POET HTJBON MOEAINIC SYSTEM AND PEOBABLE COBEELATIVES. 315 



The deposition of the silt may prove, however, to have been not in an open lake of broad 

 extent, such as the glacial Lake Algonquin, but in narrow strips of water between lobes of ice 

 that occupied the great valley-like lowlands now occupied by the arms of Grand Traverse Bay, 

 Pine Lake, Torchlight Lake, and lesser lakes of the Grand Traverse region. As noted above, 

 the deposit is found in the narrow strips of upland between these basins and is not found on the 

 inunediate floor of most of the great valleys. Possibly it is not present beneath these valleys, 

 if the ice lobes were occupying the valleys during the deposition of the clay. Buried as this 

 deposit is beneath thick deposits of till so that it is exposed in very few places, there is neces- 

 sarily such a dearth of data concerning it that its origin can not be cleared up at the present 

 writing. 



The time of its deposition compared with the overlying till is also a matter of uncertainty. 

 It is not known whether it is contemporaneous with the ice occupancy of the valley-like depres- 

 sions, or whether it was deposited in a time corresponding to Lake Arkona between the recession 

 of the ice and the readvance just discussed, or whether it is an interglacial deposit laid down 

 before the Wisconsin stage of glaciation. 



On the supposition of its being a pre- Wisconsin interglacial deposit it would naturally be 

 interpreted as a mere remnant of the original deposit with great gaps along the line of the valleys 

 made either by the pushing of the ice into these valleys or by ordinary stream action in inter- 

 glacial times, or to a combination of the two processes. 



On the supposition of deposition during an ice recession within the Wisconsin stage there 

 would be less chance for ordinary stream erosion, and the great removal of material along the 

 vaUeys would be chiefly referable to the action of the readvancing ice. 



On the supposition of contemporaneous deposition with ice lobes occupying the valleys 

 the deposit may now have about its original extent. On this interpretation the deposition of 

 the clay may have been during the waning of the ice prior to the last readvance-. It may have 

 taken place, however, during a possible readvance in which the ice protruded into the great 

 valleys before covering the upland. This would be likely to cause a heavy silt deposition, not 

 only from the edges of the advancing lobes but from the broad field of ice which was about to 

 encroach upon the upland. 



On the whole the distribution and character of the deposits and especially the freshness of 

 then appearance are thought to favor their close connection with glacial agencies either during 

 the waning or the readvance of the ice in the later part of the Wisconsin stage of glaciation. 



