22 



BULLETIN 512, U. S. DEPARTMENT OF AGRICULTURE. 



If the terrace has a fall greater than one-half foot per 100 feet, 

 erosion occurs above the terrace, a channel is scoured out and it 

 develops practically into a hillside ditch. Although this type of 

 terrace is used extensively in the Piedmont region of the South, it 

 is being supplanted rapidly by forms of the broad, cultivated terrace. 

 The hroad-hase form. — The broad-base graded terrace, generally 

 known as the Mangum terrace, has been adopted in many sections of 

 the country for the reason that the entire terrace bank can be culti- 

 vated — ^thus utilizing all land and preventing growth of objection- 

 able weeds and grass. This terrace can be crossed readily at any 

 angle in planting and cultivating crops with large farm machinery. 

 When it is intended to use such machinery and to cross at an angle, 

 the terrace must be made broader than when all farming operations 

 are in lines parallel with the terrace. The following tabulated values 

 are the results of surveys of terraced fields of the Mangum type near 

 Wake Forest, N. C. : 



Actual dimensions of Mangum 'broad-hase graded terraces. 



Dimension. 



Base width of terrace feet. . 



Height of terrace feet. . 



Vertical distance between terraces feet. . 



Length of terrace feet. . 



Grade of terrace per cent. . 



Slope of land surface per cent. . 



Absolute 

 minimum. 



25 

 .3 

 2 

 450 

 1.69 

 4.7 



Absolute 

 maximum. 



50 

 1 



8.9 

 1,250 



2.24 



18.2 



Field averages. 



Minimum. Maximum. 



30 

 .5 

 2.8 



33 

 .6 



7.7 



2.1 

 14 



These fields appeared as series of broad waves. On the steepest 

 slopes, where one terrace slope ends the next one begins, the whole 

 field being a succession of terraces. (See fig. 2, E and F.) The rows 

 were run parallel with the terraces shown in figure 2 E; on a less 

 steep slope (fig. 2 F) the rows crossed the terraces. 



Surveys were made also of a number of fields with graded terraces 

 where the crop rows always were parallel with the terraces. The 

 results obtained are shown in the following table : 



Actual dimensions of hroad-hase terraces wnere rows arc parallel iDtth terraces. 



Dimension. 



Base width of terrace feet. . 



Height of terrace feet . . 



Vertical distance between terraces feet. . 



Length of terrace foct. . 



Grade of terrace per cent. . 



Sloi)e of land surface per cent. . 



Absolute 

 minimum. 



.5.2 

 .4 



2 

 200 

 .1 



3.4 



Absolute 

 maximum. 



19 

 1.8 

 9.2 

 1,800 

 2.24 



20 



Field averages. 



Minimum. Maximum. 



7.8 



.7 



2.9 



.3 



4,4 



1.5.5 

 1.1 

 8.3 



