. Bulletin of the Geological Society of America. Geology. 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
. Bulletin of the Geological Society of America. Geology. 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
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1890
