CHARLES P. BERKEY 41 



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 pro- 

 nounced character. On the contrary, at any given horizon the 

 thicknesses are comparatively uniform and there are no breaks of a 

 higher order than those of the single unit. If, furthermore, the 

 succeeding laminae 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 laminae 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 laminae 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 fluc- 

 tuations within the season itself, and supports the other interpreta- 

 tion for the more uniform breaks. 



If the seasonal interpretation may be regarded as established, it 

 remains only to compute 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. 



Time estimates. — For computation a gray and red lamina together 

 make a unit and represent one year. At Grantsburg the deposit is 

 now 35 feet thick, and was originally greater. There are an average 

 of four units to the inch, or forty-eight to the foot, in such portions 

 of the bed as can be critically examined at that point. There are 

 then approximately 1,680 units in the total thickness. It seems fair 

 to conclude from the above computation that the interglacial lake, 



