200 PLEISTOCENE OF INDIANA AND MICHIGAN. 



sand. In passing back into the moraines on either side, however, the deposits change first to 

 a loose-textured drift with large numbers of cobblestones in a sandy matrix, and a little farther 

 back to a clayey till similar to that in the moraines outside the interlobate tracts. Commonly 

 several miles is required for the transition, but in places, as, for instance, near Pontiac, a clayey 

 till is found on the immediate borders of the gravel plain. The prominent moraine which leads 

 from Pontiac southwestward past Ann Arbor is characterized by clayey till along much of its 

 course in Oakland and Washtenaw counties. At the northeast it is near the edge of the gravel 

 plain, but toward the southwest it bears away, passing east of an older moraine. On the Sagi- 

 naw side of the interlobate tract the drift is generally loose-textured nearly to the inner edge of 

 the morainic belt, though it is interrupted in a few places by a stiff clayey till, as, for instance, 

 in the northern part of the interlobate spur west of Portage Lake in southern Livingston County. 

 In places a thin veneering of a somewhat clayey till thickly set with bowlders overlies 

 gravelly drift of great depth. This is the case on the drift of the Huron-Erie lobe in northern 

 Washtenaw and southern Livingston counties from Whitmore Lake westward to Portage Lake, 

 and hi eastern Livingston and northwestern Oakland counties on moraines of the Saginaw lobe. 



BOWLDERS. 



Bowlders are exceptionally numerous all over the Saginaw portion of the interlobate mo- 

 raine, there being scarcely a square mile outside the gravel plains in which they are not con- 

 spicuous. On the Huron-Erie side of the interlobate system they tend to segregation in belts, 

 though they are generally numerous for several miles back from the edge of the gravel plain. 

 Bowlder belts run usually about parallel to the main ridges, but are not confined to the ridges. 

 Thus one in western Washtenaw County, which has been represented on the map in the Ann 

 Arbor folio, leads southwestward from Dexter midway between two prominent moraines. 

 These belts were probably deposited along the edge of the ice in the course of its recession 

 and may mark brief halts which were insufficient to produce a moraine. The Fort Wayne 

 moraine, which crosses Huron River near Ann Arbor, has remarkably few bowlders on its sur- 

 face in Washtenaw County compared with the number in outside districts. 



Few of the bowlders are more than 5 or 6 feet in diameter and are largely granite. The red 

 j asper conglomerate bowlders are perhaps more conspicuous in this interlobate morainic system 

 than elsewhere in the southern peninsula. Many have been gathered up for dooryard orna- 

 ments, and they are frequently worked into the foundations of dwellings because of the attractive- 

 ness of the bright-red pebbles. Conglomerates of other kinds are also present in notable amount 

 and quartzites are not rare. On the Huron-Erie portion of the morainic system in Washtenaw 

 and Lenawee counties limestone blocks from the formations at the head of Lake Erie are fre- 

 quently found; some of them are large. 



The rock constituents of the drift are very largely derived from formations within a few 

 miles of the morainic system. The moraines of the Huron-Erie lobe contain limestone from the 

 Silurian and Devonian formations, which outcrop in the southeastern corner of the southern 

 peninsula and in neighboring parts of Canada; the black Devonian shale is also conspicuous. 

 The large amount of clay present in the eastern edge of the moraines of the Huron-Erie lobe 

 may be referable to the accessibility of shale material from the underlying Coldwater shale, as 

 well as to the imperfect drainage conditions. 



On the Saginaw portion of the morainic system sandstones, derived in part from the coal 

 measures and in part from the Marshall formation, form a conspicuous part of the coarser rock 

 constituents. Coal is frequently reported from all through the Saginaw part of the morainic 

 system, even from places some miles outside the limits of the coal measure formations. It 

 need scarcely be suggested that the presence of coal in paying quantities should not be inferred 

 from the finding of masses of coal in the drift. 



