ICE WORK IN SOUTHEASTERN MICHIGAN 209 
weight was not great enough to force it into the basin suff- 
ciently, or else the basin was filled with till. So far as observed 
the evidence is that the ice producing these strie acted less 
vigorously than did the early Wisconsin sheet. It does not 
seem to have produced any furrows or troughs of sufficient size 
to escape obliteration at the hands of the two later sheets. 
There may be a question as to whether this set of striaz should 
not be connected with the early Wisconsin described. Its 
general direction of movement differs, however, by some 
38° and the two sets lack 25° of coinciding at their limits. 
Undoubtedly a larger number of readings would tend to connect 
the two series in this region, but further south they diverge still 
more widely. In the rock-basin figured this set lacks that 
freshness of appearance which characterizes the Wisconsin scor- 
ings and it is believed to be pre-Wisconsin, probably Iowan. 
The general direction of movement harmonizes with what is 
known concerning the distribution of the Iowan drift, its relative 
age and the intensity of ice action are also consistent with this 
view. Further, the Iowan ice is known to have crossed this 
region, and such an independent sheet would have been much 
more liable to have left its record than would simply a minor 
advance of either the early Wisconsin or the Illinoian. 
Itinoisan.— At the northeastern corner of the Sibley quarry 
there is at present a series of partly uncovered rock-basins, one 
of which is shown in Fig. 6. This has an apparent breadth of 
twenty-five to thirty feet, is eight to ten feet deep and some 
eighty-five to ninety feet of its length are in sight. There are 
four other basins, or troughs, in this same vicinity having with 
thiswonesthe approximate: directions. S-. 3720 Wa, Sax4aWVe, S: 
20m We Ss: 2On We and S432 We 7A smallvone runmnine) S. 177 
W., belongs more probably with the preceding stage of glacia- 
tion. At the southeastern corner of the quarry a heavy double 
furrow running S. 45° W., contains pre-Wisconsin striaz and is 
to be included in the series. The average trend of these basins 
isp hound ato be S425 We aihese basins icy upon the stoss 
side of the Sibley embossment, where they were plowed out by 
