12 SEASONAL DEPOSITION IN AQUEO-GLACIAL SEDIMENTS. 
peaty layers are not always distinct, and there are beds of the clay 2 or 3 feet thick which 
do not show them, the peaty and silty matter being more or less mixed with the clay in these 
parts. It is natural to assume that the silty layers are of an annual character, and if we 
reckon that two inches of clay were deposited annually over the delta, which was 184 inches 
wide, the 94 feet required 564 years to form. 
How long the 55 feet of overlying stratified sand needed for their formation is hard 
to guess, but half a foot a year seems as rapid a rate of deposit as one can assume for so 
wide a delta. This would give 110 years for the interglacial sands.” CoLEman, 1902, p. 73. 
Coleman found other evidence to prove that these Don beds did not give 
the whole record of the Interglacial period. It is enough to record his opinion 
of the banded clays. 
In 1905 Berkey published a detailed account of the laminated interglacial 
clays of Grantsburg, Wisconsin. I quote his description of the characters of 
these clays and some of his chronological deductions: 
“These clays are all strongly laminated. The lamine vary in thickness from a mere 
film to several inches in different parts of the deposit, but are comparatively uniform in any 
particular zone. Their average thickness in the upper part of the deposit is about a quarter 
of aninch. The average thickness nearer the middle is about one-tenth of aninch. Less 
uniformity is observable near the top than in any other zone. 
“The lamination is extremely regular and approximately horizontal. Small crumplings 
or bunchings occur, but are rare. 
“The general color is red from top to bottom. On closer inspection, however, the lami- 
ne are seen to be of two types — a deep red one, and a gray, which alternate without excep- 
tion throughout the deposit. 
“Very perfect water-sorting is evident from a study of these individual lamine. The 
gray ones are comparatively coarse-grained, containing maximum diameter of 0.06 mm. 
Diameters of 0.02 to 0.03 mm are very common, while of course there is much finer 
matter. The red laminz are composed of extremely fine grains and flakes. There are no 
grains at all comparable to the sizes given above. Average diameters are less than 0.002 mm. 
“The passage from one type to the other is sometimes gradual and sometimes abrupt. 
As a rule, the gradual changes hold for all cases in passing upward from a gray to a red lamina. 
The abrupt changes are noted in passage upward from a red to a gray one. Evidently there 
is some sort of unity between each gray lamina and the overlying red one throughout the series. 
“Taking, therefore, the double lamina — i. ¢., a gray and the succeeding red one above — 
as a unit, the following facts obtain: The most irregular lines in the lamination are at the very 
base of the gray laminz. There are sinuosities on a small scale that simulate erosion uncon- 
formities. The coarsest grains seen anywhere in the material are in these small embayments 
along this line. There is an occasional streakiness of the gray laminz with the finer red 
material, but not uniformly developed. The change to red, in rising from the base of the 
gray lamina, takes place very gradually and gives a much more even line or band than that 
at the base. The change to red color is no more marked than the change to finer and finer 
grain. There is no streakiness in the red layers.” Burkey, 1905, p. 36, 37. 
Further on in this same paper (loc. cit., p. 40, 41) Berkey says: 
“The laminated clays are no doubt a lake deposit. The uniform succession of lamine, 
the relationship between the red and gray, and the constancy of their characters lead one to | 
look for some uniformly periodic cause of formation. 
