4 ON THE FRESH-WATEE, GLACIAL DRIFT 



behveen the Fox and the Wissakote rivers, the cavities are filled with trees. Near 

 the Wissakote in T. 40 N., R. 18 E. (Wisconsin meridian) at an elevation of 800 

 feet above Lake Michigan, they are broader and trough like in form ; the drift is 

 more sandy, and small lakes, ponds, or marshes, are occasionally seen at the bottom. 



Fig. 2. 



J^HoKizoNTAL Pkojection of drift cavities 15 to 60 feet deep, head waters of Oconto river, Wisconsin. 

 o o o Large Boulders of Sienite — 350 feet above Lake Micliigan. 



Of course, the boulders and the gravel are here derived from the azoic and igne- 

 ous rocks at the north. On the line of the survey for the " Chicago, St. Paul, and 

 Fond du Lac Railroad," in T. 34 N., R. 17 E., on the north of the Peshattego river, 

 at an elevation of 660 feet, the " kettles" are very numerous, and sharply defined. 

 Proceeding southerly, a series of them occur in T. 31 N., R. 17 E., about twenty 

 miles north of the Oconto, the summits of the country being 335 feet above Lake 

 Michigan and 913 feet above the ocean. 



Those on the dividing ridge, between the waters of the Avest branch of the 

 Oconto and the Wolf rivers, in T. 32 N., R. 15 E., have an elevation of 350 to 

 400 feet, and aff"ord the finest instances of steep and well defined cavities. Ter- 

 races and oblong ridges of sand or gravel might be formed by currents and eddies 

 acting upon loose materials. It is not difficult to perceive how mounds, irregular 

 elevations, and undulations, could be thus built up by gradual accretion, above the 

 general surface. But the formation of a system of depressions of an uniform 

 character, over large tracts of country, without natural mounds or ozars, is some- 



