W. Upham—Minnesota Valley in the Ice Age. 107 
them. These divisions are clearly seen through the whole 
extent of this excavation, which reaches twenty-five feet below 
the top of the clay and is four rods long. The height of its 
top is estimated to be sixty-five feet above the river. The 
excavation of Nye & Co. at Carver, where the exposure is four 
rods long and fifteen feet high, with about the same elevation 
above the Minnesota River as the foregoing, exhibits the same 
stratification, except that here the layers all have a nearly uni- 
form thickness of three inches. There is a tendency to split at 
the darker partings, which are seen to extend continuously, 
never passing one into another, and preserving a very constant. 
width of three inches apart, through the whole of the section 
exposed. They are from an eighth to three-quarters of an inch 
thick, gradually merging above and below into the less dark 
clay that makes up the principal mass of these layers. The 
bedding is nearly level, but dips one to two degrees away at 
each side. In this depth of fifteen feet there are thus about 
sixty layers, all closely alike. The alternating conditions which 
produced them were evidently repeated sixty times in uninter- 
rupted succession. The only explanation of this which seems 
possible is that these divisions mark so many years oceupied by 
the deposition-of this clay. Layers nearly like those in the 
clay at Carver and Jordan are also seen in other clay-beds in 
this valley and in that of the Mississippi in this State. The 
principal mass of each layer is regarded as the deposition dur- 
ing the warm portion of a year, and the very dark partings as: 
the sediment during winter when the glacial melting was less 
and the water consequently less turbid. ‘ 
At Chaska, situated in the Minnesota valley, two miles below 
~ 
Carver, the clay used for brick-making is modified drift of : : 
mterglacial age. It varies from twenty to forty feet in thick- 
ness, being underlain by sand and covered by till from two to 
six feet thick, holding bowlders of all sizes up to five or six 
feet in diametér, many of which are planed and striated. This 2 
till forms the surface, thirty to thirty-five feet above the river. ae 
The only fossils found here were fresh-water clam shells, which 
occurred in considerable numbers upon a space four rods in > 
diameter near the middle of Gregg & Griswold’s excavation, 
lying in the upper foot of the clay, just beneath the till. This | . 
interglacial clay, overspread by till, testifies that an ice-sheet 
covered this region after the Minnesota valley had been 
nearly as it now is. 
eroded 
Another observation which seems to give the same testi- ue 
mony, and to show that the modified drift forming high terra- 
ey = plains in this valley was deposi Ee ho neath 
Of the ice-sheet, is presented, in the notably uneven suriace © 
the broad part of oh terrace of this’ valley drift in es 
rm 
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