PHYSICAL FEATURES. 61 



bottom of the glacial deposits, and those that have cut through them commonly encounter 

 merely the crests of preglacial rock ridges; throughout the greater part of both Indiana and 

 Michigan the beds of the present streams are above the tops of the rock ridges of the districts 

 they traverse. 



The drainage varies from a very immature stage, in which the streams have scarcely begun 

 to open a channel, to a well-defined, widely branching system with well-graded channels. 

 Drainage within the glaciated region, however, is nowhere mature ; in the Illinoian drift area 

 much poorly drained surface remains even where channels are well graded; and in the Wis- 

 consin drift area many of the stream channels themselves are immature and have numerous 

 lakes along their courses. 



In much of Michigan the development of surface drainage is proceeding very slowly, because 

 the drift is sufficiently porous to rapidly absorb nearly all the rainfall. Many townships are 

 wholly destitute of surface streams and are yet so well underdrained that the soil is not swampy. 

 On the other hand, many tracts with clayey or impervious soil have plentiful surface drainage 

 and yet remain swampy in parts because their soil absorbs so small a percentage of the rainfall. 



THICKNESS OF THE DRIFT. 



The region covered by this monograph probably embraces the largest area of thick drift 

 in North America. The greater part of the southern peninsula of Michigan and the northern 

 and central portions of Indiana are so deeply covered that wells seldom reach the rock. The 

 average thickness of drift in the southern peninsula of Michigan, as determined ' by Cooper's- 

 planimeter measurements, already noted, is about 300 feet, and an equally great thickness 

 prevails over several counties in northern Indiana. It is probable that on parts of the southern 

 peninsula of Michigan nearly 1,000 feet of drift is present. In Indiana the greatest thickness 

 reported is about 500 feet (at Kendallville) . 



The geologic map of the southern peninsula of Michigan (PI. II) shows the rock altitude 

 by bedrock contours. On part of the western side of the State, from Ludington northward 

 about to Frankfort, the rock surface lies about at sea level and to reach it wells near the level 

 of Lake Michigan penetrate over 600 feet of drift. 



In Indiana the distribution of the belts of thick drift (PI. IV) is in part due to the filling 

 of preglacial valleys, notably the Whitewater, the East White, and the Wabash. In the central 

 and northern parts of the State, however, the drift is 100 feet or more thick on the preglacial 

 interfluvial tracts and 300 to 500 feet along the lines of preglacial drainage. 



