SCIENCE IN MAN’S STRUGGLE ON ARID LANDS 31 
slowly with the wind and gets piled into a variety of deposits. An 
exposed soil surface continues to lose fine soil until bedrock or 
hardpan has been uncovered; or until enough stones have been 
exposed to form a protective desert or erosion pavement. 
Fine soil material exposed to the forces of running water, in- 
cluding the infrequent but torrent-producing storms of the desert, 
is carried down to the quiet reaches of the streams. Catastrophic 
erosion with cutting and down-grading of whole regions can be 
initiated by changes in the grade of streams or by some weakening 
of the plant cover. Examples include earth movements that raise 
the land above the normal grade line of the streams. The grade 
of a stream may also be changed by cutting through one layer of 
rock into another softer one or by earth changes that shorten its 
course. The straightening of well-graded winding streams, for 
example, may so increase the new grade of the stream that down- 
cutting is stimulated through the whole of a drainage basin. Great 
changes in climate toward desert conditions have brought about 
devastating natural erosion. Even showers of hot volcanic ash 
have killed the vegetation and initiated a new erosion cycle; so 
has fire and overuse by grazing animals. 
I should not want to minimize the effects of man in causing in- 
stability of soils. But the tendency has often been to overlook 
these natural changes and potential changes and to assume that 
all active erosion has been caused by overgrazing or other wrong 
land use. Yet where the erosion is in fact due to other causes, 
grazing control or other simple soil management practices cannot 
stop it. 
Many of the predictions about the stability of landscapes have 
come from historical research. Such research can be most helpful, 
but only if done thoroughly. One can be led into serious error by 
leaving out some of the critical factors. For example, some old 
cultural ruins around the Mediterranean and in the Near East, 
now covered with sand or erosion debris, were not in agricultural 
regions. The original towns or forts were established primarily as 
military posts and to serve transport routes. Although some agri- 
culture, or rather gardening, may have been attempted on a small 
scale, the processes of sedimentation that covered some of those 
