Erosion ('i/c?es in Xorfhwesfent Illinois. — llcrslieii. 79 
Cincinnati shales still remained in considerable thickness at 
the close of cycle No. 2, the peneplain is represented by no 
ridge until the base of the Niagara limestone passes below its 
level six miles south. Thence to the Rock river this base- 
leveled plain remains only in long, narrow ridges, several miles 
apart, and occup3nng the position of the synclinal axes. In- 
deed, the regular deformations of the rock strata can be 
determined as readil}'^ by a study of the ridges which consti- 
tute the remnants of peneplain No. 2 as by the ''mounds." 
In ttie country eastward from Freeport, the uplifted pene- 
plain which we are now considering is represented by a broad 
ridge on the Stephenson and Ogle county line; and ten miles 
north it forms a hilly upland described in view 3. The basin 
between these higher upland areas widens toward the east 
until it constitutes the greater portion of the upland surface. 
In the vicinity of the preglacial valley of Rock river, pene- 
plain No. 2 has been completely destroyed, as also for many 
miles back from it. But outside of the immediate valley of 
the stream, the surface is an eroded upland plain resembling 
that of western Stephenson county; and, at first thought, the 
much lower altitude of the upland near Rock river than of 
the general surface in the more western portion of tlie district 
might be attributed to a tilting of the peneplain toward the 
east. Such tilting has occurred, but not to the amount that 
a casual survey of the region would indicate. Even in the 
eastern portion of Stephenson county, where both upland 
plains are well represented, some confusion might be exper- 
ienced in separating the upland surface into two classes; for 
the ridges which make up the lower upland are but little 
lower than the remnant ridges of peneplain No. 2 along the 
edges of the basins, while the former gently decline in a grace- 
ful curve and the two reach the Pecatonica valley proper at 
an equal hight. All the higher upland ridges form a perfect 
plain sloping gently to the southeast. The inclination is 
regular, and by projecting the plane across the Rock river 
valley it is found to be at a considerable hight above the 
general u])land surface. Proceeding on a meridional line 
southward from Freeport, the same gentle declination of the 
peneplain is indicated by the higher ridges; and, projecting 
i ts plane across the upland near Rock river, it is found to b^' 
