398 ANNUAL REPORT. 



nal occurrence of bare patches in the midst of a drift covered 

 region, like islands in the sea. Among the latest theories is that of 

 Professor Chamberlain, of Wisconsin,* who suggests that that part 

 of the broad continental ice sheet crossing Iowa from the north- 

 west, in passing over the elevated ridge of Keeweenaw Point and 

 the Wisconsin Highlands, was thinned out and held back till its 

 force was expended and the ice mass melted before this driftless 

 area was reached. As the ice sheet on either side of this high land 

 moved on unintercepted, the lobes flowed together and again formed 

 a continuous mass. But Pigeon Point is narrow and the hills 

 around Grand Portage abrupt, hence the drift accumulations left 

 upon the rocks in that corner of the state, have been almost wholly 

 washed into the lake by the snow and rain of a few thousand 

 years, t 



"In listening to your lecture before the Horticultural Society, I 

 noticed one assertion made by you, as before by Prof. Winchell in 

 the first volume of the final report of this survey, now in press, 

 which I cannot agree with. This is, that a considerable tract 

 north of lake Superior in the extreme northeastern part of Minne- 

 sota is destitute of glacial drift ! ! ! To be sure, the deposition of 

 drift there is by no means so great as in central and western Min- 

 nesota; indeed the bald rocky hills (like the surface in extensive 

 tracts near Salem and Lynn, Mass.) appear to have been swept 

 by the moving ice-sheet almost as clear of ail surface material as 

 possible, leaving the same only in the lee of the jutting ledges and 

 hills of rock. But in my opinion it is verij erroneous and mislead- 

 ing to call that a "driftless region." All the material there in 



WHICH TaEES AND VEGETATION TAKE ROOT MUST BE GLACIAL DRIFT, 



and of course timber and herbage cover all that is not bare rock. 

 We have such tracts among the hills in some parts of New Hamp- 

 shire; and I have no doubt that in some of the sheltered places in 

 the lee of hills of rock and in hollows between ledges, the glacial 

 drift in that northeastern part of Minnesota occurs in depths vary- 

 ing from a few feet to even fifty feet." 



The character of this deposit varies considerably. By far the 

 larger part is what is denominated by geologists, "Till." This is a 

 term used "to designate a compound mixture of bowlder, clay, 

 gravel and sand, formed by glaciers, constituting their ground 

 moraine." In Stearns county, bowlders of enormous size are seen, 

 sometimes weighing hundreds of tons. And all over the state 

 wherever this Till occurs, bowlders of various sizes from the 

 masses mentioned down to mere pebbles are abundant. If we 



*Geol. of Wis., Vol. I, p. 270. 



t After the MS. of this paper was prepared for the press, the following postscript to 

 a note was received from Mr. Warrea Upham, Assistant Geologist on tne Geological 

 and Natural History Survey of Minnesota : 



