174 The American Geologist. March, 1902 
B. Glacial Clays. Clays which may be traced directly to 
conditions connected with glaciation are designated by the 
three terms till, glacial stream deposits and glacial lake clays. 
All of them are abundant, each type is used in common brick 
manufacture and here and there some particular deposit has 
proven of special importance and value. 
1. Glacial Till or boulder clay is most common of all the 
glacial clays. Occasionally there has been a partial assorting 
of materials approximating the well known “modified drift” 
and an accumulation of the finer clayey constituents into de- 
posits comparatively free from injurious coarse matter. By 
far the greater number of deposits of course do carry much 
injurious matter of this sort and are not workable for this 
reason. In localities where some effective assortment has op- 
erated and where the Kaolinic constituents have been abund- 
ant these accumulations become useful and important. 
An important deposit of this kind is that near Princeton in 
Mille Lacs county where a splendid quality of common red 
brick is being made in large quantity. Similar deposits are 
worked in other localities but most of them on a smaller scale. 
In the case of these till accumulations especially a knowl- 
edge of the main features of glaciation serves as a good guide 
to certain standard qualities of the products from them. For 
example there are two contrasting types of drift in Minnesota, 
—one is called the “gray” drift which has been brought by ice 
movements from the north and northwest, the other is called 
the “red” drift which has come from the north and northeast. 
These two overlap in part in the central areas, but the two are 
seldom intimately mixed. As a result each gives its own char- 
acteristic quality to the clay beds produced from it. Gray drift 
carries an excess of lime and accordingly bricks made from 
the unweathered clay of this relationship always burn light- 
colored. Where weathering has converted this, however, into 
a residuary product, loss of lime may cause sufficient alteration 
of proportions to give a red color. The red drift carries an 
excess of iron. These clays burn red. But as a rule they are 
inclined to exhibit greater coarseness of texture and are also 
of smaller areal extent than the gray drift clays. 
2. Glacial lake clays are closely related to the modified 
condition of the last class, but in this case it is always clear 
