396 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



the river merges into the sea. The whole is epitomized in the waste- 

 laden waters which converge into the mountain gorge and then, escap- 

 ing and diverging, deposit a part of their burden on the alluvial fan. 

 The recognition of the erosion cycle and the separate processes of valley 

 deepening and valley widening has stimulated wonderfully the science 

 of physiography, but it is to be noted that no corresponding deposition 

 cycle has been elaborated to a similar degree to connect the successive 

 stages in the work of the lower parts of the river, the descriptions of 

 deltas being still essentially in the state in which they were left bv 

 Charles Lyell. 



Subaerial topset beds 

 Subaqueous topset beds 



^^^ Foreset beds • 



Bottomset beds 



Vertical scale magnified several hundred times 



Figure 3. — Diagrams showing Stages in the Delta Cycle of a large River during a 



Period of stationary Crust 



In studying the surface of the land the need for the formulation of a 

 delta cycle has not been felt, since the successive stages are concealed by 

 burial, but for the interpretation of the sedimentary record it becomes of 

 considerable importance. As a preliminary step for the evaluation of 

 the criteria of ancient delta deposits it is desirable, therefore, to formu- 

 late such a deposition cycle in reversed terms to the erosion cycle of tliG 

 upper waters. 



