SEASONAL BANDING IN GLACIAL CLAYS. : 13 
“The presence of comparatively large grains throughout the gray laminz shows a con- 
tinuance of supply of new material during its accumulation at least, while the presence of 
some streakiness of finer red matter occasionally in them indicates fluctuations or lulls within 
this period of supply. 
“The absence of all large fragments in the red lamine, their uniformity of succession, 
and their gradual increase in fineness of grain seem to argue a period of complete cessation of 
supply from without and complete protection from disturbances. 
“This periodic supply, then, must mark either successive storms or successive thawings 
of neighboring glacial ice. If the latter, then the period is seasonal, and each unit of lamina- 
tion, a gray and a succeeding red layer together, represents a year of time. On this supposi- 
tion, the summer thawings of the glaciers furnished silt to the lake then covering the Grants- 
burg area, and left. behind great quantities of coarser materials to be later spread as the 
poorest of northern soils. Winters checked the supply, coated the lake with ice, and in this 
quiet season the finest sediment settled down in the uniform red laminz of the clay deposit. 
“The deposits themselves have further evidence on this point. It is scarcely conceivable 
that a supply controlled by spasmodic periods, such as storms, could produce uniformity 
either in thickness or in distribution. Variations would be expected to be notable and fre- 
quent, and at random with occasional breaks of a much more pronounced character. On 
the contrary, at any given horizon the thicknesses ate comparatively uniform and there are 
no breaks of a higher order than those of the single unit. If, furthermore, the succeeding 
lamine represent seasonal supply from a more or less proximate ice margin, succeeding seasons 
might be expected to give fairly equivalent results, greatest uniformity being developed when 
the ice margin is distant, and greatest variation when the ice is nearest. Other things being 
equal, when the supply is near by, a greater quantity of matter would reach the lake in a given 
season, and in connection with the general tendency or character of the season there ought to 
be more irregularity and greater thickness of deposit. In terms of the clays themselves, the 
laminee of the middle zone should be most uniform and of least average thickness, while those 
of the top and bottom zones should be most variable. This is easily seen to be true in the 
deposit. The greatest irregularities are near the top. With a retreat of the ice margin to 
near its maximum withdrawal the middle zone of laminee must have been laid down. A 
seasonal interpretation accords well with the facts. Even the streakiness seen in the thicker 
gray layers makes room for-storms or other fluctuations within the season itself, and supports 
the other interpretation for the more uniform breaks. 
“Tf the seasonal interpretation may be regarded as established, it remains only to com- 
pute the number of units of deposition in order to estimate the number of years the deposit 
was in accumulating. This done, we need but to connect this episode in the general 
ice-retreat with its proper limitations in order to get the force of its bearing upon postglacial 
history.” 
In Iceland, Ferguson, 1906, mentions annual layers in the glacial deposits 
associated with igneous materials. There he found alternating clays and sandy 
layers. I quote: 
“The second moraine, at the point shown in the section, is almost hidden by the talus. 
A few feet above the moraine is a small outcrop of a much contorted tufaceous sandstone. 
This contortion seems inexplicable, except through the action of an advancing glacier, thus 
showing, with the earlier advance supplied by the ground moraine below it, a twofold advance 
and retreat of the ice during the period represented by this sedimentary series. 
“Above the contorted tufaceous sandstone is almost 100 feet of flat-lying ripple-marked 
sisi isin 
