The Development of M ississippi. Valley. — Hershey. 2(51 
In short, these basin valleys in northwestern Illinois are 
everj'where present, and such prominent features of the to- 
pography, that the observant traveler can not fail to notice 
them. In the country southward from Jo Daviess and Ste- 
phenson counties, they occupy three-fourths of the surface. 
All those which are trenched in the same formation, are 
directly proportional in size to the present streams, and the 
higher upland ridges never closely approach the canon val- 
leys. This is a rule which governs the product of a well 
balanced drainage system. It here indicates that none of 
the stream's (with one exception — the Mississippi), in north- 
western Illinois, differ materially in relative size from those 
which formed the basin valleys. The imaginary plain which 
may be drawn through the bottom of all these basin valleys 
is undoubtedly a basal plane of erosion. While it will show 
slight deformation, due to subsequent differential uplift of 
the district, it will have a level irrespective of the struct- 
ure of the rocky strata. In fact, the evidence that this plain 
will not be merely an apparent baselevel, due to the coinci- 
dence in altitude of a great number of structural plains in all 
parts of the district, is stronger than for the peneplain char- 
acter of the upper dissected plain, whose baseleveled condi- 
tion is doubted by scarely any one. 
The basin valleys do not seem to be represented in a very 
definite form north of Illinois and the same latitude in Iowa. 
The broad sandstone plain of central Wisconsin appears to bear 
a similar relation to the main Tertiary peneplain and the canon 
valleys, as the basin valleys in Illinois. But the presence of 
the very resistant Baraboo quartzite ledges crossing the course 
of the stream which baseleveled the lowland plain north- 
ward from them has suggested to Chamberlin that to them 
chiefly is due its apparently baseleveled condition. The 
"Trenton terrace" at St. Paul and Minneapolis, also bears a 
relation to the Tertiary peneplain and the caiion valleys iden- 
tical with that of the basin vallej^s in Illinois. But here it is 
impossible, with the present evidence at least, to demonstrate 
that it is not due solely to the structural inequalities of the 
formations excavated. 
llie canon valleys of the upper Mississippi retjioti. This 
very prominent topographical feature of the area under dis- 
