778 .1 . BARR.ELL MBASTJREMENTS OF GEOLOGIC TIME 



the mollusks of the deeper waters, by cuiiti-asi, do not have to withshand 

 the mechanical agitation of waves, arc not so vigorously attacked by ene- 

 mies, and are oliai-acterized ]>>■ more fragile shells. T/imestone beds, 

 furthermore, as in the (Carbon iferons, ar(!. often intercalatorl between 

 shalloA^-water shales and beds of coal. 



The great bnlk of sediments whicli have been uplifted and exposed to 

 observation were deposited in interior contineirtal seas; only as emerged 

 coastal plains on the onter rims of the continents are the records of shelf 

 seas found. These belong especially to the Cretaceous and Tertiary 

 periods, but they also are nearly always of shallow-water origin. Only 

 exceptionally, by profound orogenic movements, have the deposits of the 

 abyssal fore-set slopes of the continents been elevated and exposed by 

 erosion, and still more exceptionally are the bottom-set muds of the ocean 

 depths heaved upward to the light of day. 



Thus the sediments ^\hose interpretation, form the basis of earth history 

 bave been characteristically deposited with respect to a Jiearly horizontal 

 controlling surface. This surface of control is baselevel, but for conti- 

 nental and marine deposits the baselevel is determined by different agen- 

 cies and is a word of more inelusive content than the sense in which it 

 has generally been used by physiographers as a level limiting the depth, 

 of fluviatile erosion. Sedimentation as well as erosion is controlled by 

 baselevel, and baselevel, local or regional, is that surface toward which 

 the external forces strive, the surface at wbich neither erosion iior sedi- 

 mentation takes place. 



This emphasis on the importance of ^\'ave-basc, as the surface which 

 controls the deposition of sediment in water bodies, makes it desirable 

 to discuss its depth with respect to the water surface. Wave-base is that 

 depth at which the wave actio]i <:-eases to be strong enough to transport 

 sediment. Tf the Avave action becomes strong the particles of sediment 

 are moved to and fro, upward and clowmward, with the oscillation of each 

 passing Avave. In each lift from the bottom the sediment is subjected to 

 progressive movement by the undertow in one direction,- by currents in 

 various direction. X by the dragging effect of waves of translation in a 

 landward direction. AYitli a lesser strength of wave action the particles 

 are lifted less frequenUy and mostly during the forward pulses of waves 

 of translation. An increase in the strength of wave action leads "to a 

 down-scouring of the bottom and removal of material to deeper water, 

 especially the finer materiaL The coarsest, how^ever, mav be shifted 

 toward the shore as sand or shingle. During a following weaker phase 

 a silting up of the bottom will take place to the previous level, but tht^ 

 finer muds are still worker! sea-ward. Thus sand is kept on the con- 



