MORAINIC SYSTEMS AT HEADS OF LAKE MICHIGAN AND SAGINAW BASINS. 199 



STRUCTURE OF THE DRIFT. 

 THICKNESS. 



The thickness of drift is considerably less on the Saginaw than on the Huron-Erie part of this 

 interlobate system, for the Saginaw part lies on the table-land of sandstone belonging to the 

 Marshall formation, and the Huron-Erie part covers the lowland underlain by the Coldwater 

 shale. The general altitude of the rock surface is 150 to 200 feet greater where the sandstone is 

 the upper rock formation than where the Coldwater shale immediately underlies the drift. As 

 the general altitude of the drift surface on the Saginaw side of the interlobate is not much higher 

 (possibly 50 feet) than on the Huron-Erie side, the depth to rock is 100 to 150 feet greater on the 

 Huron-Erie side. Along the ridges of the Huron-Erie lobe it is generally about 250 to 300 feet, 

 whereas on the ridges of the Saginaw side it in few places reaches 200 feet. At the north end 

 of the interlobate system in northern Oakland and southern Lapeer counties the highest ridges 

 may carry 400 feet of drift, for the rock surface at Orion is about 825 feet above sea level and 

 several of the prominent ridges and knolls rise above 1,200 feet. 



The sandstone of the Marshall formation underlies a prominent moraine of the Huron-Erie 

 lobe in western Washtenaw County, and its eastern border seems to be within the limits of the 

 Huron-Erie lobe from Washtenaw County southwestward across northwestern Lenawee County 

 and central Hillsdale County. In that portion of the morainic system the thickness of drift 

 is about the same on the Huron-Erie as on the Saginaw side and scarcely reaches 200 feet on 

 the most prominent points. Rock outcrojis at a few points in Jackson and Hillsdale counties, 

 but among them the drift generally is 100 feet or more thick. 



The amount of drift referable to the Wisconsin invasion is to a large degree undetermined. 

 In places in Washtenaw County hi the vicinity of Ann Arbor the change from soft till to hard 

 till, thought to mark the line between Wisconsin and pre- Wisconsin drift, is found at a moderate 

 depth, but more commonly at 100 feet or more. In Hillsdale County evidence of the presence 

 of drift sheets of two distinct invasions is still clearer, beds of black muck or swampy deposits 

 being found at the base of the soft till. A few miles south of Hillsdale this black muck is 

 found in wells over an area of several square miles. It lies about 100 feet below the surface, 

 thus corresponding to the general depth to the hard till on the moraines in the vicinity of Ann 

 Arbor. The surface of the hard till in the vicinity of Ann Arbor appears to be very uneven, 

 the depth to it ranging from a few feet to about 130, either because the topography of the 

 older drift was morainic or because it was carved by interglacial erosion. It is much higher 

 in the prominent moramic ridge leading southwest from Ann Arbor and in a prominent tract 

 east of the city than it is on the lowlands between. In places east of Ann Arbor it comes 

 nearly to the surface on the morainic knolls. From the few data available it appears probable 

 that the pre- Wisconsin drift in this vicinity contained prominent ridges which have not been 

 completely concealed by the Wisconsin drift. 



It is thought that the prominent kames along the borders of Huron River near the axis of 

 this morainic system were developed during the Wisconsin ice invasion and that the drift at 

 this latest invasion was built up from a level as low as the bottoms of the basins of the lakes and 

 swamps which abound in this region. Some of these lakes have depths that exceed 100 feet. 

 This fact and the fact that the wells on the borders of the lakes penetrate great depths of assorted 

 material are thought to indicate that the deposition of morainic and outwash material at the 

 last ice invasion has produced all that is found from the level of the bottoms of the lakes to the 

 tops of the bordering knolls. At least no suggestion of a change in the constitution of the drift 

 has been noted. 



COMPOSITION. 



The drift of. the interlobate system in the area in which the ice lobes coalesced changes its 

 texture as it passes back into the districts lying beneath each of the ice lobes. Along the line 

 of coalescence the drift is gravelly and sandy with only scattered deposits of clayey till, the kames 

 are made up largely of gravel and cobble, and the outwash aprons are composed of gravel and 



