. Bulletin of the Geological Society of America. Geology. 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 d
. Bulletin of the Geological Society of America. Geology. 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 local
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