PREVEJSTTION OF EKOSION BY TEKEACING. 



39 



The terraces in use in this country are of tAvo 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 small 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 embankment 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 liable 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. 



