Fluviatile and Marine Gravels. 507 



onsly expanding platform. The thickness and width of the 

 gravel deposit, as well as its final place of deposit, varies with 

 the stage reached in the cycle. Gravel below the depth of 

 wave agitation at the beginning of a cycle may be indefinitely 

 preserved. Gravels formed in youth will constitute a thick 

 but narrow vertical wedge comparable to the accumulation of 

 talus and further distinguished by the unworn character of its 

 materials. The deposits of maturity will be thin and limited 

 in extent to the area of the platform. 



If this analysis is correct, the evidence of a rising sea pre- 

 served in the sedimentary record will be an unconformity 

 between older rock and marine sediments. Conglomerate may 

 be absent, but in most cases a basal conglomerate passing down 

 dip slopes into sandstone and overlain by liner marine sedi- 

 ments will mark the ancient shore. The conglomerate may 

 form a narrow thick band or a widespread sheet of variable 

 thickness. 



As regards maximum thickness the conclusion of Barrell is 

 affirmed : " marine conglomerates, except under local and excep- 

 tional circumstances, [are] limited to considerably less than 

 100 feet."* 



Effect of Coastal Upwarp. — Up warp of a coastal belt or its 

 equivalent, a sinking sea level, gives to the coast a simple con- 

 tour with swinging curves whose shape is determined by the 

 original inequalities of the shore. The new shore profile pre- 

 sented to the waves is in general over-flat. Under these con- 

 ditions the forces of the sea tend to crowd gravel landward, 

 and if no other agents were at work a series of abandoned 

 gravel or sand beaches would mark the retreat of the water. 

 The depth of the stranded marine gravel obviously equals the 

 thickness of deposits on the beach slope. In the youth of the 

 newly inaugurated cycle, particularly if the sea level sinks rap- 

 idly and intermittently, beaches may be preserved and notches 

 cut by waves may be present as terrace fronts. The new-born 

 subaerial landscape would therefore be characterized by ridges 

 of gravel and well-marked steps or terraces in approximate 

 parallelism to the shore line and at a high angle to general 

 surface slope. In the absence of subaerial erosion and depo- 

 sition the land surface would be veneered with a super- 

 ficial sheet of marine gravels of great durability because highly 

 porous and composed of resistant materials. On most coasts, 

 however, rivers are at work and their extension over the 

 exposed shore results in the redistribution of beach material 

 and the accumulation of land sediments. The abandoned shore 

 becomes a coastal plain, — a youthful landscape on which, if the 

 climate be humid, streams and lakes and later swamps will de- 

 *Bull. Geol. Soc. Am., 1908, p. 620, abstract. 



