=. 
a. 
ee 
_ 
Minnesota Clays.—Berkey. 17 
un 
that the material was actually laid down in quiet water such 
as a lake would afford. The clays of this class appear to be 
interglacial in point of time of accumulation and seem to have 
been in part an accompaniment of the oscillations and mutual 
adjustments of the ice streams from the east and the west. It 
is only in the zone representing the intermediate territory be- 
tween these two that such deposits have been found, Accord- 
ingly glacial lake clays are closely confined to the eastern bor- 
der of the state. Whether or not the series of lakes repre- 
- sented by lake Undine in the Minnesota river valley was ac- 
companied by similar valuable deposits is not clear from pres- 
ent development. Such as are well known seem to owe their 
accumulation more to the river than to the existence of the 
lakes. 
But in eastern Minnesota and northwestern Wisconsin oc- 
cur extensive and valuable deposits that are certainly glacial 
lake clays. In Minnesota a large development has been war- 
ranted in one of these lake beds at Wrenshall in Carlton county 
within thirty miles of the head of lake Superior. Several 
plants are firmly established and the total output is approxi- 
mately twenty millions per year. All are common brick chiefly 
of the sand mould type, but the quality is excellent. 
This deposit has an areal extent of two or three miles at 
least and recently another part of it at Clear creek has been 
opened. These beds are strongly stratified in blue and gray 
alternating layers from the bottom to within a few feet of the 
top where a red color prevails. Total thickness is more than 
forty feet. Brick made from this top stratum burn deep red, 
those from the bluish gray zone below burn cream colored. 
So extensive an occurrence of gray material which is usually 
associated with the western drift is somewhat surprising in a 
locality so far east. But the fact is plain and the presence of _ 
a capping of red clay may throw some light on conclusions as 
to sources of supply of material at that time. Of course a 
red zone at the top might be caused by weathering® as in the 
‘case Of some of our residuary products in other areas, but the 
fact that the line of division between the two colors is rather 
sharply marked and the fact that the red stratum and its 
* Minn. Geol. and Nat. History Survey, Final Rep., vol. iv, p. 21, Minne 
apolis, 1899. 
