February 22, 1900] 



NATURE 



403 



ON DESERT SAND-DUNES BORDERING THE \ 

 NILE DELTA. ^ \ 



'X'HE distribution of desert sand dunes in the neighbourhood 

 -'■ of the Nile Delta is remarkable. They form a fringe to 

 the desert (where the latter supplies a suitable sand), the 

 material being piled up in dune-tracts, or dune-massifs, where- 1 



Fig. t. — Pyramidal Dune. 



ever its flow is locally checked by ground moisture. The water 



seeps up through the sandhill, keeping it moist and compact 



nearly to the surface. The ground plan of the dune-massif has 



little relation to the wind, which, however, 



playing upon the surface, throws it into 



waves. A typical example of the progressive 



development of their form appears to be as 



follows. First, a gently-rounded swell, then 



the lee side becoming rather steeper and the 



summit of the swell no longer central, but 



nearer the lee side : the eddy under the lee 



slope gathers strength and begins to undercut 



the bank of sand, causing it to slip, forming 



a straight cliff. This process continues until 



the eddy has cut back to the summit of the 



growing dune, which is then of nearly equal 



average steepness on windward and on lee 



side. The windward side is, however, a 



smooth curve of compact sand, whereas the 



lee side consists of two portions, the upper a 



straight cliff of loose sand, the lower a curved 



surface of tolerably compact sand. It is 



common to find the central portion of a 



dune showing the fully-developed, the ends 



the embryonic form. Where the eddy cuts 



<lown to a hard bed, the slipping cliff may 



constitute the whole of the lee slope. 



The ratio Length/Height of blown ripples 

 of seashore sand (which the author found 

 to be about l8 : i) holds equally for desert 

 sand. Where, however, the air has an up- 

 ward motion relatively to the surface, the 

 ripples appear to be somewhat steeper, their 

 front is less regular, their crest more nearly 

 central, and they grow to greater amplitude, 

 chiefly by excavation. A tract of small but 

 perfect dunes (formed by the wind blowing upon dried Nile 



nd) was examined in order to see if there were any systematic 



Abstr.nct of a paper read before the Royal Geographical Society, Nov. 29, 

 ,, by Mr. Vaugban Cornish, published in the Geographical Journal, 

 , 1900. The illustrations are xf^pxoAuccAhomihcGeographical Jouriial. 



connection between height and length, such as exists in blown 

 sand ripples. The individual transverse ridges were markedly 

 undulating, and, the low portion of one ridge not corresponding 

 with the low portions of the succeeding ridges, it was evident 

 that there was no such simple relation of Length/Height as in 

 the case of ripples. A line, however, having been marked out 

 in the up-and-down-wind direction across twenty three of the 

 small dunes, it was found that the average 

 ratio Length/Height along the line was 

 identical with that of the ripples. Wherever 

 a depression occurs, the wind is concen- 

 trated, and the depression tends to in- 

 crease. On the other hand, wherever the 

 amplitude is slightly greater, the eddy is 

 stronger, and increases the amplitude. 

 The measurements show that in these 

 dunes the two processes proceeded at the 

 same rate. Although in this sense equal 

 and opposite, the two processes combine 

 to produce modification of form. Thus, 

 the sweeping round of the sand from two 

 depressions leaves the bare ground on lee- 

 ward of the intervening part of the ridge 

 surrounded by sands of such shape that the 

 pit looks like the hoof- print of a giant 

 horse. These are the structures known as 

 Fuljes, the origin of which has been much 

 discus.sed. If the wind continue with di- 

 minishing supply of sand, the crescentic 

 dune called Barchan would remain, the 

 windward slope of the Fulj being the lee 

 slope of the Barchan. Barchans, however, 

 are not necessarily, nor usually, formed in 

 that manner. 



The author describes the operations ot 

 the Suez Canal Company in planting 

 Casuarina trees to arrest the drift of sand 

 which at present finds its way into the 

 canal. The Casuarina does not require 

 rain, and its roots are capable of drawing moisture from a con- 

 siderable depth. Indifferent as it is to drought, it can endure 

 with equal indifference an excess of water at its roots, an 



important matter in the Government plantations on the west 

 of the Delta, where periodical inundation has to be reckoned 

 with. The Casuarina grows rapidly, and at Ismailia has 

 attained a height of nearly sixty feet in twenty-five years. The 

 foliage is light and feathery, waving confusedly and cheating 



t 



NO. 1582, VOL. 61] 



