42 LAMINATED CLAYS OF GRANTSBURG, WIS. 



producing the laminated clays at Grantsburg, existed 1,700 years, 

 and that the accompanying minor retreat and readvance of the ice- 

 margin consumed the same length of time. 



The whole problem of time relation to geologic process is a difficult 

 one. There is no very satisfactory formula for the solution of either 

 a process or an interval in so exact terms as years. The most suc- 

 cessful attempts at such solution are for the interval between the 

 final withdrawal of the ice from particular localities and the present 

 time. For the vicinity of Buffalo by means of Niagara Falls, and 

 for the vicinity of Minneapolis by means of St. Anthony Falls, careful 

 estimates have been made and are widely known: In both cases, 

 however, there are variable factors capable of modifying any par- 

 ticular result seriously, and even multiplying the lower estimates 

 many times. The lowest estimates are in general those either pub- 

 lished earliest or those which pay the least regard to possible variation 

 of the factors. But there is so far no more precise estimate as to this 

 interval, or any part of it, from any other source. 



It is believed, however, that the Grantsburg clay beds furnish 

 just such data, and that, so far as they go, they are more precise than 

 the two familiar examples cited above. 



The interval covered by this estimate of 1,700 years measures 

 only one of the minor oscillations of the Lake Superior ice-lobe. In 

 its whole retreat from St. Anthony Falls to the highlands of its source 

 there were doubtless several. similar fluctuations. Evidence of this 

 in part is the great development of terminal moraines piled across 

 the region, and the occurrence of similar laminated clay beds at other 

 localities. It would seem as though the time actually required for 

 the retreat of the ice from St. Anthony Falls to Lake Superior alone 

 must have been at least as great as the smallest estimates formerly 

 made for the whole postglacial interval. 



Summary. — A laminated clay bed 35 feet thick is exposed on Wood 

 and Clam Rivers in Burnett county, Wisconsin. 



The area estimated to be underlain by this deposit is more than' 

 300 square miles in extent. 



Excellent bricks are being made, and the material is capable of 

 more uses. 



The deposit is of lake origin fed by melting glacial ice. 



