182 NEW YORK STATE MUSEUM 



of gravel and coarse sand enters into the construction of the delta 

 proper. Over the bottom of the lake or bay the clays carried out 

 in suspension are constantly coming to rest at distances from the 

 delta margin determined by the presence and velocity of the 

 currents and the time taken for the particles to fall through the 

 water. For some distance over the bottom in the path of the 

 stream-made current, the finer particles of sand which have not 

 at once been drawn by gravity down on the delta talus will come 

 to rest, forming a deposit of very fine sand extending outward 

 from that part of the base of the delta. Around the remaining 

 portion of the area confronting the delta base, clays will deposit 

 as elsewhere over the floor of the water body. In the course of 



Fig. 22 Cross-section of interstratifled clay and sand on Jake or bay bottom iu 

 advance of a delta 



a few days or weeks or months, dependent on velocity, load, and 

 the area of its delta fan, the stream will have moved laterally 

 across its delta to the opposite side. The fine sands will now 

 have been deposited over the entire area in front of the delta 

 base while clays will have been deposited on that side where sand 

 was previously going down. Still later, the stream will have 

 swung back to the left of the delta and sands will be depositing 

 along that portion of the basin floor, while clays are deposited 

 over all the area on the right. The stream thus swings to the 

 left and right of its delta, strewing fine sand over the bottom in 

 advance of the delta. These changes will continue so long as the 

 stream is building up its delta and the water body is unfilled with 

 sediment. There will thus be built up on the floor of the basin 

 an alternation of layers of clay and fine sand, whose stratification 

 seen in a cross-section drawn transverse to the axis of the delta 

 will l»e llijit shown in figure 22, in which the black line represents 

 the sand layers, the white banding, the clays. 



Where the stream halts, the sand layer will be thicker than 

 where the stream has moved steadily along in its lateral motion. 

 At tlie extreme right and left, where the stream has halted and 

 turned back on its course, the sand bands should be thicker than 

 in the middle of its shifts. 



