302 
The American Geologist. 
May, 1896 
4. Retreat of the ice-front to the east side of Rock river, 
giving free drainage to the Pecatonica basin, and hence de¬ 
stroying the lake. Extensive subaerial erosion of the blue 
silt, with the formation of a soil at its surface, while the for¬ 
mer lake area was occupied by marshy vegetation. Climate 
apparently cold temperate, such as would exist as long as the 
ice lay within 50 or 75 miles distant from this district. 
5. Re-advance of the ice-front to the west side of Rock 
river. Formation of the later lake Pecatonica, on the site of 
the previous and similar extraglacial lake. Deposition of 
variegated clays, with iceberg-carried pebbles and boulders, 
named the Lake Pecatonica formation. Climate evidently 
arctic. 
6. Continued advance of the ice-front, driving the lake in 
front of it, and finally destroying it altogether by filling the 
valleys with ice. 
7. The front of the continental glacier reaches its farthest 
limit, 20 miles west of Freeport; remains stationary, in the 
culmination of this Kansan epoch, sufficiently long to accu¬ 
mulate a boulder belt and faintly defined moraine; then be¬ 
gins to melt away, slowly at first; afterward it dissolves 
faster under the genial conditions of a milder climate, until 
within a few hundred years it entirely abandoned northwest¬ 
ern Illinois. 
It is desired to call the attention of glacialists to the fol¬ 
lowing points: 
1. The Kansan ice-sheet, in occupying the territory that is 
now northern Illinois, advanced or moved as glaciers do at the 
present day, not being formed in situ as some have supposed. 
2. During the general advance, there were fluctuations of 
the border of the ice, similar to those which produced the 
successive moraines during the general recession of the ice- 
sheet in the Wisconsin epoch. 
3. The climate was constantly severe during the accumu¬ 
lation and general advance of the Kansan ice-sheet, but varied 
from cold temperate to arctic in periodic changes, of which 
the several recorded in this paper were probably only a small 
portion. 
4. Even during the advance, the summer season was at¬ 
tended with more or less melting of ice and of the yearly ac- 
