SEDIMENTARY ROCKS 271 



In the shifting, bars or reefs are formed, most of which have a 

 rather strep face in the direction in which the sand is shifted. The 

 sand carried over the top of the bar finds lodgment on the sloping 

 face beyond. The inclined laminae thus formed constitute a kind 

 of bedding, but its planes do not conform to the general horizontal 

 attitude of the formation as a whole. The structure is called 

 cross-bald ing, or, more accurately, cross-lamination (Fig. 244). 

 The same structure is developed on delta fronts, and generally in 

 w;iU-r shallow enough to be subject to frequent agitation at the 

 bottom. Sandstone is cross-bedded more commonly than shale 

 or limestone. The bedding of conglomerate is due chiefly to varia- 

 tions in coarseness. Laminae or beds of sand occur between the 

 layers of coarser material in many places. The beds of conglomer- 

 ate are likely to be thick, and in conglomerate cross-bedding is 

 common. 



Lateral gradation. When the varying nature of the agitation 

 of the sea at different depths and along the different parts of a 

 coast is considered, it will be understood that deposits of one kind 

 may grade into others horizontally. Thus a bed of conglomerate 

 (gravel) may grade laterally into sandstone (sand), and this into 

 shale (mud) or limestone. It is indeed more remarkable that sedi- 

 mentary strata are as regular and persistent as they are, than that 

 they grade into one another in some places. 



Position of strata. At the time of deposition, beds of sediment 

 conform in a general way to the slope of the bottom where they are 

 laid down. Since the slope of the sea bottom near shore is very 

 gentle, as a rule, beds of sediment are, in most cases, nearly hori- 

 zontal when deposited. Their slope is rarely so much as 20, and 

 commonly less than 5. 



Special markings. The rhythmical action of waves gives rise 

 to ripple-marks (Fig. 196), which are also made by streams, stream- 

 like currents, and wind (Fig. 13). They are usually only a few 

 inches from crest to crest, but in rare instances they attain much 

 grealer size. Under proper circumstances, ripple-marks are pre- 

 served indefinitely. 



Some sediments are exposed between tides, or under other cir- 

 cumstances, for periods long enough to permit drying and crack- 

 ing at the surface. On the return of the waters, the cracks may be 

 filled and permanently preserved. (Figs. 200 and 201) Such rec- 

 ords of sun cracks affect shales chiefly, but are seen occasionally in 



