RHYTHMS IN -.^EDlMExXTATlO^N 789 



as the latest phase suggests that it is a nioAemeiit of sealevel. At a 

 slightl}' earlier time, the retreat of tlie great glaeieis from the last 

 Pleistocene advaiiee lias added between 100 and 200 feet of water to the 

 sea and would appear to be an important factor in. the general snb- 

 rnergence of coral islands to a depth of 20 to 30 fathoms. 



These movements liave resulted in the recent depression of the sur- 

 faces of sedimentation in deltas and the bordering seas and a correspond- 

 ing acceleration in upbuilding. Estuaries and tidal bays such as the 

 Persian Gulf, the Adriatic and Yellow seas^ are being filled ; coral growth 

 has received a great expansion; for the time being the rate of sedimenta- 

 tion approaclies the rate of supply of waste. The lack of recognition of 

 tlie variables nature of these conditions has permitted geologic thought 

 to project them backward through past time as the normal procedure. 

 It is seen-/ -however^ that before geologic retrospection can be sound it 

 must recognize in the principles of perspective the markedly cyclic or 

 rhythmic nature of geologic activities^ both in denudation and in deposi- 

 tion, and the acceleration of both these processes at the present time, 

 owing to the concurrence of several controlling factors. 



PROGRESSIVE TILTING COMBINED WITH RHYTHMIC OSCILLATION OF 



BASE LEV EL 



Sediments are deposited either in geosynclines, as prisms thickening 

 toward the axis, or as thinner, lenslike formations in shallow down- 

 warped basins, or on the contiiiental sholves in the form of wedges 

 thickening seaward. In any case room for further sediment is made by 

 down warping. On the slope of the depressed area this is expressed as 

 a tilting of the floor about an axis. On the landward side there is uplift 

 and erosion; on the seaward side depression and deposition. Thi^ tilting 

 of the floor about an axis is, then, a general phenomenon of diastrophic 

 origin connected with sedimentation and is expressed by the thinning 

 down and dropping out of stratigraphic members when folloAved toward 

 the limits of the formation. 



The tilting, like all other crust movements, is discontinuous. Pulses 

 of erosion occur on the landward side. On the seaward side of the axis, 

 by depression of the baselevel, room is made more or less simultaneously 

 for more sediment. These rhythmic pulses, provided that sedimentation 

 is controlled by baselevel, will result in divisions in the stratigraphic series 

 separated by breaks, A'isible or invisible. 



At the same time that the floor is being warped, there are, owing to 

 otlier causes, rhythmic elevations and depressions in the baseleAcl Avith- 

 ont lilting. For example, the rhythm of the seasons causes J'iver aciion 



