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STUDIES FOR STUDENTS 



epicontinental sea, protected more or less effectually from ocean cur- 

 rents and possibly from tides; a sea which for that reason would be 

 unable to cut away the rapidly forming deposits of the encroaching 

 delta. Such a sea existed over much of the northeastern portion of 

 the United States during much of the Neopaleozoic times. Within 

 this basin of sedimentation former shallow-water conditions are indi- 

 cated in certain formations by mud- cracks and supplemented by 

 ripple-marks, cross-bedding, and inconstancy of arenaceous strata. 

 This must signify that the upbuilding power of the ancient rivers was, 

 on the whole, equal to or in excess of the progressive subsidence of 

 the basin. There is not likely to be long maintained an exact balance 

 between the two, and hence in general it may be said that such struc- 

 tural feeatures indicate a capacity in river-building to more than com- 

 pensate for the progressive subsidence of the geosynchne. But, having 

 granted this excess of sedimentary power, it is seen that, especially in 

 the protected bays, the surface of deposition should be very largely 

 a land surface ; occasionally flooded, as a result of the rainy seasons, 

 by the fresh waters from the land; occasionally flooded over the 

 seaward portions during combinations of storms and high tides by 

 the waters of the sea. 



Diagrams illustrating the relations of foreset and topset beds under 

 ideal conditions are given in Figs, i and 2. In Fig. i a delta is sup- 

 posed to be built out into a quiet lake with constant water-level. The 

 thicknesses perpendicular to the planes of stratification of the foreset 

 and topset beds will vary approximately with the angle of dip of each. 

 The ratio of volumes will depend upon the area multiplied by the 

 thickness, and although the topset beds under these conditions may 

 not be more than one-tenth as thick as the foreset beds, their area in 

 a large delta, especially in a shallow sea, may be ten times that of the 

 latter, so that the volumes of sediments deposited above and below 

 the sea may be of the same order of magnitude. In Fig. 2 a subsiding 

 delta is supposed to be built upward and outward, with just such 

 speed that the front of the delta is maintained at a constant line. In 

 this hypothetical case the gradient of the delta would be less than in 

 the first case, the currents more sluggish, the delta more frequently 

 flooded, and a greater proportion of the waste would be dropped upon 

 the topset surface. Under such conditions the topset beds might 



