AVERAGE THICKNESS OF THE DKIFT IN ILLINOIS. 543 



There are two quite different methods by which the thickness of the 

 drift may be estimated. The first is that employed some years ago by 

 Chamberlin and Salisbury, which is based upon a comparison of the 

 Driftless Area of southwestern Wisconsin with drift-covered areas on its 

 borders. 1 The second is based upon wells or other borings and upon rock 

 outcrops within the drift-covered areas. By either method only a rude 

 approximation can be expected. In comparing drift-covered areas with the 

 Driftless Area by the first method, some allowance must be made for depar- 

 tures within the drift-covered areas, from the type or types of topography 

 exhibited in the Driftless Area. A region underlaid by shale is likely to 

 have a topography somewhat different from one underlaid by limestone or 

 sandstone, while among the several classes of limestone and sandstone much 

 variation in topography may be found. To make discriminative estimates 

 it is necessary to have a fair acquaintance with the several types of topog- 

 raphy likely to be encountered within a given drift-covered area, as well as 

 with the types found in the Driftless Area. Such an estimate will also be 

 made more reliable through an acquaintance with the distribution of the 

 main preglacial lines of drainage in the drift-covered area under investiga- 

 tion and the extent of each preglacial watershed. In an estimate based 

 upon borings and outcrops there is room for much error, since they may 

 inadequately test the thickness of the drift. Indeed, it is to be expected 

 that there will be a larger number of outcrops and a greater proportion of 

 borings which reach rock along the line of the preglacial ridges than along 

 the preglacial valleys, especially where the drift has filled the valleys to the 

 level of the ridges. By employing both methods the results are more likely 

 to be reliable than by either method alone. The borings throw some light 

 upon the preglacial topography and thus help in an estimate in which pre- 

 glacial topography is made the main basis of calculation; while in an estimate 

 based largely upon borings and outcrops a knowledge of the topography of 

 neighboring driftless tracts is of great value. The data from well borings 

 and the results of estimates based upon them are first presented. These 

 results are then considered in the light of an estimate based upon the 

 requirements of the topography. 



The thickuess of the drift has been ascertained at 1,339 points within the 

 district covered by the Illinois lobe, of which 1,179 are within the State of 



' See Sixth Ann. Rept. U. S. Geol. Survey, 1884-85, pp. 257-258. 



