ABSTRACTS OF TAPERS 63 



RANGE AND RHYTHMIC ACTION OF SAND-BLAST EROSION PROM STUDIES IN 



THE LIBYAN DESERT 



BY WILLIAM H. IIOBBS 



(Ahstract) 



Although rock debris of a coarseness usually designated as sand is during 

 sand storms elevated by the wind to considerable though as yet undetermined 

 heights, there is evidence that the effective action of the sand-blast is limited 

 to a zone extending from the surface to a height of a few feet only. It is this 

 limited range which explains the characteristic mushroom rocks as well as the 

 luisymmetrical low ridges, with steep windward and flat leeward slopes, which 

 are found in many desert regions. Within the zone of effective action of the 

 blast the air is given a rliythmic motion which accounts not only for the ripple- 

 marks of dunes, but for a peculiar "ruffling" of the polished rock surface 

 strikingly similar tu the ruflied surface of a Inllow of water. 



Presented by title in tlie absence of tlie author. 



CORRASIVE EFFICIENCY OF NATURAL SAND-BLAST 

 BY CHARLES KEYES 



(Abstract) 



Tn the analysis of the effects of the erosive processes under tlie stimulus of 

 aridity sharp distinctions are to be drawn between those products whicli are 

 the result of weathering alone, those which are due to the transportative 

 capacities of the winds, and those which are strictly corrasive in character or 

 originate through natural sand-blast action. 



The potency of natural sand-blast action is rendered particularly impressive 

 by recent engineering difliculties imposed by blowing sands that in various 

 parts of the world have had to be overcome. Since man has now entered 

 vigorously and successfully on the conquest of the desert, whicli occupies more 

 than one-fifth of the entire land surface of the globe, these difliculties iii the 

 arid regions multiply amazingly. The precautions taken to master them have 

 an important geologic bearing. 



As is well known, the small sand-jet driven by compressed air is one of the 

 most efficient abrading tools at the service of man. In nature, also, there is a 

 near approach to the artificial sand-blast in the action of the rapidly propelled 

 sands of the desert. The extent of this action in arid lands attracts small 

 attention until it begins to interfere with human plans and works. Any geo- 

 logical effects that the power may have are largely obscured until, by the 

 elimination of the influences of the attendant powers, it is possible to iiuaiiti- 

 lativcly measure them. 



Wiien attention is p:irticularly directed to the i>lienonienoii, I lie abiiuiing 

 fffect of wind-blown sands, especially in arid regions, is shown in ni:iiiy ways. 

 Glass windows of iiniiscs on the windward side of sand-wastes soon lose their 

 transi)aiTii('y liy tlu- conslaiit play of llic sands against them. The rapidity 

 witli wliicli tlie process takes itlace is indicated by the laMtcrii lenses of liglit- 

 hous'es being rendered useless by the action of wlnddrixcn siinds during a 

 single gale. When r.-irernlly I'xamincd. the e.xposed sides ot' tiu> irons of desert 



