414 J. BAERELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



side by the land and on the other by the sea, each advancing into the 

 other and dovetailing in intricate fashion, thongh one or the other may 

 much of the time be absent, and when present seldom of equal develop- 

 ment. Looking down through the depths of the solid rocks the surface 

 of separation is seen to be generated by the ascent of the strand-line 

 through the geologic ages, osicillating back and forth and often com- 

 pletely across the zone of sedimentation. 



From the historic aspect the process of sedimentary infilling may be 

 viewed as the record of a contest between land and sea. The strand-line 

 separates the territory of the opposing forces — Poseidon, the god of the 

 ocean, warring against the earth-born Titans. For a time the field may 

 be held by each; the contest oscillates back and forth, recorded by delta 

 conditions. Then, owing to an advantage gained by the sea, the strand- 

 line may be pushed far inland and victory rests with Poseidon. But at 

 other times the strand is driven back to the margin of the continent, the 

 Titans in their turn are temporary victors, and the lord of ocean is com- 

 pelled to rule within his proper realm. 



Part II. — Evaluation or stratigraphic Criteria 



COMPLEXITY OF THE PROBLEM 



The strata of ancient deltas are now exposed for study as a consequence 

 of uplift and partial erosion, and the broader concepts in regard to their 

 original nature are derived from a synthesis of the observations on indi- 

 vidual outcrops. Increase of knowledge rests on an accurate interpreta- 

 tion of the conditions of origin of the strata as seen in tliese outcrops. 

 But, as noted in the introduction, it is the strand-line which separates 

 the two widel}^ contrasted zones of life, and the most fundamental use of 

 criteria is therefore to distinguish deposits accumulated on the land sur- 

 face, either by wind or water, from those originating under permanent 

 bodies of water. As applied to deltas the initial problem is in conse- 

 quence to determine the distinctive stratigraphic characters of tlie sub- 

 aerial plain as contrasted to all the other parts of the delta, but espe- 

 cially to the rather closely related subaqueous portion of the topset plain. 

 In order to prevent undue reliance from being placed on indeterminate 

 criteria it is necessary to discuss the degree to which they may occur in 

 various situations and to find if possible in such cases minor distinctions 

 which may be of determinative value. In general it may be remarked 

 that considerable caution must be used in drawing distinctions between 

 water-made structures of the land and sea. Evidences of exposure to the 

 air, on the other hand, as shown by special structures or by fossils, are 

 inherently of higher value. 



