PREVENTION OF EROSION BY TERRACING. _ 91 
unable to withstand even a moderate period of drought. The broad- 
base level-ridge terrace tends to correct this by the detention of water 
above the terrace. 
While most upland soils suffer from a lack of moisture, it is true 
that “ worn-out,” impervious clay soils with an impermeable subsoil 
foundation will not permit a ready percolation of surface water. 
With such soils the rain water would coliect on the surface above 
the terrace, if pockets exist, and remain sufficiently long to injure 
plant growth. It is essential in such cases that the ends of the ter- 
races be left open; then if the terraces are laid out absolutely level, 
the water will flow slowly toward the ends by virtue of the higher 
elevation of the water surface midway the length of the terrace. 
The subsoil plow or explosives are used sometimes to loosen up the 
soil above the terrace and thereby increase the amount of percolation. 
A complete system of tile drainage, as before mentioned, forms a 
valuable adjunct to a system of level terraces. 
The testimony of a number of farmers experienced in the use of 
the broad-base level-ridge terrace is, that in dry seasons or periods 
of drought’ they obtained the best crop yields from level-terraced 
fields as compared with adjacent unterraced and graded-terraced 
lands, and that their crops were equally good in seasons of abundant 
mama ec 
_ The broad-base level-ridge terrace is best adapted for use on open, 
pervious soils and on slopes up to 15 per cent. But it can be used 
successfully on any type of soil if the vertical spacings are as shown 
by the curves in figure 7 and means are employed to remove any 
surface water which may collect in depressions above the terrace. 
This terrace is used often on slopes steeper than 15 per cent, where 
such slopes occur in portions of fields having a smaller average slope. 
Wherever used, it is vitally important that proper methods be em- 
ployed and care exercised in the laying out, construction, and main- 
_ tenance of the terrace system. 
THE GRADED-RIDGE TERRACE. 
The narrow-base form.—Like the level terraces, the first graded 
terraces were small, with narrow bases, and the terrace embankments 
were seeded to grass or allowed to grow up in weeds to protect the 
terrace against erosion due to the flow of the water above. The 
narrow-base graded-ridge terrace is built usually from 3 to 6 feet 
wide at the base and from one-half to 1 foot high, with a fall of one- 
half foot to 2 feet in the 100. Some objections to this terrace are, 
the necessity of growing a protective covering on the terrace, the 
erosion along the upper side of the terrace due to the flowing water 
and the failure of the small terrace embankments to withstand the 
water pressure above or the effects of erosion due to overtopping. 
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