614 



A. W. GRABAU TYPES OF SEDIMENTARY OVERLAP 



the region unaffected by the shore detritus, even during the retreat, a 

 continuous and uniform, or nearly uniform, series of deposits is accumu- 

 lating. Even within the outermost zone affected by the retreat — that 

 is, the region reached by the shore detritus at the end of the regressive 

 movement — continuous deposition of offshore sediments is accumulating, 

 until near the end of the movement, when shore detritus will replace the 

 more open-water sediment. It thus becomes apparent that an additional 

 series of strata is forming in the offshore district of which there is no 

 representation in the nearer-shore area. When the retreat is a slow 

 one a considerable amount of sedimentation may result in the offshore 

 area. If the retreat of the sea is rapid, a relatively small amount of 

 sedimentation records it. In any case the amount of sedimentation in 

 any area over that of another area in the line of retreat becomes a meas- 

 ure of the interval occupied by the retreat of the sea from one to the 

 other point. This implies, of course, that the amount of sedimentation 



Figure 6. — Diagram showing Planes of Sedimentation and Relationship of Seashore Bed. 



measured in each sectionbegins at the same datum plane — that is, the plane 

 of sedimentation at the beginning of retreat. It becomes, furthermore, 

 apparent that the retreatal sand or conglomerate bed is not of the same 

 age throughout, but rises in the scale seaward. Hence, if the gradation is 

 a gradual one, the retreatal bed will grade down, near the old shoreline, 

 into and may contain the fossils of a bed very much older than the bed 

 into which it will grade at a distance from the old shore; for during the 

 period of retreat a considerable space of time has been consumed and a 

 certain amount of sediment has collected at the point eventually reached 

 by the farthest retreat. The relationships of the beds are indicated in the 

 diagram, figure 6. 



In this diagram each bed from a to d was in turn laid down during the 

 retreat, each later bed reaching to a less extent upon the old shore and 

 each bed ending in a sand member. Thus bed b does not extend as far 

 as bed a, nor c as far as b, but each ends landward in a sand facies; and 

 these sand ends together constitute a more or less continuous bed of sand 

 passing diagonally across the beds a-d. It is evident that the thickness of 

 the beds a-d at the point B is a depositional measure of the time consumed 



