IOWA ACADEMY OF SCIENCE 
139 
Another phase of lake-basins of the arid regions and one that is now thought 
to he mainly formed through agency of the wind is the playa. There are many 
transitions, among these interment basin-plains, from those holding extensive 
and permanent bodies of water to those which are perfectly drained. The 
stage known as the playa is one in which there is a broad expanse of barren, 
silt-covered flats. “Dry-lakes” they are often termed. Waters are only brought 
down once or twice a year from the surrounding mountains. At such times the 
mud-flats are covered by water to depths of a few inches or a foot or two. 
Lakes of the playas are very short-lived and last only a few weeks at a time. 
For nine to ten months out of the year the areas occupied by them are parched 
and dry and support no vegetation whatever. 
Playas and similar mud-flats of the arid plains are areas of great degrada- 
tion as well as of aggradation. It is probable that in the majority of cases the 
first mentioned process lags a little behind the latter. When the waters are 
finally evaporated the playas . are mud-flats in every sense of the word. The 
bottom-mud as it dries curls up into thin leaves a millimeter or two in thick- 
ness. The first strong wind that comes along blows these away as dead 
leaves before the first blast of winter. Much of the material is carried bodily 
out of the playa-area, often a distance of many miles, or gathers in great wind- 
rows about the margins. Much of the dried mud is ground to dust in the 
moving and is carried off in the air as other dusts of the plains. With every 
summer shower that falls over the playa there is a new mud layer formed and 
further exportation. 
When old playas have been exposed in section through recent stream action 
the soft deposits appear, in a number of observed cases at least, to have no very 
great thickness. In the Meadow valley, in southeastern Nevada, for example, 
100 feet beneath the surface of the old playa the hard rock floor of the plains 
appears; ancient limestones, sandstones and shales highly inclined and hori- 
zontally bevelled evenly. The Armagosa river valley of southeastern California 
presents similar phenomena. In the remnants of the old bolson surfaces along 
the Rio Grande there are often displayed the ancient rock-floor surface high 
above the present level of the river channel. 
Many salinas exhibit similar conditions. The great Hueco bolson of central 
New Mexico has already been mentioned. The central flats of the interment 
plains of the arid regions are not then always areas of constant aggradation, as 
has been commonly regarded, but they are areas of most rapid degradation 
as well. 
Eojian erosion, even in dry regions, has been generally overlooked. In the 
arid parts of the globe wind is probably not only the most potent of the grada- 
tion agencies, but its efficiency is greater than all other geologic processes com- 
bined. Its main activity is manifestly degradational in character. The con- 
structional effects are very local. It is now believed that in the arid regions of 
western America the wind has been a levelling agent, the importance of which 
has been little considered. Its general effects in this role has been second 
only to that of normal base-leveling. 
Throughout the dry regions “dust-storms” are violent and frequent. While 
they endure their effects in producing personal discomforture have commonly 
blinded all, even the trained geologist, to their real geologic significance. Dur- 
ing their progress and even for several days afterward the air is so filled with 
