308 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 8 



One other major work of reconstruction is needed, though it is being 

 taken in hand on a large scale in the United States, and that is the 

 establishment of shelterbelts to break up the fierce winds that rage 

 over these open, treeless spaces. It is not an easy matter to get trees 

 started, especially in the north in Saskatchewan and Alberta, where 

 the range of species that will stand up to the extremes of climate is 

 limited, but a certain number of suitable species have been found. 

 Besides the regional planting of cross-country belts, farmers are being 

 taught to protect their own holdings. 



So far I have only been describing to you cases where the eroding 

 agent is the wind on the flat plains, but erosion by rain is even more 

 common wherever the cultivated land is set on any slope and the 

 rains are heavy. The danger does not lie in a large annual rainfall; 

 some of the worst destruction is wrought in regions where a low total 

 is concentrated into short intense spells. A high rainfall will, as a 

 rule, generate such a forest vegetation as will protect the soil, though 

 as tea planters in Ceylon and Assam have found to their cost, any 

 piece of cultivated soil on a slope is in danger of erosion, sometimes a 

 slow continuous removal of the good soil, sheet erosion, sometimes a 

 catastrophic wash-out by gullying. Preventive measures are now 

 well known; terrace the worst slopes and cultivate along the contour 

 lines so as to avoid setting up watercourses. The object is to get the 

 soil to absorb the rain as it falls without giving it a chance to set up a 

 run over the surface, but in bad cases it may be necessary to break 

 the contour terraces at intervals with spillways which lead accumu- 

 lated water into an unobstructed drain or watercourse. Alternatively 

 where washing is severe, occasional belts of unplowed vegetation 

 should be left, in order to break any run-off that may have been set 

 up. By such grading of the slope, coupled with the growth between 

 the bushes of a temporary crop of some leguminous plant, which can 

 be dug in so as to add to the stock both of nitrogen and humus, the 

 tea planters of Ceylon have been able to check soil erosion even with 

 the excessive rainfall that often prevails there. It should be remem- 

 bered that such a system of contour terracing has been practiced in 

 China from time immemorial. 



But wherever torrential rains occur there is always the danger of 

 starting gullies in bare soil. We can see the process operating in 

 this country when rain falls on any spoil bank or bare cutting; as 

 soon as the rate of fall becomes sufficient to cause an actual flow of 

 water at any spot, it acquires an excavating power and begins to cut 

 a channel in the loose earth, a gully which not only enlarges itself 

 by the undercutting of its banks but tends to eat its way backward. 

 Gullying can be initiated either by careless management of culti- 

 vated soil, or on grassland by overgrazing, which bares the surface. 

 Even before actual overgrazing sets in, both cattle and sheep will 



