EVALUATION OF STRATIGRA I'll IC TRTTERTA 427 



of the mud cracks. This poorly laminated bedding is especially charac- 

 teristic of the thick red shale members of the Catskill formation, which 

 there is reason to believe is largely fluviatile and contrasts with the better 

 lamination in the olive shales of the Chemung Avhich represent the off- 

 shore deposition. 



Thus, to sum up the previous discussion, it is seen that higlily ])orfect 

 lamination in pure pelites is characteristic of quiet subaqueous deposi- 

 tion, but an absence of such lamination is no proof of subaerial condi- 

 tions, though most extensively developed in such situations. It is to be 

 noted, however, that studies on the character of lamination in modern 

 sedimentation is a subject which has received but little attention. 



Stratification of sandstones — Effects of waves. — The transporting 

 agencies of marine sands are primarily waves, secondarily currents. In 

 fluviatile work, on the contrary, currents are the controlling agency and 

 the work of waves is limited. In neither region, however, is either one 

 entirely absent and locally the minor activity may dominate the resulting 

 structure. 



Normal wave action tends to sweep sand in a direction opposite to that 

 of the surface wave motion, ^^ but where the bottom shallows the wave 

 becomes to some degree a wave of' translation and carries the coarser 

 bottom material which it can move with the water toward the shore and 

 results in the building of bars. Waves over a broad bottom which is flat 

 tend therefore to maintain an even deptli of water and develop a regular 

 bedding, the sand being swept under normal wave action from the 

 slightly higher places to the quieter water of greater depth. Tlie differ- 

 ing direction and force of storm w^inds tends also to shift the bottom 

 materials. The action near the shore is different in character from that 

 on the flat offshore bottom, since the material tends to be moved partly 

 to deeper water, partly to shallow^er water, and the shoreward slopes are 

 steepened, the outer slopes flattened. The shifting of bars results, fur- 

 ther, in a continual cutting out and redeposition elsewhere of the sand 

 beds. This is well illustrated off the Xew Jersey and Maryland coasts, 

 irregularities of the bottom extending to depths of 10 fathoms at dis- 

 tances up to 15 or more miles from the coast. 



It is important, however, that a quantitative estimate of such effects be 

 given, and a study of the coast charts shows that the maximum slo]ies of 

 the submerged sand banks off the open coasts, where waves and not 

 currents are the controlling forces, is not over 15 feet in 1,000. Whore 



^ N. M. Fenneman : Development of the profile of equilibrium of the subaqueous shore 

 terrace. Journal of Geology, vol. 10. 1902. pp. 1-^2. 



