GLACIAL GEOLOGY OF SOUTHERN MICHIGAN. 105 



Survey, that the oldest striae iu the region between Lake Superior aud 

 Hudson Bay have a southeastAvard bearing, which he interprets to 

 "prove that the Keewatin ghicier overran the region north of Lake Su- 

 perior before it was covered by the Labrador ghieier."* The present 

 evidence, therefore, seems to indicate that the first invasion into this 

 peninsula may have been by ice coming across Lake Superior from the 

 region Avest of Hudson Bay, and that it was not until after this ice 

 field melted from Michigan that the Labrador ice field camr' into this 

 State. 



EVIDENCES IN MICHIGAN OF SUCCESSIVE ADVANCES OF THE LABRADOR ICE 



FIELD. 



Inasmuch as the older sheets of drift Avere completely overridden in 

 Michigan iu the Wisconsin stage of glaeiatioii, and covered deeply by 

 its drift sheets, this is not so favored a place to gather evidence of 

 successive ice advances as certa'in localities nearer the margin of the 

 Wisconsin aft'ord, for there the weathering and erosion of the lllinoian, or 

 of the lowan drift sheet may be brought into comi»arison with changes 

 in the surface of the Wisconsin. The southern peninsula of Michigan 

 lies in the path passed over by the Labrador ice field in the lllinoian 

 stage of glaciation and no doubt it received a notable contribution of 

 drift at that time'. It is also probable that the State was covered by 

 ice at the lowan stage of glaciation, for the lowan drift sheet is well 

 displayed on the west side of Lake Michigan outside the limits of the 

 Wisconsin drift. It is to be expected, therefore, that tAvo drift sheets 

 formed by the Labrador ice field, and possibly another by the Keewatin, 

 will be met with under the Wisconsin drift. To identify these sheets 

 properly one must be familiar with the characteristics of each sheet, 

 and must haA-e a sufficiently extensive exposure or large enough sample 

 to bring out the characteristics. At present only a fcAV borings have 

 been recorded with sufficient definiteness to even suggest where division 

 lines betAveen drift sheets should be placed, and no samples of the lower 

 sheets have been i)reserA'ed for study. Beds of nnick and peat ha\'e 

 been found betAA^en drift sheets in deep borings as far north as Hop- 

 kins in Allegan countv, and near Shelbv in Oceana countv. These beds 

 of muck and peat occur at a depth of about 150 feet, but it is not known 

 whether the overlying drift is entirely Wisconsin ; it may include also 

 the lowan. At Ann Arbor a boring at the court house and also a well 

 made by the Water Company struck a much harder sheet of till in the 

 lower part of the drift than in the u})per part, or Wisconsin drift sheet. 

 This harder drift sheet may be either the lllinoian of the Labrador field, 

 or an earlier deposit of the KecAvatin ice field. Its thickness at Ann 

 Arbor is between 30 and 40 feet, but north of Ypsilanti it reaches a 

 thickness of 200 feet. 



EVIDENCE OF THE STRIAE; 



Glacial striae exposed in the southeast corner of the State are found 

 to shoAV great diA-ergence in bearing. The latest are directed north- 

 w^estward toward a moraine formed on the west side of the Huron- 

 Erie basin. The remainder are directed south of AA'est, some of them 



*Geological Survey of Canada, Vol. 13, 1902, part D.. p. 81. 

 14 



