The Bevelopment of MissiHslp])i Volley. — Hershey. 255 
leys, giving the border of the plateau a crenulate outline. 
These marginal ridges do not form a perfectly even plain, but 
they do not appear to reflect the attitude of the slightly cor- 
rugated strata as perfectly as a mere structural plain should. 
The summit line of the escarpment gradually rises, with ref- 
erence to the Tertiary i:)eneplain east of it until, near the 
mouth of the Turkey river, there is a vertical difference be- 
tween them of about 200 feet. 
The Niagara escarpment which approaches very close to 
the canon valley of the Mississippi river, south of the mouth 
of the Turkey river, again recedes from it, and in northeastern 
Dubuque county, the Tertiary peneplain is developed over a 
width of 10 or 12 miles. It is interrupted however, by outliers 
of the Niagara plateau of which Sherrill mound is the most 
prominent. At the city of Dubuque, the Tertiarj'- peneplain 
is represented by the general upland surface at an altitude of 
about 900 feet above the sea. Half a dozen miles w^estward 
from the city, the Niagara escarpment is seen, with a concen- 
tric course, hounding the plain like the perpendicular walls of 
a high crater. Its crest line is remarkably even if it represent 
a structural plain, for it is crossed by a number of slight an- 
ticlinal folds. The nearly level plateau whose eastern border 
this escarpment is, has an altitude which barely reaches 1200 
feet above the sea, or something over 250 feet above the Terti- 
ary peneplain east of it. The Niagara plateau slopes gently 
westward, and is overlain near Rockville in Delaware county, 
by a gravel formation which McGee has provisionally corre- 
lated with the Cretaceous strata of southeastern Minnesota. 
This correlation being probably correct, seems to indicate 
that the Niagara plateau in Dubuque and Delaware (jounties 
is not merely a structural plain, but also corresponds with the 
land surface of the Cretaceo is period. In fact its extension 
without any decided change in altitude and only a slight and 
gentle slope, from the undoubted Cretaceous peneplain of 
southeastern Minnesota and Howard county, Iowa, warrant 
our reference to it as the Cretaceous peneplain. This pene- 
plain everwhere slopes westward, which is necessary that it 
may go under the ("retaceous strata in the western part of the 
state. 
Southward from Dubuque, the Niagara plateau sends sev- 
