Waves of sand and snow and the eddies which make them . es of thedimensions of small gravel, ultimately collects ina sheet, which covers and completely protects theupper part of the ripples. The completeness ofthis protection depends upon the circumstance thatindividual particles of coarsest sand or very finegravel lie wholly above the general surface and,being thus exposed to the horizontal pressure ofthe wind, roll freely, whereas when they arecollected in a sheet the wind glides ineffectivelyover their surface. The rate of their subsidence,or fall, in air is, moreover, such that it wouldnee
Waves of sand and snow and the eddies which make them . es of thedimensions of small gravel, ultimately collects ina sheet, which covers and completely protects theupper part of the ripples. The completeness ofthis protection depends upon the circumstance thatindividual particles of coarsest sand or very finegravel lie wholly above the general surface and,being thus exposed to the horizontal pressure ofthe wind, roll freely, whereas when they arecollected in a sheet the wind glides ineffectivelyover their surface. The rate of their subsidence,or fall, in air is, moreover, such that it wouldneed a very strong eddy to suspend them. Thuswhen this sheet of coarse sand, or lag gravel,has been formed it protects the sand below it andthe ripples grow by deepening of their interveningtroughs. These are dug out in sand which hasbecome somewhat compact owing to superincum-bent pressure. This kind of erosion-ripple not ?nsjj»,—:y - ?jafci—^ T S Plate ii.—Aeolian s^and-ripples at Soutlibourne, in wiiich the sandgrains have been much Plate ii.—Aeolian sand-ripples at Ismailia to leeward ot a sand-dunewhere there is an upward current of air. 85 AEOLIAN SAND-RIPPLES 87 only attains a greater wave-length than that ofripples formed in quite loose sand, but a greatersteepness also. Thus on the nearly flat top of asandhill near Lake Timsah, in Egypt, I measuredsuch ripples with a wave-length of 7 feet 2 inchesand a height of 6 inches, being a ratio of lengthto height of 14* 3. This kind of rippling is quicklyproduced in positions where the supply of driftingsand is small and the wind is swirling lower photograph of Plate XL shows thiserosion rippling in such a position to leeward ofa sand-dune at Abu Racan, near Ismailia. Thecrest of the ripple is more nearly in the middlethan is the case with the ripples in quite loosesand, and this form suggests that these erosionripples were almost or quite stationary. The fact that there is a sorting of sand-grainsdur
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Keywords: ., bookcentury1900, bookdecade1910, bookidwavesofs, booksubjectwaves
