E. H. Mudge — Pre-Glacial Drainage in Michigan. 383 



Aet. XLI. — Some Features of Pre-Glacial Drainage in 

 Michigan; by E. H. Mudge. 



To reach a definite and satisfactory conclusion in regard to 

 the condition of the pre-glacial surface of a region now deeply 

 covered with drift, is not an easy matter. The best, perhaps, 

 that can be done is to examine the surface of an unglaciated 

 region of like geologic age, and with the knowledge thus 

 gained examine such data as may be obtainable from the cov- 

 ered region. 



The lower peninsula of Michigan is such a covered area. 

 In all the glaciated area of North America there is, I think, no 

 region of equal extent, approximating 40,000 square miles, 

 that is so deeply and uniformly covered with drift as this. 

 Except within the southern half of the Carboniferous area, 

 and in the vicinity of its southern rim, there are practically no 

 outcrops in the interior of the state. The glacial mass seems 

 to have concentrated itself upon this territory. Though the 

 general direction of the glacial movement was southwest, that 

 portion of the ice sheet which would naturally have passed 

 over Wisconsin was largely deflected to the south by the val- 

 leys now occupied by Lakes Michigan and Huron, and joined 

 with the great body which passed over Michigan,* mingling 

 its volume of drift from the Lake Superior region with that 

 derived more locally from the sedimentary terranes of the 

 lower peninsula. The drift-mantle resulting from these con- 

 ditions is so deep and so uniformly distributed that any conclu- 

 sions in regard to the pre-glacial surface must be largely 

 speculative, though we are not entirely without data bearing 

 on the subject. 



One thing seems certain. We are quite sure that during 

 the several millions of years between the close of the Car- 

 boniferous and the beginning of the ice invasion the surface 

 was exposed to the constant influence of the agents of denuda- 

 tion and erosion — rains, frosts, winds, chemical action and run- 

 ning streams. We conclude, therefore, that the condition of 

 the surface just previous to the ice invasion was not very 

 unlike that which may now be seen in unglaciated regions of 

 similar age. The unglaciated region most suitable for com- 

 parison in this case is doubtless the driftless area of Wisconsin, 

 above referred to. With this as a criterion, we may infer that 

 our field was traversed by broad base-level valleys and sharp, 

 ridge-like divides, the latter sometimes cut through by the 



*" Driftless Area of "Wisconsin," T. C. Chamberlin, Sixth Ann. Report U. S. 

 Geol. Survey. ♦ 



