80 GEOLOGY OF OHIO. 
removing ice-dams, may have produced the great changes in our drainage 
system of which the evidence is so unmistakable. The accompanying 
map, which is compiled from data furnished by Dawson, Logan, Whit- 
tlesey, and my own observations, will probably aid in making the views 
advanced on the preceding pages a little more intelligible than they 
would otherwise be. 
