800 J. BARBELL MEASUREMENTS OF GEOLOGIC TIME 



taken away than is brought to the region, clo^^'n-scour may take the place 

 of upbuilding for shorter or longer periods, and the break, or diastem, 

 between two similar beds represents an unknown time interval. The 

 crescent form of dunes brings in further local complexities, but these may 

 be eliminated in this discussion. The subject lends itself to diagrammatic 

 illustration Avhicli brings out impressively the relatively large time value 

 of the lost intervals represented by the division planes between beds. 



In figure C) are illustrated the principles wlricli enter into the building 

 and marcliiiig of dunes, but the same reasouiug applies in n general man- 

 ner to all sedimentation where there is alternate fill and sconr. T]ie wind 

 is shown blowing from left to riglit. \ i ]'c>lls and jumps the sand particles 

 up the gentle ])ack slope of the dune and at the crest meets a return edd}'. 

 When the wind is strong the sand is seen suspended in the air just to the 

 lee of the crest and the dune is said to smoke. At the top of the lee slope 

 the sand at the crest is kept by the return eddy on a slope of about 30 



Figure 6. — Diagrammatic Cross-section to illustrate Time Values of true hedding 

 Surfaces hetween Beds of false-'bedded Dune Sands 



degrees or a little more — the angle of repose. The curvature of the wind 

 currents gives broad sweeping surfaces, passing from the lee slope up 

 the flat back of the next dune to its crest. The dune grows by adding 

 successive beds to the lee side, and these are progressively cut out by the 

 scour on the windward side. Where there is a progressive accumulation 

 the down-scour does not cut on the average quite to the base of the dune. 



In the diagram the wave length of the dune is divided into eight stages. 

 A21, B21, C21 are laminae of the same age; but lamina C21 lies above 

 B13 and this in turn above A5. The time value of the breaks represented 

 by the planes of separation is given l)y the time which is taken for a dune 

 to advance one wave length compared to the thickness of the beds spared 

 by the down-scour. Thus, in a vertical section just back of the left-hand 

 dune, time intervals 3, 4; 11, 12; 19, 20 are represented by sediments; 

 the intervening times are represented by tlie breaks. 



The dunes of the great deserts are often several hundred feet in height. 

 As seen in plan, the typical form is the crescentic barchane, the wings of 

 which slope downward. The wind shift'^ in direction, and different years 

 are inarked l)y notably different inaxiiuuiu intensities. The great bulk 

 of sand movement is accomplished dui'ing relatively brief periods of in- 

 tense wiiids. 'I'liese factoi's. as picN'iously noted, mnke the actual struc- 



