| Show ThA AND f I v IB w remarkable le san sand dunes of ot GIs Peruvian desert have attracted the attention of ot e every ery ry visitor to this arid region The striking re Le of or these formations Is that slowly but constantly shift their position always In the same direction In the Geographical Journal of ot London W. W S. S Barclay thus describes and exilus ex- ex ilus these traveling tra sand saml dunes Although the majority of the dunes are to be found In the western half halt of ot oft the Pampa the weatherIng weatherIng weatherIng weather- weather t Ing of the conglomerate bed from which the they take their origin Is noticeable noticeable no no- elsewhere along the Peruvian t coast One of my most vivid recollections lection on the tle steamer VO voyage along the desolate coast from fLom Lima LImu to Mollendo Mollendo Mollendo Mol- Mol lendo Is n a vision against the curly early morning sun of a great flame colored hill bill that seen through the coast fogs fogy presented almost the appearance of a n city on fire lire This flame effect was asI asI as I j jI I afterwards ascertained duo due to sand and dust blowing across Its upper ridges Starting from Mollendo the railway I winds Its way up the deeply eroded flanks of the coast hills The hot air rising from the ilie I desert keeps back or dissolves the clouds which attempt to tomake tomake tomake make their way across the Andean crests to the sea and rain Is reckoned I to fall faU on this coast not more than once in seven seyen years When this occurs however however however how how- ever Its erosive e effect on the tho soft sott loose soil soli Is striking Watercourses 15 to 20 feet deep are cut In a few ew hours One of these rare phenomena had taken taken ta tn ken place a few days before my visit It does not pay the railway when constructing constructing constructing con con- its track to allow openings for these the infrequent cloudbursts and andone andone andone one sees In cons consequence solid soUd earth banks built bunt along the hill hlll flanks damming damming damming dam dam- ming of great depth extending sometimes a mile nille or more on the up up- 81 side de Wind That Forms the Dunes On reaching the Pacific fringe of the Pampa or Desert del Sacramento after after after af af- af- af ter an ascent from the coast of some feet teet one begins to feel the wind which Is responsible for the formation of the dunes The general direction lon of this wind is from the northwest being caused by radiation fr from m the desert surface anI and aud the consequent Inrush inrush in In- rush of cooler air from the Pacific It blows at about 20 miles an hour on the hard flat sur surface between the ridges The wind l starts as soon as the desert warms up and Increases In violence with the rising temperature attaining t Its maximum force between 2 p p. p. p m m. m and sunset The conglomerate bed of this desert P Pampa is chiefly formed of the follow- follow r fm In constituents in order of their Importance Importance Importance Im Im- Quart Quartzite gypsum dl- dl oril with iron hon and crystals and quartz The whole is pack packed d ane and blended b by the gypsum which has run into l to and filled fractures made perpendicularly thus facilitating weathering The tuta is found In beds also In the them i m neighborhood of Arequipa and aud Is cut Into blocks Mocks for tor building purposes The They ring fine when struck like well-burnt well brt kJ The quartzite forms the mala main t m- m lI mass ass Of ft the hills hUll surrounding the desert des des- ert It weather weathers Into a brick redonde on on n Ute tile surface which gives the desert Its Ita tone making the white slaty-white sand noticeable by hy contrast The quartz Is Infrequent nt DIorite Is Interspersed in- in Jn In n the thi conglomerate but In angular r form ho Ung no signs of or water water water wa wa- ter wearing it mu- mu must t t therefore be attributed at at- tr bt ted ed to 01 volcanic nic The ge- ge nests of the sand dun dunes o Is best seen seen on on tho thu Pacific or western i e tera fringe frIno of or the desert where the winds are eating Z Il away the conglomerate ridges possibly possibly bly the remains of an ancient higher level which run out from the quartz quartz- quartzIte Ite hills in a direction at right a angles les lesto to the prevailing winds On the western border of the desert the sand Band remains inchoate but after alter about 15 or or miles rones dunes begin to appear and these the Increase In number number number num num- ber till on nearing the Inland fringe some 25 miles from they can enn cant t be observed la to serried battalions accompanying ac aCe companying the railway Once termed the dunes tate take their l moon half shape and d In La stately sequence southeast a across acres the Pampa until they reach some large obstacle or abrupt change of leveL If U the harder barrier la Is negotiable they adapt then elves to cro cross It the dune pro- pro ecUng itself into the easIest passes asses ad nd ad elongating up to the limit of ot i to Us ts sand mass Once the ol obstacle is s crossed the vanguard lard of ot the dune marks time until the rear catches u up forms once more into a moon half shape nape and proceeds proceed on its way These rhese dunes often attain a II considerable sl size lD measuring up to CO W yards between the points of ot the horns I IThe The march of the dunes is asses assisted by y the sand ripples whose more ex ez exposed exposed posed surfaces are blown onward l by y the he wind The sand climbs the he steep steel back of ot the dune more slowly than the I low 17 salient wings hence the halfmoon halfmoon half half- moon shape Small particles travel at r relatively high s speed d near ground level as at far tar as the projecting points or horns borns but as soon as they reach them they are on the lee of the wind and their farther progress Is la checked until un un- til w the mass of the dune catches up Thus the distance the sand travels travel along the points is dependent on the maximum height of ot the back of the dune Rune and this in turn depends on the d u force of or wind The he stronger the wind the tho higher It will force up the sand particles in a direct right angle to the axis of ot the tho dune I I. I e. e to a line drawn 1 between the horns The shape of any t gl given n dune Is therefore the result of a perfectly graduated balance between the wind force torce and the floor level The railway track in crossing the desert 1 I shows an average rise of ot slightly over I 1 in so that although their shape is is 18 governed o by surface variations the I dunes are arc on the whole marching up- up 1 hill f j Move Yards a Year Owing O to m my short sta stay In the Pam Pam- pa I was not nut able to check the dunes dunes' j I j I rate of ot march by personal observation i i but I was Informed by plate layers layS j I and other railway men whose duties i necessitate watching them that th the rate rote is about yards per annum I The high hills flanking the desert near Arequipa are too steep to allow t the P dunes to proceed farther tarther They there ther therefore fore pile up in a sort of a sand ometery om- om ru at the eastern edge of the Pam Pum pa pat Before they reach the end however however however how how- ever and as the hills deflect the air In different directions dunes o occasionally occasionally occasion occasion- ally advance over or threaten the railway railway railway rail rail- way line The method of avoiding th the threatened obstruction is simple but effective A couple of ot men go out with long-handled long spades and a wheelbarrow row and collect loose pebbles pebble's s and grit from the surface of the Pampa This they proceed to scatter in a thin layer over the back of ot the unfortunate dune The pebbles arrest the action of the ripples and so Interfere with the even circulation of ot the sand particles which Is apparently as essential to the progress progress pro ress of the dune as the circulation of lit blood to a human being Very I ly th the dune assumes a lopsided shapsa shapes shap sa sagging gin where here the debris has been cast casi upon it and offering Instead of a well- well rounded back a breach to the action of the wind The progress of disintegration disintegration gration is fairly rapid and at the end nothing is left on the Pampa except except ex ex- the original wheelbarrow loads o of grit and pebbles which suffice to exercise exercise ex ex- the monster The dunes a advance over advance over pebbles and stones without disturbing their poss tion I noticed immediately behind several large dunes scattered stones that obviously ob had not shifted from their bed just as pebbles may lie pe- pe e- e cure on the seashore In the swell of the breakers Generally speaking the smoother the surface of ot the Pampa and the higher the wind the larger and more perfect becomes the dune dun |
