210 WEL ( SELLAR ZA 
the ice as it was forced up and over the slope. The rock here 
is of uniform hardness over the area affected so that the basins 
were the result of differential ice action. A glance at Fig. 1 
shows that the general direction of these basins corresponds 
perfectly with that of the troughs previously described, and the 
conclusion is irresistible 
that they were all made 
by the same ice-sheet, 
which) miowe diaiveny, 
steadily straight out 
irom they center, sok 
Laurentide accumula- 
tion to the southern- 
most limit of glaciation 
in southern Illinois. It 
is apparent that this 
sheet acted with a vigor 
farvin excess-om that.or 
any previous sheet, that 
it showed an utter dis- 
regard of topography; 
indeed, it seems prob- 
Fic. 7.— Diagram of the principal rock-basin 
at present exposed at the Sibley quarry. The arrows able that it had more 
indicate the direction of each general movement, tg do with the making 
numbered in order from the first, or Illinoian. The 
central part of this basin is still covered with till. 
Ol OUT =present Moe 
topography, than the 
other three sheets combined. These facts suggest that it was 
the most massive of the Laurentide sheets and presumably able 
to reach a more southern latitude than any of the other three, 
that it was in reality the Illinoian. The following diagram will 
serve to make clear the relation of the striz to the principal 
basin. 
Rock-shattering.— It has been observed in all the quarries and 
natural exposures in southeastern Michigan that the upper three 
to five feet of limestone strata are much shattered and apparently 
thin bedded. The strata become more solid and have the 
