.REGRESSIVE DEPOSITS 735 



posed. The shore end of b is also composed of quartz sands, since 

 during the formation of ^ the shore has migrated seaward. In hke 

 manner c does not entirely cover b, and d does not wholly cover 

 c, each ending in a sand facies. Thus the sand ends of all the beds 

 will be exposed at or just above sea-level, and constitute a continu- 

 ous sand formation, which, however, is not of the same age at 

 any two points along a line transverse to the direction of the shore. 

 Such a sand formation will, however, be mapped as a unit, and re- 

 ceive a formational name. If the basal portion of such a sand for- 

 mation is fossiliferous, it will contain in a seaward direction the 

 fossils of successively higher formations. Thus the portion of the 

 sandstone formation .r, 3; m Fig. 148 will at x contain the fossils 

 of formation a, and at y the fossils of formation d, while between 

 these points it will contain the fossils of b and c, where it forms 

 the end of these respective formations. When the land is suffi- 

 ciently elevated during the retreat of the sea, stream erosion will 

 set in and the material left by the retreating sea may be removed 

 by the streams. Furthermore, if elevation of the land is responsible 

 for the retreat of the sea, the streams coming from this higher 

 land will bring more detritus, and hence, where erosion is not 

 going on, deposition by rivers will further elevate the surface of 

 the emerging coastal plain. The same thing is true also if the 

 regressive movement is due to an increase in the supply of detrital 

 material. In either case, pebbles and sands derived through the 

 erosion of the old land or of old conglomerates and sandstones 

 may be carried out for great distances over this emerging surface, 

 while wind-assorted sands, with their grains rounded and pitted 

 from attrition, may also accumulate over this surface. The peculiar 

 structures of both torrential and eolian sands, i. e., cross-bedding, 

 ripples, etc., may thus be incorporated in this retreatal sandstone. 

 Remains of land-plants and of land and fresh water animals may 

 readily be entombed in the deposits thus accumulating upon the 

 flat plain of retreat, and even coal beds ma^ form and become em- 

 bedded in sandstones the bases of which vary in age from place 

 to place. 



Burial of Retreatal Sandstones by Subsequent Transgressive 



Movements. 



When the regressive movement of the shore has come to an 

 end, and transgressive changes recommence, the upper portions of 

 these migratory shore deposits may be worked over once more, and 

 now partake of the character of a basal sandstone or conglomerate. 



