14 BULLETIN 142, U. S. DEPARTMENT OP AGRICULTURE. 



ice, it is more than probable that a very large part of the earthy mate- 

 rial which was reworked into the later till sheet was contributed by 

 this older till and its associated sandy and gravelly deposits. Through 

 this unconsolidated material the local country rock had been exposed 

 by water erosion occurring between the different stages of glaciation. 

 As the ice readvanced over the surface of the region it picked up 

 earthy material from the older till, mingled it with earthy and stony 

 material from the various rock outcrops, and contributed varying 

 amounts of extraneous material carried into the region from the 

 areas of crystalline rocks which were exposed to glacial action in 

 the territory north and east of the Great Lakes. 



It is probable that a large part of the material derived from local 

 sources was carried under the ice and within its lower sections. 

 There is considerable evidence that such local materials were moved 

 only short distances and rearranged and deposited by the melting ice 

 to form the deeper part of the surface till covering. For this rea- 

 son the local country rock exerts a strong influence upon the litho- 

 logical character of the till sheet, in some cases giving rise to 90 per 

 cent or more of the coarser stony particles which may be identified. 



Thus, the ice of the Wisconsin stage of glaciation in its advance 

 collected material from the limestones and shales of western Ohio 

 and central and eastern Indiana, from the limestones, shales, and 

 sandstones of the lower peninsula of Michigan, and from the lime- 

 stones and associated shales which cover large areas in southeastern 

 Wisconsin. Even where the limestone rocks do not directly under- 

 lie areas now occupied by the soils of the Miami series, broad areas 

 of limestone lay directly across the path of the advancing glacial ice, 

 as in the neck of the Saginaw Bay region, and, from exposed out- 

 crops, contributions of earthy and stony material were obtained. 

 It is also probable that materials derived from the remaining de- 

 posits of previous glaciation contained a fair percentage of such 

 calcareous material. In Plate II, figure 2, a part of a ledge of the 

 limestone which underlies portions of the Miami soils is shown. 



The soils of the Miami series contain varying quantities of lime- 

 stone bowlders, gravel, and rock flour in nearly all areas where they 

 are mapped. This is particularly true of the deeper subsoil, which 

 it is presumed most nearly represents the materials existing on the 

 surfaces over which the ice moved. Examinations of the deeper 

 subsoils show a general calcareous condition below depths of 1 to 2 

 feet, especially in the areas surveyed in Ohio, Indiana, and Wis- 

 consin. The presence of the limestone or calcareous shale in the 

 drift is not nearly so marked in southern Michigan as in other re- 

 gions dominated by the Miami soils, but analyses of the drift indi- 

 cate that as much as 25 per cent of the material is of a calcareous 

 nature over considerable areas.* 



