IOWA ACADEMY OF SCIENCE 
159 
The utter inadequacy of water-action as an erosive agent in an arid climate 
is made manifest by a number of well-known phenomena. In the desert regions 
of the West that which strikes the scientific traveler with greatest wonder- 
ment is the prodigious amount of erosion which is plainly evidenced on every 
side. At first thought, all is readily ascribable to the action of running water 
just as it naturally would be in the more familiar moist quarters of the globe. 
However, soon the impression is gained that the process of water-sculpture 
must be excessively slow. After a considerable sojourn in the region there may 
be encountered one of those sporadic heavy downpours of rain called “cloud- 
bursts,” and it is at once surmised that the real secret of the vast erosion dis- 
played by the desert is disclosed. Should the sojourner actually live for a few 
years in the dry country, he finds that notwithstanding the numerous evidences 
of fresh, extensive, and vigorous erosion, the “cloud-burst” is too infrequent, 
too local, too ephemeral in its effects, to be seriously considered as an important 
agent of general erosion. By inquiry among the older dwellers of the country, 
and by marshalling together the deductions from his own observations, the 
geographer reaches finally the conclusion that in the desert there is at best far 
too insufficient water to account for even a small fraction of the erosional effects. 
Ample evidences of great erosion there surely are; but as surely no water to 
produce it. 
The verity of the utter impotency of w^ater-erosion to accomplish the prodig- 
ious erosional effects everywhere displayed throughout the dry region is deeply 
impressed upon the mind of every desert dweller. After falling below about 10 
inches annually, rain-waters quickly lose their general corrading powers. 
In the arid region of Western America most of the precipitation takes place in 
the form of light showers which immediately are absorbed by a loose and thirsty 
soil. Only once or twice a year, perhaps, is the rain-fall abundant enough in a 
locality to form any run-off at all. 
Sharply contrasted are whatever erosive effects there are on desert mountain 
and desert plain. Should rain-fall on the former be so much as 10 inches each 
year it is often less than half as much on the latter. Desert mountains are 
usually made up of very hard rocks almost devoid of soil; the desert plains of 
non-resistant materials, with deep, porous soils. On the mountains, the effects 
of torrential stream-action is seemingly often as marked as it is in the high- 
lands of m.oist countries. On the plains evidences of stream-corrasion are in 
reality almost entirely wanting. In its effects and in its vigor even the sheet- 
fiood is quite local, sporadic and relatively unimportant.* * * § Unless the itme ele- 
ment be made excessively long, water as a corrasive agent is utterly inadequate 
to produce the tremendous ariiount of sculpturing which, throughout the desert 
region, is on every hand so apparent. 
The extent, rapidity and vigor of wind-erosion under the favorable climatic 
conditions of aridity have not been, until quite lately, appreciated as fully as they 
seem to merit. Recently in the writing of Passarge*, Walthert, Penck§, and 
others, the great importance of this geologic agent in desert regions is urged. In 
*Bull. Geol. Soc. America, Vol. XIX, p. 81, 1908. 
fZeitschrift d. deutschen geol. Gesellschaft, Ivi Bd., Protokol, p. 108, 1904. 
iDas Gesetz der Wustenbildung, Berlin, 1900. 
§ American Jour. Sci., (4), Vol. XIX, p. 165, 1905. 
