582 THE GEOLOGY OF MINNESOTA. 



[Interglacial modified drift. 



reached is the general level of the vast prairie of gently undulating till, through which the Min- 

 nesota valley is excavated. The grade cuts to a depth of about 40 feet at the edge of the bluff 

 and thence ascends, with decreasing depth of cut, along a distance of some twenty-five rods, to 

 the surface of the drift-sheet. This section exhibits two beds of true till, separated by modified 

 drift which is probably an interglacial formation, supplied, as already stated, at the time of final 

 melting of the earlier ice-sheet and spread beyond its receding margin upon the unchanneled sur. 

 face of the till that had been formed during that earlier part of the ice age. The upper bed of till, 

 apparently representing the total thickness of the drift deposited here in the last glacial epoch, is 

 16 to 18 feet thick, and is an entirely unstratified yellowish gravelly clay, containing occasional 

 rock-fragments up to six or eight inches in diameter, but showing only two or three of larger size, 

 these being two or three feet in diameter. Portions of this till, within six to eight or ten feet 

 below the top, are gray, with limy concretions and limy layers that have been gathered by 

 percolating waters. The bottom of this upper till, seen clearly exposed along a distance of about 

 250 feet, is an almost exactly level line. Next below is the earlier modified drift. Its thick- 

 ness is also 16 to 18 feet, levelly stratified throughout, but having the horizontal layers often 

 obliquely laminated. The dip of this lamination, which marks the direction of the current 

 of water that brought this sediment, is to the east or northeast, toward the Minnesota river, 

 and varies in amount from two or three to fifteen or twenty degrees. Floods produced by glacial 

 melting, and carrying gravel, sand and clay that had been contained in the ice-sheet, appear to 

 have taken their course along the central depression of the Minnesota basin, coming from ice- 

 fields which still covered its upper portion, with their retreating border probably only a few 

 miles distant at the time when this stratum was deposited. Its largest pebbles are six to eight 

 inches in diameter. The underlying till was seen along an extent of 100 feet, the greatest depth 

 cut into it being about eight feet. Its upper line, separating it from the modified drift is ap- 

 proximately level but undulating, with its highest points two or three feet above the lowest. 

 This till, like the upper bed, bears no marks of stratification; and neither shows any interbed- 

 ding or transition, but both are bounded by definite lines, at their junction with the intervening 

 gravel and sand. The lower bed of till is dark bluish, excepting for about twenty feet from the 

 face of the bluff inward, where weathering has changed it to the same yellow color that charac- 

 terizes the modified drift and upper till. 



FIG. 47. SECTION OF THE MINNESOTA VALLEY, SOUTHWEST FROM THE RIVEK, AT NEW ULM. 



The modified drift below till, mentioned in Courtland, ten miles southeast from this local- 

 ity, is made known by a well that was bored to a depth of 100 feet at the house of Carl llichert, 

 in the S. W. J of section 11, upon the upland or general level, even in hight with the top of the 

 bluffs of the Minnesota valley, from which its distance is about an eighth of a mile. Its section 

 was soil, 2 feet; yellow and gray till, 30 feet; sand, mainly yellow, but in considerable part white 

 near the bottom, 54 feet; and yellowish gravel, 14 feet and extending lower. No water was 

 obtained, and the well was given up. 



Modified drift of tlie last glacial epocli. Upon the sheet of till which covers these counties 

 are frequently noticed mounds and knolls or short ridges of gravel and sand, 10 to 20 feet, or 

 rarely 30 feet or more, in hight, which in any excavation are seen to be irregularly interstratified 

 and obliquely bedded. These deposits appear to have been formed by streams that llowed from 

 the drift-strown surface of the departing ice-fields of the last glacial epoch; having a similar 

 origin with the eskers or kames, which form prolonged ridges, or series of interlocking ridges and 

 mounds, in Ireland and Scotland, in Sweden, and in New England. Conspicuous kame-like de- 

 posits of modified drift in Redwood county were observed in the N. E. } of section 33, Swede's 

 Forest, where a mound of this class rises some 30 feet above the general level; in the northwest 

 part of Vesta, which has numerous hillocks and short ridges of gravel and sand, 10 to 40 feet in 

 hight, trending from north to south more commonly than in other directions; and in T. 11 1, B. 

 38, and thence southwestward to the Cottonwood river. In Brown county a notable series of 

 kames, or short ridges and knolls of gravel and sand, 25 to 40 feet high, occurs about a mile east 



