6i2 PRINCIPLES OF STRATIGRAPHY 



testifies to the tough, resistant character of this clay, which when 

 wet is ahnost inerociable by pure water. (29.) 



Structure and Composition of the Delta. Theoretically the delta 

 consists of bottom-set, fore-set, and top-set beds. These are all 

 well developed in the small deltas formed in Pleistocenic time in 

 temporary ice-dammed lakes and now open to examination after 

 the draining of these lakes. In the deltas on the sea coast, however, 

 one or the other of these beds is often absent, or two may merge, 

 this being most frequently the case with the fore-set and bottom- 

 set beds. 



The fore-set beds of small or young deltas are generally steeply 

 inclined, as already noted. This is especially the case when the 

 supply of detritus is large. As the delta increases in size the later 

 fore-set beds become more flattened and bend over at the bottom 

 into the horizontal bottom-set beds. The upper ends of the fore- 

 set beds, on the other hand, show more or less of erosion and 

 across their truncated edges are deposited the top-set beds. There 

 may, of course, be at times a bending over of the top-set beds into 

 the fore-set, but in the young delta the contact is more or less sharp. 

 In the larger deltas of fine material, on the other hand, the top-set 

 beds may be more or less continuous with the fore-set, and, indeed, 

 the two may imperceptibly grade into each other, without even a 

 change in angle. 



The top-set beds of small deltas consist of the coarser material 

 laid down in nearly horizontal beds. If we assume that the normal 

 delta begins as a subaqueous detrital cone or semi-cone growing 

 in circumference, it is apparent that fore-set and bottom-set beds 

 alone exist during the earlier stages. As tlie radius of the cone 

 increases, its summit is invariably truncated by the waves and by the 

 stream itself, which carries the detritus out to the front of the cone. 

 Thus a level plane is formed partly by non-deposition and partly by 

 contemporaneous erosion truncating the fore-set beds. Its surface 

 will be to some extent below water level, the depth depending on 

 the strength of the wave activity. Upon this plane will be deposited 

 the top-set series which the current can no longer carry to the edge 

 of the growing delta. The top-set series will continue to grow in 

 thickness and extent as the delta grows, and its landward part will 

 begin to emerge and become subaerial. The extent to which this 

 subaerial part of the top-set series will grow is determined by 

 the strength of the river and the slope which it can control. In 

 smaller deltas the frontal angle of the delta is also the junction 

 between top-set and fore-set beds and marks the extent of sub- 

 mergence of the delta. In larger deltas the top-set beds go deeper. 



