657 - 
Photo 3 
Fourier spectra of the dune area depicted in photo 2: (a) unfiltered 
(see photo 2); (b) with the dominant oblique direction filtered out 
(see photo 4); (c) with the horizontal direction filtered out (see photo 5). 
Photo 2 represents an unfiltered picture of the area investigated showing the main 
characteristics of the dune pattern. The Fourier spectrum of photo 3a clearly 
reflects the general dune structure just described and particularly the dominance 
of the oblique dunes stretching obliquely over the picture. This dominant dune 
direction was subsequently eliminated by using two overlapping wedge-shaped 
"pie-slice" filters. Only then was it evident that dune ridges stretching horizon 
tally across the picture are also of considerable importance (photo 4). They re 
present dune ridges formed under the influence of winds that occasionally blow 
from top to bottom across the picture. The corresponding Fourier spectrum of 
photo 3b shows that the regularity of the spacing of the horizontal dune ridges is 
insufficient for spatial analysis of the pattern. Photo 5 has been obtained after 
elimination of the horizontal dune direction. The oblique direction, making an 
angle of about 25 degrees with the former, is now strongly dominant. The related 
Fourier spectrum of photo 3c clearly shows the effect of the wedge-shaped filter 
on the diffraction pattern. 
The oblique dunes are not perpendicular to the large longitudinal ridges in the area 
of study, but are approximately so in nearby parts. The pattern of dunes is be 
lieved to be caused by dominant oblique winds alternating with less frequent winds 
blowing vertically across the picture. The longitudinal ridges owe their irregular 
ities to this alternance. They may predate the transverse ridges but this needs 
further confirmation.