32 PLEISTOCENE OF INDIANA AND MICHIGAN. 



Certain features seem to indicate that the substages may eventually need subdivision, 

 but this will not be attempted at this time. Thus certain moraines within the group embraced 

 in a given substage show a slight forward movement into territory that had been vacated by 

 the ice sheet; also a crowding together of subsidiary ridges in certain complex moraines is 

 followed by wider spacing. This is notable in part of the St. Johns moraine of the Saginaw 

 basin and is not uncommon in other moraines. 



CALCULATION OP TIME. 



The calculation of time involved in the disappearance of the ice sheet at the last stage of 

 glaciation may be worked out on this basis. If the time required for the development of a 

 morainic ridge or of a subsidiary ridge can be satisfactorily determined a rude estimate may be 

 made of the entire time required for the retreat of the ice from the southernmost to the north- 

 ernmost of the moraines that have been mapped. 



The calculation must be based on areas where the moraines show the best development and 

 on moraines winch show the fullest development of subsidiary ridges. Account must be taken 

 not only of the visible subsidiary ridges in the moraine which displays them best but also of 

 ridges that were possibly overridden in the course of a readvance to the culminating position. 

 Similarly, it is insufficient to count the visible moraines of a substage; calculations must also be 

 made for moraines that were overridden in the next succeeding substage. Calculations based 

 on overridden material are necessarily less definite than those based on visible features, and 

 must be given correspondingly less weight. 



RETARDATION OF MOVEMENT IN THE SAGINAW LOBE. 



The area covered by this monograph is probably less suitable for demonstrating the oscil- 

 latory movement of the ice border and the periodicity of the expansions and contractions of the 

 ice than are districts in which the ice had greater freedom for wide deployment. The area was 

 one of conflicting ice movements, the Saginaw lobe being crowded on each side by neighboring 

 lobes. Studies in the field have brought to light evidence (see p. 128) which points to the rela- 

 tive stagnation of the movement or to a steady decrease in the Saginaw lobe in sharp contrast 

 ■with the oscillations of the border displayed by the Lake Michigan lobe on the west and the 

 Huron-Erie lobe on the east. 



The oscillations in the Lake Michigan lobe are also somewhat peculiar, for they show a tend- 

 ency to a westward shifting of the axial movement. Thus the axis of movement at the time 

 the Shelbyville morainic system was being formed was along a line pointing toward Shelby- 

 ville, III., from the southern end of the Lake Michigan basin, whereas at the time the Bloom- 

 ington morainic system was being developed the axial line was directed more nearly toward 

 Peoria, 111. Still later, the movement shifted so much farther to the west that in southern 

 Wisconsin the moraines override those of the Bloomington system. It is probable that this 

 shifting resulted from the great accumulations of drift on the eastern side of the Lake Michigan 

 basin, especially in Oceana and Mason counties, where the trend of the east shore changes from 

 southwestward to southward. The eastern side of the Lake Michigan lobe had thus less freedom 

 for deployment than the western, and the ice tended to hold its position on the east while it was 

 advancing on the west. 



The conflicting movements just mentioned may have served to prevent obliteration of 

 records by burial of the glacial features such as occur in regions of oscillating ice border, and 

 in this way the Saginaw and neighboring districts may be of exceptional value in working 

 out the full series of moraines. As will be shown later (p. 123), the number of moraines pre- 

 served in this region is certainly greater than in the region of freer movement to the west. 



