He 
u 
ia 
- 
% 
iE 
ee a — ee —_ r a a ne erase rane eet teate le a a 
60 SEASONAL DEPOSITION IN AQUEO-GLACIAL SEDIMENTS. 
of the minute structure of the banding in the clays which has been published. 
His evidence has a most important bearing in favor of the theory of seasonal 
deposition. 
Cyclonic and anticyclonic conditions, although fairly regular during parts 
of the year, with an average period of about seven days, are too irregular, when 
the whole year is considered, to produce the regular banding seen in the clays. 
If the coarse components of the banding are due to gentle wave action on the 
bottom, and the fine layer above is due to the settling of the sediment after the 
storm, how can the layers show such even thicknesses? Wave action produced 
by storms varies enormously in intensity. Furthermore, if the fine component 
means settling after a storm, and nothing more, what record is there left of 
the winter period? If there were seasons during glacial and interglacial times 
surely the deposits should show some record of them. It must also be noted 
that wave action of sufficient intensity to stir up one tenth of the amount of fine 
sediment found in the fine components of the banding is not even suggested in 
the layers of the coarse components. The disturbances noted in the coarse 
components denote extremely gentle action, and there is every reason to believe 
this was current action. 
Although wind action of a regular seasonal nature is very possibly respon- 
sible for some of the material and for certain definite layers of coarse sediment 
intercalated in the banding, the findings of Berkey in the coarse components 
of the Grantsburg clays appear to show conclusively that currents of water 
were responsible for the coarse components of the banding as a whole, whether 
or not some of the coarse material reached the basin of deposition by the wind 
or by glacial streams. In either case seasonal deposition would be indicated, 
for in the winter the water body would be frozen over and no wind-blown sedi- 
ment could be deposited in the water at that time. On account of the juxta- 
position of some glacial and loess deposits it is not at all unlikely that the wind 
played an important part in the seasonal deposition. (See p. 36). 
It is very evident that tidal action had nothing to do with the clays at 
Grantsburg. The laminated glacial clays here and in other places, present, as 
far as can be observed, the same details of structure. 
De Geer, Emerson, Taylor, Berkey, and others have written about the 
seasonal deposition of clays in widely separated regions, and their accounts have 
been quoted. It remains to note one point which has not been sufficiently 
appreciated by those who have studied the clays. I refer to the layers in the 
clays described by Emerson and Coleman which contain leaves, twigs, spruce 
