254 
The American Geolocjist. 
October, 1.S97 
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Fig. 1.— Section drawn north and south across the basins of ihe Leaf river 
AND Elk Horn Creek in northwestern Illinois. 
Legend— a, Tertiary peneplain, b, Lafayette base level, c, Present stream level. N, Niagara lime- 
stone. H R, Hudson River shales. G, Galena limestone. T, Trenton limestone. St. P, 
St. Peter sandstone. L M, Calciferous limestone and shale. P, Potsdam sandstone. 
outlier in Iowa is important as corroborating the evidence 
previously presented of the practical correspondence between 
the apparent structural plains represented by the Trenton and 
Niagara outcrops in this portion of the Mississippi basin, 
with the ancient land surface which has been designated the 
Cretaceous peneplain. 
Southward from Allamakee county- to the Turkey river, the 
upland within 20 miles of the Mississippi river, consists of 
the flat topped ridges whose crests merge, in the far distance, 
into an even sky-line — the remarkably even surface of the an- 
cient Tertiary peneplain. It passes from the 1'renton lime- 
stone on to the Galena limestone, and bevels the edges of the 
latter. It also slopes gently and evenl}^ in this southward di- 
rection, to about 1,000 feet above the sea near the mouth of 
the Turkey river. It extends almost to the very verge of the 
canon valley of the Mississippi and beyond this stream it forms 
the general upland surface throughout southwestern Wiscon- 
sin. Because of its ignoring the gentle folds of the indurated 
formations, its passage without deformation from one terrane 
to another, and its even sky-line in the far distance, it is gen- 
erally recognized as a dissected base-levelled plain regardless 
of the fact that some portions of it present as strong evidence 
of being ''structural plains" as any in America. 
Returning now again to our 1300-foot plain, in tracing it 
southward from Winneshiek county, it becomes the well known 
Niagara plateau of northeastern Iowa, whose eastern limit is 
the Niagara escarpment, the most prominent topographical 
feature of tiie territory. This plateau gently declines south- 
ward, until near the mouth of the Turkey river, its altitude 
slightly exceeeds 1200 feet. The escarpment which bounds 
it is nearly everywhere precipitous, but is deeplj^ cut by val- 
