334 E. H, Mudge — Pre- Glacial Drainage in Michigan. 



headwaters of streams, leaving detached erosion blocks, which 

 in some cases had become reduced to fragile erosion towers, 

 ready to tumble to decay. The more level surface was cov- 

 ered with disintegrated rock, not yet borne away. There were 

 no lakes or marshes ; these, if any there ever were, having been 

 drained by the cutting down of their outlets. If, however, we 

 would determine the location of the old streams, and join 

 them into a consistent drainage system, we must depend upon 

 such knowledge as an examination of the territory itself may 

 yield us. This is not inconsiderable, though quite fragmentary 

 and indefinite. 



Notwithstanding the vigorous glacial action to which the 

 lower peninsula was subjected, the more prominent features of 

 the old topography do not appear to have been entirely oblit- 

 erated. The greater valleys were not entirely filled, nor were 

 the greater eminences smoothly planed down. The leveling 

 process was left incomplete. Of the old valleys thus left in 

 recognizable shape, that which crosses the state from east to 

 west, and which is now occupied in part by the Saginaw river 

 and its branches, and in part by the Grand and Maple rivers, 

 is the most clearly discernible. It is still a striking topo- 

 graphic feature, and during the departure of the ice sheet it 

 was for a long period the natural outlet of the great glacial 

 lakes farther to the east. The theory that this valley is the 

 modern representative of a far greater pre-glacial valley is by 

 no means new, the same having been set forth especially by 

 Dr. J. W. Spencer.* The evidence on this point is clear. It 

 may be inferred from the great lateral extent of the depression, 

 indicating that it is not of post-glacial origin, and it is clearly 

 proven by a deep boring at Alma, which penetrated about 500 

 feet of drift and failed to reach the rock surface. There is 

 reason to believe that the axis of the ancient valley was some- 

 what farther north than the center of the modern depression. 

 The Alma boring is well to the north, while at one point in 

 the bottom of the present river valley near Ionia the rocks 

 come to the surface and project several feet above the river, f 

 Two recent borings at Ionia, near the valley margin, found 

 rock at moderate depths. This would indicate that the modern 

 valley is located over the southern edge of the ancient one, 

 the northern portion having been more completely filled by 

 the flood of glacial debris which was swept into it from that 

 direction. I have, therefore, on the accompanying map, 

 located the ancient river somewhat to the north of the modern 

 streams. 



* Quar. Jour. G-eol. Soc, November, 1890. 



f See the writer's paper in this Journal for November, 1895. 



