RHYTHMS IN SEDIMENTATION 795 



makes for a slow rate of a'ccumiilation of a formation, associated with 

 a more rapid ra.te of accmiiidation of the individual beds. 



NATURE OF STRATIGRAPHW RECORD RESULTING FROM COMPOSITE 



RHYTHMS 



The making of a sedimentary series is conditioned on an oscillating but 

 progressive rise of j^aselovel. Xature is not so even in her course as to 

 give truh" harmonic motion, hut in a theoretical discussion that may be as- 

 sumed as the simplest form, and it will be shown that it does not exagger- 

 ate the amount of time whicli is represented by the intervals. In this 

 type of oscillation the vertical motion slows gradually to nothing at the 

 limits and is most rapid at the middle of the cycle. But nature pulsates 

 with many rhythms, small and large, fast and slow. Their combination 

 gives a varied curve whicli, if the rhythms are incommensurable in period, 

 may never recur in quite the same combination. How will such composite 

 rhythms affect the stratigraphic record ? In figure -f a diagram was given 

 which compared for various locations the continuity of tlie record due to 

 a simple rhythm comlhned with continuous warping. Here another aspect 

 is considered. It is represented by a diagram which will serve to visual- 

 ize the nature of the stratigraphic record for one place, as dependent on 

 the relations of ])eriod arul amplitude of the several components of com- 

 posite rhythms. Figure 5 is drawn for that purpose. In this diagram 

 time is measured from left to right and the rise of baselevel is measured 

 in the vertical direction. The curve A A represents a long term rise of 

 baselevel, beginning slowly, gaining in rate, and then slowly ceasing. 

 This is a portion of a harmonic curve, but for this fundamental curve it 

 could not continue except with respect to a sloping axis of coordinates, 

 as otherwise the following downward sinking would destroy tbe -entire 

 series made during the 2:»revious rise. 



Such a general rise of baselevel will be accomplisli^d by diastrophic 

 oscillations. In the Paleozoic these larger oscillations are indicated by 

 the coming and. going ol' the epeiric seas or the shifting of their areas. 

 They were usually so slow that erosion and sedimentation ke])t pace ^xiih 

 them and widely extended deposits were laid down, alternating Avifh dis- 

 conformities. 



In the Pleistocene there has been \)\ contrast a general sinking of base- 

 level, showji by the ejiiergence oi' the hind, but the movement here also 

 has been strongly oscillatory, as shown \)y the drowned river valleys, and. 

 above the sea level, hy the several Plcistoccjie J'ormations, partly of alluvial 

 and estuariiic natmc, pa illy juarijie, which skirt the margin of the con- 

 tinent. In .Afaryland the late Pliocene Lafayette deposits were uplifted 



