Erosion C'ljcles in Xorfhire.sfern TJliuois. — Hershey. 95 
widely extended Tertiary penei^lain of the Appalachian prov- 
ince be correct, the Lafayette plane would belong in the for- 
mer district at the level of the higher upland ridges constitu- 
ting peneplain No. 2. There would then intervene, between 
the Lafayette period and the time of canon valley erosion, a 
considerable length of time represented by the excavation of 
the broad basins of northwestern Illinois. It is possible that 
erosion cycle No. 3 in our district is the time equivalent of 
the deposition of the Lafayette formation in more southern 
districts. It is also possible that its products in the southern 
Appalachian and Ozark provinces are topographic forms of so 
little importance that the existence of this as a distinct base- 
level of erosion has not yet been recognized. Because of the 
above doubts I would locate the Lafayette plane in north- 
western Illinois as probably equivalent to peneplain No. 2, or 
possibly to No. 3. In either case the caiion valleys are post- 
Lafayette and may properly be classed as Quaternary in age. 
The buried valleys constituting the product of cycle No. 5 
are not indicated beyond this district except in a few widely 
separated localities. Mr. Frank Leverett, in securing well- 
sections along the Mississippi valley, encountered evidences 
of a buried rock-shelf.* He says: "In places a preglacial val- 
ley is found to carry shelves of considerable breadth, now con- 
cealed beneath the present valley bottom. Thus at Quincy 
and St. Louis a rock shelf extends entirely across the present 
bed of the Mississippi, with an altitude 60 feet or more above 
the deeper portion of the valley. * * * * It is probable 
that such shelves as these ai-e remnants of an old valley floor. 
Their full breadth is not known, hence we cannot judge 
whether they are mere fringes on the border of the deep chan- 
nel or are of such breadth as to greatly reduce the width of 
the deep channel." While we cannot completely demonstrate 
the synchronism of these Mississippi rock-shelves and those 
in our district, the valleys M'hich are trenched below their sur- 
face may be correlated in a general way; for it is evident 
that the termination of preglacial erosion was practically con- 
temporaneous throughout the central Mississippi basin. The 
first etl'ect of the advent of an ice-sheet in northwestern Illi- 
nois was to depress the area and cause a silting up of tlic 
valleys in some instances even above the present water-level. 
^Journal of Geology, vol. iii, Nuinhor 7. p. 7(52. 
