STUDIES FOR STUDENTS. 713 



distinctly nearer the former than the latter. This is not to be con- 

 strued to mean that the drift is not often thick both without and 

 within the zone here specified, or that it is never thin in this zone. 



Any theory of the drift must take account of these facts. 

 The agent or agents to whose activity it is to be attributed must 

 have been able to leave many small patches, not always higher 

 or lower than their surroundings, essentially without drift. They 

 must have been such as could have left the drift now in thick 

 beds, and now in thin, over limited or extensive areas, without 

 close dependence on topography or altitude, yet without com- 

 plete independence of either. 



Thickness of the drift along its borders. At its margin, the drift 

 sometimes thickens so as to constitute a considerable ridge. 

 This is the exceptional condition of things rather than the gen- 

 eral. In other cases the drift grows thinner and thinner toward 

 its edge, its limit being still well defined, and constituting a defin- 

 itely traceable line. In still other cases the drift feathers out at 

 its edge, ceasing to exert any observable influence on topography. 

 In such cases the border may become so ill defined that it is 

 traceable only with difficulty. 



Driftless Areas. 



Besides the many small patches of bare or nearly bare rock 

 over which the drift forces acted without leaving deposits of 

 much thickness, there is, far within the outermost limit of the 

 drift-covered country, an area several thousand square miles in 

 extent, where drift is altogether absent. This area lies mainly 

 in southwestern Wisconsin, but embraces small adjacent areas in 

 Illinois, Iowa, and Minnesota. The absence of even a meager coat- 

 ing of drift from this area has led to the conclusion that the agency 

 or agencies which produced the drift were not here operative. 

 There is probably a second, smaller driftless area, also, in the axis 

 of the Mississippi basin, occupying a part of the elevated land 

 between the Illinois and Mississippi rivers, near their junction. 

 This area is much nearer the border of the drift, and appears 

 much less anomalous than the other. Neither of the driftless 



