LARGER RELATIONS Ol- DKLTAS :>lll 



Under present physiographic attitudes of high continental relict tlie 

 marine, lacustrine, and tiuviatile phases are abnormally sharpened. 

 Under the physiographic states of low relief, which have been more usual 

 through geologic time, this sharpness of separation is seen to have been 

 absent. This blending together of unlike parts in connection with a 

 tendency to interpret sedimentary formations as marine or lacustrine has 

 permitted an unjustified assignment of certain delta deposits to other 

 modes of origin, with the result that the idea of delta formations is 

 almost absent from the interpretation of the stratigraphic record. In 

 approaching the subject of the criteria of ancient delta deposits, it is 

 therefore to be held in mind that the several parts of a delta may vary 

 in importance; they may vanish, or they may become blended into each 

 other and lose their individuality. Xevertheless, the distinction of parts 

 in the ideal delta is a necessary analytical step and serves as a basis foi- 

 discussing the application of criteria. 



At the beginning of this section a delta was defined as a formation 

 built by rivers against a permanent body of water and maintained jjaitly 

 as a land surface. The discussion has served to show the great variety 

 of conditions under which this may take place and the non-essential 

 character of certain features which have been regarded not uncommonly 

 as essential marks of deltas. This is especially true of the foreset beds, 

 which assume great dominance and marked steepness in the case of small 

 deltas built by torrential streams into deep lakes. From Gilbert^s classic 

 studies of deltas of this type^ and the common occurrence of similar dis- 

 sected deltas connected with Pleistocene glaciation, this particular variety 

 of delta has become commonly accepted as the standard type, and the 

 specific criteria by which it is identified have been unwarrantably broad- 

 ened into generic value. 



NEGATIVE VALUE OF OVERLAP 



PreU7ninary statement. — The successive strata of a formation if pre- 

 served to their original limits will be found to have different horizontal 

 extents. The higher beds may overlap farther toward the regions of 

 erosion than the lower, or they may be more restricted ; or, turning to 

 the opposite direction, they may extend to greater distances from the 

 source of material. Grabau has discussed these three types under the 

 titles of marine transgressive, marine regressive, and non-marine fluvia- 

 tile progressive overlap. Certain exceptional cases he further classifies 



3 The topographic features of hike shores, nth Ann. Rept.. I'. S. Gool. Siirv.. ISS.j, 

 pp. 104-108. 



