ANCIENT WATER LEVELS OF CHAMPLAIN-HUDSON VALLEYS 181 



depositing grounds. Emerson^ has invoked this seasonal change 

 to account for the alternate lamination of bands of fat and lean 

 claj in the Connecticut valley, making each layer of lean clay 

 correspond to a summer, and each layer of fat clay to a winter. 



It is difficult to see how either this or the preceding variation 

 in clay discharge will account for the essentially even deposition 

 of alternately coarse and fine layers of clay and particularly 

 alternations of layers of clay with layers of fine sand over a 

 large area of deposition far from the mouth of the discharging 

 streams, for the fine sand would go to the bottom within a short 

 distance of the edge of the water basin where the streams entered. 



Astronomical changes. Gilbert, noting the remarkable rhythmic 

 succession in the alternation of clays and sands in certain sedi- 

 ments of the .West compared the phenomenon with the supposed 

 effect of periods of minimum and maximum variations in the 

 ellipticity of the earth's orbit, the geologic effects of which were 

 first pointed out by Sir John Herschel. But as the period of such 

 maxima and minima in ^the theory proposed by Croll cor- 

 respond to entire periods of glaciation and deglaciation, it is 

 not to be supposed that the glacial clays of a single episode of 

 deposition manifest any control exerted by these changes and 

 we may therefore dismiss the view as having no bearing on this 

 group of clays. 



Prodelta clays. There are several other conditions controlling 

 or interfering with the deposition of clays, particularly in bodies 

 of water lying within or adjacent to a retreating ice sheet. One 

 of these conditions is inherent in the method of delta construc- 

 tion by which a stream swings from side to side of its delta. 



For illustration the simplest case will be taken, that of a glacier 

 discharging its drainage by a single stream into the head of a 

 bay or lake on the border of which it has already built a delta 

 across whose surface the stream swings in the process of dis- 

 charging its load of gravel, sand and clay. 



While the stream is aggrading its delta, it swings from side 

 to side through the arc whose trace is the free margin or shore 

 line of the deposit and whose center is the mouth of the glacial 

 stream. Take the stream at a moment when it lies at one side 

 (say the left) of its delta contiguous to the ice front. Its burden 



^Emeirson, B, K. Geology of Old Hampshire County, Mass. U. S. Geol. 

 Sur. Monogr. 29. 1808. p.706-7. 



