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NEW YORK STATE MUSEUM 



obtain, even though temporary rest periods may intervene. A 

 rising sea level means a landward advancing- shore and this im- 

 plies that each later deposit reaches farther up on the shore than 

 the preceding ones. In other words there is, under normal con- 

 ditions of deposition, a constant and progressive overlapping of 

 the later over the earlier layers. Any given formation will thin 

 shoreward, but this is due to the fact that only the later beds of 

 the formation are involved in the thinning wedge, the thinnest 

 portion far up on the shore consisting only of the highest beds 

 composing it. This is illustrated in the annexed diagram [fig. 4a]. 



c 



Fig-. 4a Diagram illustrating progressive overlap 



Again, in a normal series of deposits against a shelving shore 

 each formation from the lowest up will in turn come to rest on 

 the old land, the earlier formations having come to an end. Thus 

 in the left-hand portion of the diagram [fig. 4a] formation c rests 

 on the old land without the intervention of a or b. Conversely 

 away from the shore, each formation becomes gradually under- 

 laid by earlier beds which appear between it and the old land. 

 It must also be borne in mind that apart from exceptional cases 

 there is a progressive shoreward change in the lithic character 

 of a bed and (hat in any given region tins change may be uniform 

 in successive beds. Tims a bed may consist of silicious pebbles at 

 the shore, become a silicious sand farther out. and a calcareous 

 sand or a clay at a distance from shore. Each succeeding forma- 

 tion will carry its pebble portion farther up on the shore, its sand 

 facies thus coming to lie more or less directly above the pebble 

 facies of the preceding bed. As the progressive advance of the 

 seashore may lie assumed to be a uniform one. the pebble facies of 

 successive formations will have the appearance of a continuous 



