Erosion Cycles in Korthirestern Illinois. — I/ersJiei/. 87 
valleys are very narrow at the base. Indeed, there is no evi- 
dence that the streams had trenched their new caiions to the 
new baselevel before the close of the cycle. 
The depth of the lower canons in the Pecatonica valley is 
probably not less than 200 feet. If we may judge from the 
imperfect data at hand, these valleys increased in depth to- 
ward the northeast. This seems to indicate a general tilting 
of the district toward the southwest. As this district is but 
a small portion of a great geologic province, we may presume 
that this tilting was extended over a much larger area. May 
it not have proceeded from some point far northeast of our 
district, as, for instance, the Laurentian area of uplift? May 
it not have been a portion of the temporary uplift of the 
northeastern part of North America, to which the inaugura- 
tion of glacial conditions is by some considered to have been 
due? 
The altitude of northwestern Illinois at the close of erosion 
cycle No. 5 is undeterminable at the present time. It is as- 
sumed that the sea coast was even farther distant than dur- 
ing the preceding cycle. An addition of 500 feet to our last 
estimate, or a maximum altitude of 1,441 feet, is probably not 
too much. The general upland surface would then be 1,200 
feet above the sea, and the water level about 900 feet. This 
would not be sufficient to bring on glacial conditions in our 
district, but it is assumed that the presence of an ice-sheet in 
northwestern Illinois was due to introduction from a more 
elevated area to the northeast. 
Type Section. 
The conclusions expressed in the preceding pages may be 
summarized into the accompanying ideal section (figure 1), 
designed to illustrate the ancient baselevels developed in the 
vicinity of the city of Freeport. It will serve an important 
purpose in the matter of reference, and may be known as the 
Preeport baselevel section. It is exaggerated vertically, but is 
true to the impression which an examination of the topogra- 
phy of the area would make on the average student. 
A subject of considerable interest is the relative length of 
the several cycles of erosion here indicated. The solution of 
this problem is dependent on too many factors of unknown 
♦quality to give any present estimates much value. It is nee- 
