300 The American Geologist. 
May, 1896 
able; but I do not think that any long time was required for 
it. The land still stood at a comparatively high altitude, and 
as soon as the lake had ceased to exist, the streams, then 
much enlarged under the increased precipitation of the Glacial 
period, rapidly attacked the silt, and perhaps a few hundred 
years were sufficient to accomplish the work. The significance 
of this erosion interval lies in the fact that, after the existence 
of the first lake due to the presence of the ice-front on the 
west side of Rock river, it must have been melted back to the 
cast side of that stream so as to open the Pecatonica basin to 
a free drainage through the Rock river valley. This fluctu- 
ation in the border of the ice may have amounted to only ten 
miles, or it may have been hundreds. I am inclined to believe, 
as above stated, that the time of erosion was not long, and 
that the ice had not retreated far. 
The brown silt forming the upper part of the deposit is an 
old interglacial or, rather, inter-lacustrine soil. The change 
of color has been effected by an atmospheric oxidation of the 
iron ingredient contained in the blue silt. This latter, on ex- 
posure to the air, now rapidly changes to a dull light red, the 
iron being converted into the red oxide. But at the time of 
the formation of the old soil it was converted into the brown 
oxide or limonite, indicating apparently a cold moist climate. 
It was before stated that this soil runs down over the old 
erosion slope of the silt, thereby being formed from succes- 
sively lower layers of it. Within one foot of the surface the 
lamination has been mostly destroyed. On the surface of the 
soil there grew a flora consisting mostly of grasses and other 
herbaceous plants, the remains of which, found in a very thin 
layer between the soil and the overling black clay, have the 
aspect of the vegetation found in northern marshes. 
Carbonaceous, laminated clay (No. 6 of the section), having 
a thickness of about four inches, occurs immediately over the 
remains of the plants which grew on the old soil, and dips 
down over the erosion slope of the silt, being thus unconform- 
able to it. but conformable with, and in fact onl} T the lower 
portion of, the variegated clays. When the re-advance of the 
ice-front had again established a lake in the Pecatonica 
basin, the rising waters found a vegetation in possession of 
the surface, and perhaps a thin black mucky soil at low 
