J. T. Jut son — Sheet-flows or Sheet-floods. 439 



flat-bottomed "valley"; or they may be extensive areas 

 without any definitely observed relation to the drainage, 

 except that they are low-lying portions which may com- 

 municate with lakes or which may practically form 

 shallow basins with no outlet. Probably most of these 

 flats possess a certain fall for at least part of their areas, 

 and this fall enables the water, after heavy rain, to flow 

 along in broad thin sheets. 



The absence of definite channels is a marked feature, 

 but in lieu of these, much of the flat may be scored by 

 very numerous furrows averaging perhaps 9 inches in 

 depth and 18 inches or 2 feet in width. These furrows 

 frequently join one another, and as frequently die out, 

 fresh ones taking their places. The absence of trees and 

 tall shrubs and the presence of the salt bush and other 

 small shrubs, mostly not more than 2 feet high, together 



Fig. 2. 



Fig. 2. Cross section illustrating furrowed-floor type of sheet-flow. 

 Bedrock below with fine detritus containing occasional pebbles above. 



with the covering of detritus of sands and muds, facili- 

 tate the formation of these furrows. The water sweeps 

 and swishes around and among the small shrubs, scour- 

 ing the detritus out to form furrows, and depositing it 

 around these shrubs until they may be half buried. The 

 furrows must be ever changing as the ever varying small 

 streams gouge out in one place and fill up in another. 4 



The irregular distribution and heaping up of the detri- 

 tus enable, on the cessation of the rain, numerous small 

 pools to be formed, the water of which soon soaks into 

 the ground or evaporates. The net result of much of the 

 water action seems to be merely the redistribution of the 

 detritus over much of the area, although rock waste must, 

 of course, be continually brought from the higher por- 

 tions of the country. Much of the fine waste, however, 

 is no doubt removed by the wind. 



Perth, Western Australia. 



4 Wind action has probably some effect in both the formation and 

 removal of these hillocks. The latter are comprised in the "Neulinge" 

 of Walther. See "Das Gesetz der Wustenbildung, " 2d Ed., pp. 70-71, 

 1912. ^ 



