As these are clothed in subtropical montane 

 vegetation and are ringed by orchardbush and the Savannah 

 Belt, we exclude them here, and draw the periphery of the 

 Chihuahua Province along the line between them and the edge 

 of the South Scrub Belt. 



The flora and fauna of this province are homogeneous 

 throughout — typical desert forms centrally and indigenous 

 scrubland forms peripherally. The latter may extend to 

 considerable altitudes up the inner faces of the Sierras, due 

 both to the aridity of the central upland plateau and also to the 

 following fact: Under absolutely theoretical conditions, the 

 Desert and South Scrub Belts should run from east to west 

 between about 15 degrees and 25 degrees north latitude. On the 

 west side of this continent, as we have repeatedly pointed out. 

 they are pushed far north. However, here, between those 

 latitudes, they have a chance to assert themselves, and, aided b} 

 the exceptional aridity, they perform what is called a major 

 "re-entrance" (from their normal position toward the south, in 

 the form of this long, tonguelike projection). 



The province is almost 1000 miles long from northwest to 

 southeast, 500 miles wide at its northern end, and about 

 250 miles wide between the Sierras, narrowing to less than 

 100 miles in the south. The northern part is on an average 

 about 3500 feet above sea level, the tongue some 6000 feet high. 



However, along either side of the valley it remained as a hori- 

 zontal bed, running along the upper faces of the mountains. As 

 rains continued to fall upon these mountains, the gypsum was 

 washed down to the floor of the valley, along with detritus from 

 the other strata. 



At one point on the valley floor there happened to be a small 

 lake, and in this the gypsum accumulated in solution. Then, 

 during extended dry periods between the rains, this lake under- 

 went excessive evaporation and large, platelike crystals of a 

 glassy, transparent material called selenite grew around its 

 margin. Next, hot and powerful winds blew over these and broke 

 them up into little grains, which rolled away over the floor of 

 the valley. These grains were angular and were also heavier, 

 though smaller, than the other sand grains thereabouts, so that 

 they became segregated and formed dunes, while smaller-grained 

 material was blown onto the plain beyond. 



This process has been going on here for thousands of years. 

 The result is a monumental pile of glistening, pure white sand. 

 in dunes that may reach a height of sixty feet, extending in a 

 belt some five miles wide for some twenty-five miles to the 

 northwest of little Lake Lucero. These dunes are perpetually on 

 the move, their grains being blown up their gentle windward 

 slopes by strong winds and then falling over their sharp crests. 

 The dunes sometimes crawl over each other and there are lesser 



dunes on greater ones. Their windward faces may be covered 

 with beautiful ripple marks. 



We have mentioned many dunes before but most of these 

 were either fully vegetated or supported at least some plants. It 

 so happens that in the Tularosa Valley there are no such plants 

 available. Although the highly alkaline floor of the valley sup- 

 ports stunted growth which can use the subsurface water that is 

 highly charged with gypsum in solution, there is no vegetation 

 here that can root in and live on pure gypseous sand. However, 

 seven kinds of plants have by a most novel method partially 

 adapted themselves to these singular conditions. They grow 

 normally in the soil of the plain, but if a dune approaches them — 

 and the movement of a dune is irresistible — they somehow man- 

 age to put on a spurt of growth that just equalizes the rate at 

 which the dune piles up, thus keeping their heads above the 

 sand. Botanists have been amazed to find that some of these 

 plants had roots going down forty feet. 



The dunes themselves are of great beauty. They also have a 

 quality that I can describe only as one of friendly eeriness. As 

 you stand among them, especially when only the tops of the 

 greater mountains far to the west and east can be seen over the 

 larger dunes, you can readily imagine that you are on another 

 planet. Yet, despite the total absence of anything familiar, the 

 place is somehow not an unfriendly one. Around the edges of 



270 



