734 



PRINCIPLES OF STRATIGRAPHY 



represent the seaward spreading shore zone of a rising sea-floor, 

 unless they are referable to continental deposits. 



Characteristics of Regressive Deposits. 



As the retreat of the sea from the land may under normal condi- 

 tions be considered a process occupying a greater or less amount 

 of time, it is evident that deposition at a distance' from shore need 

 not be interrupted. Thus, while within the zone of retreat for any 

 given time period, no sedimentation will occur, such sedimentation 

 may, nevertheless, go on at a regular rate beyond this zone. In 

 other words, a certain thickness of off-shore deposits must be 

 considered the depositional equivalent of a given time period, 

 which in the shore zone is represented by a given amount ot re- 



FlG. 148. Diagram illustrating regressive overlap (off-lap) and the forma- 

 tion of a sandstone of emergence (x-y) into which the shore- 

 ends of the successive members of the rctreatal series (a-d) 

 grade. 



treat. Thus it is brought about that each successive formation of 

 the retreatal series extends shoreward to a less extent than the 

 preceding one. As each formation or bed passes shoreward into 

 a coarser clastic it is evident that the shore ends of all the forma- 

 tions deposited during the retreat will together constitute a stratum 

 of sandstone or conglomerate which in age rises seaward, since in 

 that direction it is progressively composed of the ends of higher 

 and higher formations. These relationships are shown in Fig. 148, 

 where it will be noted that the diagonal rise of the shore-formed 

 stratum is from the shore seaward, whereas in transgressive move- 

 ments the shore-formed stratum or basal bed rises diagonally land- 

 ward. In the series shown in the diagram, the beds a-d were suc- 

 cessively laid down during the retreat of the sea from A to B. 

 Each later formed bed comes to an end before it has reached the 

 shoreward end of the preceding one, and each formation grades 

 from a clayey or calcareous character landward into silicoarena- 

 ceous character. Thus b reaches landward to a less extent than a, 

 the shore end of which, composed of quartz sands, remains ex- 



