390 J.BARRELL RECOGIS'lTIOK OF ANCIENT DELTA DEPOSITS 



mtnt, separate rivers converging toward a shallow sea, and stationary 

 crust favor great breadth of subaerial deposits. 



Deltas may, furthermore, vary from miniature examples built into 

 small lakes to deltas of subcontinental size. It is, consequently, mis- 

 leading to discuss delta structure and delta grow^th chiefly from the 

 easily studied small examples, with but brief mention of the larger deltas. 

 In the interpretation of ancient delta dej^osits a similar or perhaps 

 greater range in variations of structure must be anticipated, but that 

 departure from present conditions which was most common was in the 

 direction of shallow interior seas, whose bottoms for indefinite distances 

 were nearly level and worked on by the waves. This physiographic set- 

 ting eliminates the presence of bottomset and foreset beds, the clastic 

 material of tlie subaqueous plain growing progressively finer in texture 

 and smaller in quantity with distance from shore. Under such condi- 

 tions the waves may spend their force on the bottom before reaching the 

 shore, and the shallow sea fades out into shallow brackish lagoons and 

 the fresh-water lakes, swamps, and playas of the river plain. Thus even 

 the dividing line between the subaerial and subaqueous plains may be- 

 come impossible to delineate and the deposits of a river floodplain grade 

 through a delta into the contrasted deposits of an open shallow sea. 



Widespread limestone deposits, thinning out gradually against the 

 lands, imply high sealevels and low surrounding lands of such small 

 relief as not to supply much clastic material to the water. Under such 

 conditions, also, the sea must have shalloAved out indefinitely, the waves 

 spending their force on the bottom rather than the shore. If, however, 

 as during the formation of the coal measures of the Pennsylvanian, ero- 

 sion was vigorous on the land, then alluvial deposits must have replaced 

 large parts of the sea. But on indefinite outward growth the deltas must 

 have been developed over w4de areas at such a low gradient as to result 

 in stagnant drainage. The Yangtse Eiver, for example, at the present 

 time is said to have a fall of but an inch per mile for 200 miles from its 

 mouth. Under such conditions large areas, shifting from time to time 

 and corresj^onding on a larger scale to the border swamps of existing 

 deltas, would be covered with permanent river water, giving rise to asso- 

 ciated lacustrine conditions. During the time of river overflow the flood- 

 plain may be viewed as a temporary lake, with a sea instead of a land 

 boundary on its outer side. With such flat gradients, shallow offshore 

 waters, and consequent weak wave action, the shiftings of the shoreline 

 will be more largely controlled by crustal movement and river-building 

 than by marine action, and the horizontal movements of the strand 

 through each succeeding stage will be at a maximum. 



