392 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



as irregular overlap.* The discussion in that paper develops the princi- 

 ples that during the advance of the sea in any cycle of inundation a 

 similar lithologic facies marks the relation of the sea to the shore, and 

 in the case of transgressive overlap the facies farther inland represents a 

 higher geologic horizon. On a retreat of the sea such a shore facies will 

 also retreat while still ascending in the time scale. In the building out- 

 ward of a river fan, Grabau points out that the upper beds extend beyond 

 the lower and their outer parts rest in turn on the underlying formation. 

 These principles are used, however, not only in discussing the discordance 

 between the lithologic facies and the chronologic succession and in sepa- 

 rating the several types of overlap, but as definite criteria for distin- 

 guishing marine from terrestrial formations. It is this question of the 

 positive or negative value of overlap, not as general principles of rela- 

 tion, but as definite .criteria for separating marine and fluviatile deposits, 

 wliich it is desired here to discuss. 



Transgressive overlap. — It is held in the present paper that transgres- 

 sive overlap is most commonly marine, but may also be fluviatile and 

 may arise from several causes : first, progress in the normal erosion cycle, 

 especially through the stages of youth and maturity; second, subsidence 

 of the whole region, producing a rise of the ultimate baselevel ; tliird, 

 relative subsidence of the fields of erosion, producing a landward aggra- 

 dation by rivers, even if there is no invasion by the sea ; fourth, a climatic 

 change of such a nature as to steepen the grades of the upper parts of 

 the rivers. A brief discussion of each of these causes is needed, and the 

 citation of examples will make the conclusion more evident. 



In the progress of the erosion cycle the waves plane inland to a greater 

 or less extent, save where dammed out by the sediment of powerful 

 ri\ers. They are eroding and only incidentally depositing agents. If 

 ri\ers are able to build deltas outward, however, against the sea the in- 

 land parts of the delta must also build upward and backward. As the 

 river grade grows flatter and lower with advanced maturity, however, this 

 process is limited and finally comes to an end. With stationary sealevel, 

 therefore, transgressive overlap is unimportant and exceptional, but may 

 be either marine or fluviatile. 



A rising sealevel may be due either to marine sedimentation or to 

 crustal changes, lowering the land or raising the sea bottom. The first 

 is a slow and world-wide effect, accompanying the progress of the erosion 

 cycle during a period of quiet. The second is usually more rapid and 

 regional. In either case there will be transgressive overlap. The first 

 commonly but not necessarily results in marine overlap. 



♦Types of sedimentary overlap. Bull, Geol, Soc. America, vol. 17, 1906, p. 560, 



