OFTHENORTHWESTERNSTATES. 3 



The Moraine hillocks and cavities that are represented on the map near the line of 

 the profile, in Northern Ohio, Wisconsin, and Minnesota, are in this member. It 

 may be considered strange that the coarsest material should occupy the highest 

 drift summits, but such is uniformly the case. These cavities extend below the 

 general surface ten, fifteen, and even one hundred feet, their outline being rudely 

 circular, and their sides as steep as is consistent with stability of the soil. 



Deipt Cavities, ok "Potash Kettles," near Greenbnsh, Wisoonsiu. Range of drift hills looking west. 

 000 Boulders of Northern rocks — base 150 feet above Lake Michigan. 



About Lake Winnebago, the pebbles and boulders of the subjacent Niagara lime- 

 stone constitute a large portion of the mass, with which sand and gravel are inti- 

 mately mixed. I have traced them one hundred and fifty miles farther in a northerly 

 direction to the Wissakote or Brule river. 



After passing northward beyond the sedimentary rocks above Lake Winnebago, 

 the proportion of sand increases, and also the size and number of the boulders, 

 which are mostly of igneous origin. To form an idea of the appearance of the 

 " potash kettle" country, we may imagine a region of drift moraines inverted, and 

 instead of a surface thickly set with rounded hillocks, suppose it to be occupied by 

 cavities of irregular size and depth. If the grinder of a mastodon were impressed 

 upon a piece of clay the depressions which result would represent the drift cavities as 

 contrasted with drift elevations. In travelling through such a region the explorer 

 frequently finds these hollows so near together that he no sooner rises out of one 

 than he is obliged immediately to descend into another, the diameter of which 

 may not be more than twice or thrice its depth. 



There is very seldom any water in the bottom, owing to the loose and porous 

 character of the gravel drift. Boulders are found at the bottom, on the sides, and 

 on the surface around them. Where these cavities are thickly set, as at the source 

 of the Oconto river, and are without hillocks, the rim or edge between them is 

 sometimes so narrow, that large boulders have not base enough to rest upon, and 

 tumble down the sides. (See Fig. 2.) 



The internal slope is occasionally straight like a funnel, or inverted cone, but 

 oftener cup-shaped or curved in a manner correctly represented by the form of a 

 kettle. In the prairie regipn of Southern Wisconsin, timber grows within the 

 cavities ; as it does on the adjacent lands in clumps, or as separate trees, under the 

 local name of "oak orchards." Farther north, in the thickly timbered country 



