GEOLOei OF EASTEEN WISCONSIN. 



OHAPTEE I. 



TOPOGEAPHT. 



When Eastern "Wisconsin first emerged from the ocean, it doubtless 

 presented an essentially plane surface, having a slight inclination to 

 the east and southeast. The ii:regularities which it now presents are 

 due to subsequent changes, the results of three classes of agents, act- 

 ing at different times and under different conditions. 



1. During the long ages between the emergence of the land and the 

 drift period, the streams were cutting their beds deeper and deeper 

 into the rock, and rendering the former level surface more and more 

 irregular. The softer rocks were more readily eroded than the harder 

 ones, and this helped to increase the unevenness. There was a ten- 

 dency of the streams, as far as the slope favored, to follow the less 

 resisting belts of soft rock, and as these run in a northerly and 

 southerly direction in this region, the main streams had that direction. 

 The little streams gathered into the larger ones in a manner not un- 

 like that by which the branches of a tree are united into the trunk. 

 The unevenness of surface produced by erosion of this nature pos- 

 sesses a certain kind of system and symmetry readily recognizable. 

 As this erosion occupied the time preceding the Glacial period, we 

 may conveniently designate the features produced by it Pre-Glacial. 



We have the best example of this kind of surface configuration in 

 the Lead region, over which the drift forces did not act, and which 

 has not been resubmerged, so that we have the results of this class of 

 action pure and simple. As we proceed eastward into the region of 

 drift action in the central part of the state, these features are 

 modified more and more by the results of glacial action, until in east- 

 ern Wisconsin they become wholly obscured, except in their grander 

 outlines. 



Wis. Sub.— 7 



