J. T. Jut son — Sheet-flows or Sheet-floods. 437 



which is yet absolutely smooth and unfurrowed, 3 forms 

 one of the most striking features of the country. 



The rills of water have no power to directly move for- 

 ward the angular fragments, of various sizes, of quartz, 

 jasper and ironstone, which are frequently spread over 

 the surface of the ground. 



On soil-covered gentle slopes, these rills of water help 

 to form minute soil-terraces from 1 to 6 inches in height. 

 These terraces are primarily due to the occurrence of a 

 surface film of soil, more compacted than the portions 

 beneath, owing to the deposition of material on evapora- 

 tion of water* brought to the surface by capillary attrac- 

 tion. 



2. Smooth-bottomed valley type.— This type occurs in 

 wide flat-bottomed valleys which generally have a mod- 

 erate amount of vegetation in the form of small trees, 

 and large and small shrubs. These valleys possess a 

 fairly well-marked drainage line along their floors, by 

 means of one or more channels, which are from 6 to 20 

 feet wide, are usually not more than 4 or 6 feet deep, and 

 are almost wholly cut through detritus. It is seldom 

 that there is but one channel, although there may be a 

 main one, for the tendency is to form several, which, how- 

 ever, frequently die out, unite and reform. In other 

 words the drainage line is "braided." 



These channels occur in wide, smooth, level, soil- 

 covered flats, which consist of detritus (chiefly fine), 

 brought down by the water and spread evenly over the 

 floor of the valley, the long gently sloping sides of which 

 rise gradually from such floor. As the valley is followed 

 downstream the flats become wider and wider, and the 

 valley sides less and less pronounced, until practically a 

 slightly undulating plain results, often without distinct 

 drainage lines. 



The channels of the better defined portions of the val- 

 ley are the main carriers of the water. The latter comes 

 from the hillier country, where of course in the narrow 

 valleys the erosive power of the water is great, also from 

 the numerous rills, which form the first type of sheet- 

 flow; and it naturally collects in and follows the chan- 

 nels. The rains need not be very heavy or lasting before 

 the water overflows into a broad, shallow sheet, which 



3 If the country be of the sandy loam or loamy sand type with abundant 

 ' ' mulga ' ; scrub much of the water will rapidly soak into the ground. 



