PREVENTION OF EROSION BY TERRACING. t 39 
The terraces in use in this country are of two general classes, the 
bench terrace and the ridge terrace, each having variations which are 
adapted to particular conditions of topography and soil. 
The true horizontal-bench terrace is not used widely in the United 
States, while the sloping-bench terrace is quite common. The dis- 
advantages of the bench terrace are that it can not be crossed by 
modern farm machinery; the banks can not be cultivated, while each 
- bench must be cultivated as a separate field; weeds and objectionable 
grasses which grow on the banks tend to sow the entire field. It is 
best adapted to slopes too steep to permit the use of any form of 
cultivated terrace, but it can not be recommended for use on slopes 
exceeding 20 per cent. 
The narrow-base level-ridge terrace is used extensively in the Pied- 
mont section of the South. It is cheap to construct and easy to main- 
tain. However, attempts to cultivate this type of terrace have not 
been successful generally; consequently, as in the case of the bench 
terrace, considerable land is lost to cultivation, and the growth of 
weeds and grasses on the embankments tends to seed the entire field 
as well as sap the strength of the adjacent soil. Outside of these 
objections, the narrow-base level-ridge terrace, where heavily sodded, 
renders satisfactory service on pervious soils and slopes not greater 
than 8 per cent. 
The broad-base level-ridge terrace has been developed from at- 
tempts to render cultivable the narrow-base form. It has all the 
advantages of the latter terrace with the added one that no land is 
lost to cultivation. By the use of this terrace little or no soil is re- 
moved from the field. It is best adapted to use on open, pervious 
soils on slopes not exceeding 15 per cent, but under proper conditions 
of design, construction, and maintenance can be used on any soil and 
on slopes somewhat greater than 15 per cent. 
As in the case of the level-ridge terrace, the first graded-ridge ter- 
races were smali with narrow bases, and they are subject to the same 
objections that apply to the narrow-base level-ridge type. More- 
over, the velocity of flow of the water, due to the grade of the terrace, 
tends to erode the upper side of the eiercal to an extent which 
a narrow-base terrace can not withstand. 
The broad-base graded-ridge terrace (the Mangum terrace) has 
been adopted in many parts of the country. This terrace, properly 
constructed, not only can be cultivated but it can be crossed at any 
angle with large farm machinery. Its broad base and flat embank- 
ment slopes render it less hable to damage by the flowing water than 
is the case with the narrow-base type. The grade may be either uni- 
form or variable, but both practice and theory indicate the variable- 
graded terrace to be superior to the uniform-graded type. 
