CHARLES P. BERKEY 39 



most approved paving quality. Ornamental terra-cotta, however, is 

 the most promising line of development to which this clay lends itself. 



In practice the chief problem is to secure complete intermixture 

 of the two types of laminae. They have very different shrinkage and 

 fusibility. When the mixing is thorough, however, the result is a 

 clay of superior quality and capable of use in higher grades of wares. 



Geological relationships. — Evidence as to the age and origin of 

 this deposit has been in part involved in the preceding description. 

 The laminated clays rest upon a thick accumulation of red glacial 

 till of typical late Wisconsin character. At the mouth of Wood 

 River this is well exposed. Occasional horizontal markings seen 

 here throughout a thickness of about 40 feet of this till are thought 

 to indicate standing water during its deposition. On the St. Croix, 

 half a mile away, this same till in turn rests upon the eroded surface 

 of white and yellow friable Cambrian sandstone. 



Everywhere above the clays rests a sand or till capping, or a 

 mixture of both. It contains coarse sand in greatest quantity, but 

 also gravel and bowlders. At the clay pits near Grantsburg, the 

 overlying gravelly till also contains an occasional pyritiferous and 

 carbonaceous nodule very like those sometimes seen in lignite hori- 

 zons. There are also numerous chunks, angular fragments, of 

 laminated clay, of the size of small bowlders which, in connection 

 with the furrowed or uneven surface of the clay deposit itself, argues 

 a plowing up of some of this material by subsequent advance of 

 glacial ice. The capping drift does not show water marking, and 

 is of variable thickness, with occasional typical morainic aspect. 

 Wherever the surface is level for any considerable area the sloughs 

 and meadows are found. These are a surprising phenomenon in a 

 country that appears to be all sand, but no doubt the effective bottom 

 is formed by the underlying laminated clay beds. 



It is clear that the clay deposit was accumulated some time in the 

 late Wisconsin stage of the Glacial period. Heavy deposits of Wis- 

 consin till preceded the clays, and there was a readvance of the ice 

 covering the whole area again after the laminated clays were laid 

 down. The thin cap left by this last advance, and the little disturb- 

 ance of previous deposits, argue a place at the immediate close of 

 the stage for this locality. 



