54 



The geological conditions of the country greatly influence the course 

 and action of streams. The heavy curved line across the map represents 

 the southern limit of the ice sheet. Thus this drainage area is partly in 

 the glaciated and partly in the unglaciated portion of the State. It is in 

 the unglaciated region that we have the most picturesque scenery. The 

 entire area, subjected to the processes of weathering and stream erosion 

 for millions of years, was maturely dissected into a complex network of 

 valleys, ridges and isolated hills. Over this surface the ice^sheet passed 

 several times, extending as far as the boundary shown. Its effect was to 

 smooth off the hills, till up the valleys and to leave the surface covered 

 over with a great mass of loose, foreign material from the northern re- 

 gions. Since glacial times the streams have to some extent removed the 

 loose material from some of the old valleys and are forming a system of 

 new drainage in the surface of the drift. Geologically speaking, this glacial 

 accumulation is of very recent origin and the streams seem to have made 

 only a small beginning in the work they will be able to perform. 



An accurate topographic map of the drainage area would show the 

 contrast in the physical features of the glaciated and unglaciated portions 

 better than any other description or illustration that could be given to a 

 person who had not been over the area to investigate the contrast. In the 

 glaciated area the contour lines would run in large regular curves and 

 far apart, showing the smoothness and regularity of the surface. South 

 of the drift limit the lines would be very close together, with a very wind- 

 ing course and sharj) curves, showing a region of deep, narrow valleys, ir- 

 regular divides and abrupt cliffs. 



In attempting to work out the geographic history of an area whose 

 drainage has been arrested by the invasion of an ice-sheet, we find that 

 the story of the life resolves itself into four fundamental parts. First: 

 What are the topographic characteristics of the area during the pre- 

 glacial history. Second : What changes took place during the glacial 

 history. Third : What has happened since the disappearance of the ice- 

 sheet ; its post-glacial history. Fourth : What was the effect produced 

 by the above events on the unglaciated parts of the area. 



It is doubtful if the entire glacial area in Indiana was covered by 

 the ice-sheet at any one time. At its extreme limit the ice deix)sited but 

 little drift; and as a rule there is not a well-defined ridge of drift along 

 the glacial boundary, though some drift is to be seen — as in Chestnut 

 Ridge, in Jackson County, and a similar ridge in southern Morgan County. 



