Pleistocene Deposits of Illinois.—Hershey . 301 
places in the valleys; but the lake partially destroyed this 
vegetation and soil, and incorporated them with the first few 
inches of its own deposits. It may be of interest to note 
that a precisely similar succession of conditions occurred 
during the Trenton period on the site of what is now the Elk 
Horn valley of Ogle county, Illinois.* During the deposition 
in surrounding areas of the Buff or Pecatonica limestone, the 
surface of the St. Peter sandstone rose above the sea level as 
a small island, bearing a soil about as deep, and in the same 
condition, as the one above described. Subsequently, at the 
opening of the epoch of the Blue or Trenton (proper) limestone, 
when the island slowly sunk below the sea, the water found a 
quantity of peaty matter or perhaps a locally developed black 
soil, which it broke up and redeposited as a few inches of 
laminated black shale, succeeded immediately by the usual, 
but there very fossiliferous, Blue limestone, just as the lake 
Pecatonica, after having disposed of all the mucky soil that 
it could reach, continued with its normal deposition, of varie¬ 
gated clays. It is not intended to presume that the climatic 
conditions were similar in these two areas at so widely separ¬ 
ated periods, but simply to show that the deposits supposed to 
represent that Lower Silurian soil have so much resemblance 
to undoubted buried soils of Pleistocene age as to make the 
existence of the former well-nigh a certainty. 
Summary. 
The general course of events during the earlier portion of 
the Pleistocene period in the Pecatonica basin, according to 
the study presented in this paper, was as follows: 
1. Great epeirogenic uplift, resulting in excavation of deep 
valleys in the bottom of the Tertiary troughs. 
2. Gradual subsidence of the land. Accumulation of the 
Kansan ice-sheet. 
3. Advance of the ice border to the west side of the Rock 
river, causing obstruction of the Pecatonica valley and the 
formation of a lake in which was deposited a blue silt or loess 
(Kansan Buried Loess), the material being derived chiefly 
from the debris contained in and under the peripheral portion 
of the ice. Climate apparently almost frigid. 
*Am. Geologist, vol. xiv, pp. 169-179, with map, Sept., 1894. 
