ANCIENT WATER LEVELS OF CLIAMPLAIN-II (JDSON VALLEYS 183 



Sand partings will ordinarily be thinner than the clay part- 

 ings for the reason that the fine sand is depositing over the basin 

 only beneath the laterally shifting, stream-made current, while 

 clays are making everywhere else in the longer time during which 

 the stream fails to cover the much larger segment of the arc 

 traversed by its swings. The thickness of clay layers and sand 

 Layers will be greater the slower the rate of lateral swinging of 

 the stream; the sand layers will thicken toward the delta, the 

 clay layers will thicken away from it; and at a distance beyond 

 which the fine sand is carried in suspension, The deposit of clay will 

 be from this cause alone continuous. The rate of lateral shifting 

 will increase directly as the load carried by the stream since the 

 excess of detritus left on the delta plain over that carried to its 

 edge fills up the bed and causes the current to slide off on to the 

 part not so much built up or to give off distributaries which will 

 naturally start out from the side toward which the stream is 

 shifting. Thus increase in load and marginal discharge will not 

 give rise to a proportionate increase in thickness of the prodelta 

 sand layers for the reason that the stream will not deposit sand 

 for so long a time over a given space, because its cycles of swing- 

 ing will be more rapid. 



Delta streams tend to break up into minor streams or an inter- 

 lacing of streams, so that, there will frequently be many lines 

 of prodelta sand deposition, introducing minor bands of sand and 

 clay. The breaking out and shutting off of a distributary which 

 ends independently on the delta edge will give rise to lenticular 

 partings of sand over the prodelta floor. 



The above statement is somewhat ideal, but the prodelta clays 

 of the small esker fan at Drownville R. I. appear to the writer 

 to illustrate the theory here presented. It is doubtful if the regu- 

 lar banding of larger bodies of clay miles beyond a delta margin 

 with an even lamination of sandy partings can be so explained. The 

 criterion of the applicability of the explanation to any given area 

 will be found in the thickening of the sand partings in the 

 direction of the delta and their passage into the segment of the 

 " foreset " beds of the delta with which they are contemporaneous 

 along any given portion of the delta front. Observation and 

 experiment are required to determine the distance over which 

 fine sands may be carried in suspension in fresh, salt and 

 brackish water. 



