82 The America)) Geolotii.sl . August, 1896 
tion to tlie higher peneplain antl to tiie present water level, 
are strong presumptive evidence that they indicate the posi- 
tion of an uplifted baselevel of erosion. 
The depth of the basins averages 100 feet throughout the 
district. Their width was dependent not only on the size of 
the stream but also to u certain extent on the nature of the 
rock ex(;avated, and to a less extent on the position of the 
synclinal axes. While the Pecatonica river above Freeport 
eroded a basin only about three miles wide in Galena lime- 
stone, its tributary, the ancient Yellow creek, excavated a 
basin four to five miles wide in Cincinnati shales. In the 
same formation, however, the size of the basins is proportion- 
ate to the size of the present streams. In the vicinity of the 
main lines of drainage, like that of the ancient Rock river, 
the basin was very broad and the central portion of it a fiat 
plain miles in width (a true peneplain though a narrow one) ; 
but, in ascending to the headwaters of the streams, these ba- 
sins narrow until they become mere gentle slopes of the upper 
portion of the present valley sides. This is their nature 
throughout a large part of the southern portion of the Drift- 
less Area in Jo Daviess county, Illinois. But even here the 
rather abrupt change from a gentle slope to a nearly perpen- 
dicular bluff face will impress one as indicating a baselevel of 
erosion. 
Assuming the existence of an uplifted erosion plane at the 
level of the lower upland ridges as demonstrated, we find that 
its present altitude at Freeport is 820 feet above the sea. It 
is represented in the city by the ridge on its southern edge, 
Avhich ridge is overlooked by the remnant of the higher pene- 
plain constituted by the western ridge before mentioned. I 
have not the data for accurately locating its altitude in any 
other portion of the district. If it always coincided with the 
summit of the valley bluffs, its altitude could be determined 
by adding the known hight of a blulf to the altitude of the 
stream at its base. On this principle I should locate baselevel 
No. 3 near Rockford as 780 feet above the sea; at Galena, 
800 feet; at Savanna, 800 feet; at Kock Island, 700 feet; at 
Sterling, 750 feet; and at Oregon. 800 feet. But as there is 
usuiilly a short slope from the summit of the ridges which 
form the eroded peneplain to the top of the blufit' proper, the 
above altitudes are probably somewhat below the true hight. 
