Tlie Development of Mississippi Ynlley. — Hershey. 259 
or several widely developed peneplains. The conditions un- 
der which a "structural plain" is formed are limited, and un- 
less it also corresponds to a basal plane of erosion, it will nev- 
er have great areal extent. Peneplains are well developed 
where the strata removed above them were soft shales or but 
slightly indurated sandstones, and are best preserved where 
the strata under them are hard limestones or still more resis- 
tant formations. 
The basin valleys of northwestern Illinois. In the small 
geographical district which is enclosed by the Mississippi and 
Rock rivers, and the Wisconsin line, there is a fourth class of 
topographical features which, although not confined to this 
area, is here quite prominent and easily studied. In south- 
eastern Minnesota and northeastern Iowa, the canon valleys 
seem to be trenched beneath the surface of the Tertiary pene- 
Ijlain which extends with its normal altitude nearl}' to the 
verge of the bluff. But in northwestern Illinois a long gentle 
slope intervenes between the edge of the higher upland ridges 
and the summit of the bluffs which bound the canon valleys. 
These long slopes, on opposite sides of the streams, form broad 
shallow basin valleys, which are in strong contrast to the 
steep-sided caiion valleys trenched beneath their floors. The 
basin valleys are not confined to any formation, but their 
width varies according to the resistant nature of the strata ex- 
cavated. When the strata removed in eroding them were the 
Galena or the Niagara limestone they are nevermore than two 
or three times as wide as the canon valleys within them. But 
over the outcropping areas of the Hudson River shales, they 
occupy nearly the entire surface, sometimes even the main 
divides having been reduced from the level of the Tertiary 
peneplain. In the northern portion of the district the depth 
is often 100 feet, but in Whiteside count}' it does not much ex- 
ceed 75 feet. 
In the southern one- third of Steplienson county, where the 
Hudson River strata outcrop, a belt of country from five to ten 
miles in width, has been reduced below the surface of the main 
Tertiary peneplain, forming a lower, undulating, dissected 
plain, literally a small peneplain. Throughout one entire town- 
ship in the southeastern corner of the county, the main Ter- 
tiary peneplain is represented by a single elevation — Bunker 
