PROGRESSIVE OVERLAP 615 



in the retreat of the sea from A to B; and that the retreatal sandstone 

 bed x-y is of much later age at B, where it represents bed d, than at A, 

 where it represents bed a. If this retreatal bed contains fossils in its 

 basal portion, they will be fossils of successively higher formations when 

 traced from A to B. 



Where the land is sufficiently elevated during this retreat of the sea 

 stream erosion will set in and the material left by the retreating sea 

 may be removed by this process. Furthermore, since elevation of the land 

 is responsible for the retreat of the sea, the streams coming from the 

 higher land will have their slope, and hence their velocity, accentuated. 

 As a result, more detrital material is carried down, and where erosion is 

 not going on deposition of land-derived detritus will take place. Thus 

 pebbles derived from the old-land or from old conglomerates may be 

 carried out for great distances over the emerging coastal plain. Wind 

 deposits of assorted sands with rounded and pitted grains will likewise 

 accumulate on this plain; and remains of land plants and of land and 

 fresh-water animals may be buried in these sands. These sands, being 

 wind or river deposits, will often show cross-bedding and wind ripples. 



Examples of regressively overlapping or, better, off -lapping formations 

 are frequently met. Since, however, in the most typical cases available 

 for investigation the conditions are complicated by the structures result- 

 ing from the readvance of the sea, a brief account of these complicated 

 phenomena may first be given. 



Compound Eegressive and Transgressive Overlap 

 statement of the principles 



After the retreat of the sea and the washing seaward, during this re- 

 treat, of the land-derived detritus, a period of readvance, we may assume, 

 invariably sets in, because stationary conditions in nature, if they ever 

 occur, are so rare as to be negligible. 



The readvance will, of course, have all the characteristics of a first 

 advance, except that the material of which the basal bed of the read- 

 vancing series is formed is that of the retreatal bed deposited during the 

 regressive movement and the river deposits and sand dunes accumulated 

 on the recently emerged coastal plain. Thus the retreatal sands and 

 pebbles will be reworked by the advancing sea and incorporated in the 

 progressively overlapping beds of this readvance as a basal or shore facies. 

 If the deposit by wind and streams on the emerged coastal plain was a 

 heavy one, the advancing sea will work over only the upper portion, 

 leaving the middle and lower portions undisturbed. Thus the resulting 

 bed may be a wholly non-marine deposit in the middle, and yet grade 

 LVI — Bull. Geol. Soc. Am., Vol. 17, 1905 



