Krosion C'l/cles in Xovfliiresfern TlUtiois. — Hershei/. 85 
the portion of the valley above the flood-plain with that under 
it, so far as the latter can be determined by well-sections, 
indicates that the bluffs were formed during a cycle of erosion 
when the streams flowed at a level, at Freeport, 20 feet lower 
than the present water level. The existence of the rock-shelf 
at Freeport is undoubted, but in other portions of the district 
the evidence rests largely on the fact that, while the bluff's 
above the present stream level are frequently precipitous, their 
buried portion has often a moderate slope. Nor is this sloping 
basal portion of the bluff a talus, for the wells upon reaching 
it penetrate solid undisturbed rock strata. Now a perpen- 
dicular bluff in such a formation as the Galena limestone can 
only be produced by the undermining of its base by a stream. 
Hence, the base of the bluff' indicates the stream level during 
its formation, and the slope below belongs to a succeeding 
cycle of erosion. 
While the evidence of the existence of a rock-shelf a short 
distance below the present alluvial plains is not conclusive in 
the greater portion of the district, it appears to the writer to 
be so in the vicinity of Freeport. And as the rock-shelf at 
the latter place can hardly be an isolated phenomenon, it may 
be assumed that it is represented in some form or other and 
in some position throughout the district and prf)bably far be- 
yond it. 
At the close of the cycle No. 4, what is now northwestern 
Illinois was a dissected gently rolling plain with topography 
essentially as now. As the valleys whose excavation was 
accomplished during this cycle were many times wider than 
the streams which flowed in them, it may be presumed that 
their base was a baselevel plane of erosion. The uplift which 
terminated cycle No. 3 was comparatively rapid so that the 
streams, quickly cutting down to the new baselevel, meandered 
on a gradually widening flood-plain, and, undermining the 
valley sides, produced the present blufl's. The preceding sys- 
tem of valleys — the broad basins of cycle No. 3 — had no bluffs, 
although excavated into the same formation. While this cycle 
was undoubtedly several times as long as that which succeed- 
ed it, the strong contrast in the topographic forms produced 
is explainable under the supposition that the uplift during 
cycle No. 3 was slow and extended through practically the 
