616 



A. W. GRABAU TYPES OF SEDIMENTARY OVERLAP 



downward into a marine series belonging to the lower and upward into 

 a marine series belonging to the upper formation. Such a sandstone will 

 occupy a stratigraphic gap which widens progressively toward the shore; 

 for it was this region that the retreating sea first laid bare, and it is this 

 region that the advancing sea covers last. Thus the time interval repre- 

 sented by the top and the bottom of the sandstone formation widens more 

 and more toward the shore of the period, while seaward it decreases until 

 it finally dies away, and with it, generally, the sandstone. These relation- 

 ships are expressed in the following diagrams : 





Figure 7. — Diagrammatic Illustration of 

 compound Overlap; actual relationship. 



Figure 8. — Diagrammatic Illustration of 

 compound Overlap; shotving the hiatus. 



Figure 7 represents the conditions as they will actually appear after 

 a period of combined retreat and readvance. Beds a to d are deposited 

 during the retreat of the sea; beds e to i during the readvance. x-^y is 

 the retreatal sandstone reworked by the advancing sea and made into a 

 basal bed. At A it fills the interval between a and i; at B it forms the 

 dividing line between d and E and is no more than the basal part of bed e, 

 the stratigraphic break of A having disappeared entirely. At B, then, the 

 sandstone x—y is wholly marine and may contain fossils intermediate be- 

 tween those of d and f, or the fosssils of the deeper-water bed e, farther 

 out. This relationship is expressed in figure 8, where the widening gap 

 from y to x—x' represents the increasing time interval comprised within 

 the sandstone member. It need hardly be said, that in nature the beds of 

 the lower and upper series will be so nearly parallel as to seem abso- 

 lutely so. 



It is evident that such a retreatal- transgressive sandstone can not serve 

 as a horizon marker, since it not only varies in age in different localities, 

 but also includes within itself a hiatus which widens progressively toward 

 the source of the material. 



APPLICATION OF THE PRINCIPLES 



The Saint Peter sandstone.* — Although there are numerous examples 

 of retreatal-transgressive beds, only two cases, the Saint Peter sandstone 



* C. P. Berkey 

 229-250. 



Paleography of Saint Peter time, Bull. Geol. Soc. Am., vol. 1' 



pp. 



