66 PLEISTOCENE OF INDIANA AND MICHIGAN. 



All these ridges appear to be made up mainly of waterworn assorted material and should 

 perhaps be classed as kame moraines, though they are not parallel with the neighboring parts 

 of the glacial boundary. The Mount Auburn Ridge trends north-northeast and south-southwest, 

 directly toward the glacial boundary instead of parallel with it. The trend of the other two 

 ridges diverge only slightly from that of the neighboring glacial boundary, and they are each 

 within about 5 miles of the limits of the well-defined drift sheet. Mount Auburn Ridge appears 

 to have been formed at the junction of converging ice currents, for it stands back of a sharp 

 bend or reentrant in the glacial boundary. 



Detailed mapping of the knolls of this earlier drift has not been attempted. In the hilly 

 districts along the drift border it is difficult correctly to interpret some of the hills, for some 

 which had been supposed to have a rock nucleus have been proved by boring to be made up 

 very largely of drift. Even the detailed studies by Fuller and Clapp in the quadrangles in 

 southwestern Indiana were not carried far enough to warrant a separation of all the drift hills 

 from those with a rock nucleus. 



Ridges of pre- Wisconsin drift some distance inside the limits of the Wisconsin, though 

 known by borings to be present, are so deeply buried under the later drift that little is known 

 concerning their topography. Those in southeastern Michigan appear to be broad, massive 

 belts rather than sharp ridges. They will be considered in connection with the discussion of 

 the later drift. 



STRUCTURE OF THE PRE- WISCONSIN DRIFT. 



COMPOSITION. 



The glacial deposits found outside and also beneath the Wisconsin drift for some distance 

 inside its limits consist very largely of a clayey till oxidized to a brownish color near the surface 

 but becoming a blue-gray unoxidized till at a depth of 20 feet or less. The till as a rule is 

 thickly set with small stones and contains some bowlders, though few large ones; often several 

 exposures must be examined to find one that exceeds a foot in diameter. Yet some 4 to 6 feet 

 in diameter have been found clear out to the limits of glaciation. 



The character of this drift sheet varies to some extent in accord with the underlying rocks, 

 yet in other respects it is constant. Thus the calcareousness of the till is striking even in 

 districts where it overlies sandstone. The rock constituents include limestone derived from 

 the formations along the path traversed by the ice. The character of the drift sheet varies 

 with the drainage conch tions along the ice border; thus gravelly and sandy material is scarce 

 over much of the area of exposure in western and eastern Indiana but is more common in central 

 Indiana. In Owen, Morgan, Johnson, and Brown counties, Ind., sand and gravel form the 

 bulk of the drift knolls and ridges as well as the filling in the preglacial valleys. The attenuated 

 border in Pike and Gibson counties, as indicated by Fuller and Ashley, 1 contains considerable 

 sand and gravel, but the preglacial valleys of that region, except the Ohio and Wabash, are 

 largely filled with silt or fine sand. Records obtained in several counties in southwestern 

 Indiana show that the pebbly glacial material forms only a thin capping of the surface. 



The silt filling is attributable to deposition in ponds caused by ice blocking the mouths of 

 valleys that drained northwestward, and hence were favorably situated for receiving the 

 turbid discharge from the ice sheet but not for receiving the coarser outwash. These silt 

 deposits were eventually covered by the advancing ice as far out as the limits of glaciation. 

 Their calcareousness in a sandstone region, such as that of most of the valleys thus filled, shows 

 clearly that the ice sheet contributed the material. Wells in these silts yield decidedly harder 

 water than those sunk in the rock formations of the region, and this condition of the water has 

 furnished an index of the nature of the silts at many points where they have not been exposed 

 to view. 



Valleys with this sort of filling are conspicuous in Posey, Gibson, Vanderburg, Pike, 

 Dubois, Davis, Clay, and Vigo counties and occur in other counties. In places the silts underlie 



i Ditney folio (No. 84), Geol. Atlas V. S., TJ. S. Geol. Survey, 1902, p. 3. 



