788 J. BARBELL MEASUREMENTS OF (iEOLOGIC TIME 



several thousand feet in maximum thickness. These limestones, 1 low- 

 er er, show abundantly and recurrently the marks of shallow water aiitl 

 even exposure to the air, hut the poverty of clastic material indicates 

 that either the lands were very low and possibly somewhdt distant, oi- 

 that there was deeper water, with a bottom below wave-base, between the 

 ]ime deposits and the land — -a temporarily unfilled geosynclinal axis east 

 of the present outcrops. 



Although, theoretically, deposits of muds below wave-base settling from 

 suspension in geosynclines should be expected to occur, yet they clearly 

 are not a common type of sediments. They are the only deposits, how- 

 ever, from which a continuous record of deposition may be read, and 

 should be sought from this point of view. 



The discussion of this topic has thus far turned on the factors con- 

 trolling the rate of sedimentation. In the light of this discussion, how 

 does the present rate compare with the rates in the past? There are 

 two world-wide features characteristic of the present which determine 

 the ansAver : One is the present high degree of relief of the earth^s surface, 

 as shown by the contrast of the present and past curves of relief as 

 illustrated by figure 3 and discussed in Part I; the other is the direction 

 of the last movement of sealevel. 



To take up these in order : The present high relief is a feature which 

 means the rapid downwarping of many catchment areas for sediment 

 during the Neocene and Pleistocene. Very thick formations have been 

 made in a short geological time. A few are upturned and exposed, as 

 in the younger Tertiary formations, which make foothill ranges ; for ex- 

 ample, the Siwalik formations of India, 14,000 feet in thickness. Other 

 sites of sedimentation, as intermontane basins, are in many cases stil] 

 somewhat below grade, maintained so by subsidence, and are being 

 rapidly built up. Where no catchment basin is crossed by the rivers 

 flowing from mountains, a large amount of land waste is rapidly sup- 

 plied to the sea, building out deltas or adding to' the continental plat- 

 forms as a submarine terrace of construction. 



To turn to the other factor — the direction of the last movement of 

 the sealevel; the evidence of the shorelines shows that the last stage has 

 for most shores been. one of submergence. Eiver mouths are drowned 

 and transformed into harbors and estuaries ; the sea is cutting vigorously 

 at the headlands of embayed shores. The. motion has been more or less 

 intermittent and there is little evidence that it is now going forward. 

 The evidence around the northwestern shores of Europe indicates, how- 

 ever, that there the last movement of submergence occurred not more 

 than .'>,000 or (\J)Of) ypars a.oro. The generality of recent submergem-e 



