top of the escarpment. Farther north we have a true cuesta formed of 
the Knobstone, as the yale or back-slope is composed of the upper part of 
the Knobstone rock and this back-slope has a dip comparable to that of the 
strata of which it is composed. At the extreme north this cuesta is much 
dissected and destroyed by the short streams flowing north into the Musca- 
tatuck River. Probably more than half of the mapped area has the Mitchell 
limestone as the surface rock. The great thickness of this limestone and 
the development of subterranean drainage have allowed a plain of consid- 
erable width to extend along the strike of the outcrop. This plain in places 
attains a width of 25 miles. To the west of the Mitchell limestone area 
and mainly at the south, the Mitchell limestone is partially covered by the 
clastic Chester members. The Chester in the area overlying the Mitchell 
limestone is mainly in the form of ridges and outliers.. 
The lower geologic units extend farther west along the east and west 
streams than their general upland outcrop, and often extend along the 
streams into the general outcrop region of the succeeding units. Thus, 
along Muddy Fork of Blue River the Knobstone strata extend several miles 
west of Pekin, whereas in the upland area adjacent the succeeding lime- 
stones form the surface rocks. 
TOPOGRAPHIC CONDITION OF THE KNOBSTONE CUESTA REGION. 
Factors Involved in the Development of Topographic Condition. 
The explanation of the topographic condition of a region brings in an 
inventory of the responsible active and conditioning factors. The active 
factors are simply the physiographic processes, viz., weathering and erosion. 
The conditioning factors consist of both material and time elements. The 
material elements are the yarious lithologic units exposed to the physi- 
ographic processes. The expression “time elements” as used here needs 
brief explanation. The physiographic development of any region involves 
certain changes in the lay of the land with respect to the sea. These 
changes may be due to regional elevation or depression, or warping and tilt- 
ing. These things may. take place in the region itself or in an adjacent 
region which is intimately related to it. Thus, a slight regional depression 
or a tilting in the lower Mississippi valley would allow the sea to come 
much nearer the southern Indiana region, and certain important changes 
in stream regimen would result. In addition to land movements, glacia- 
tion has been an important factor in the physiographic development of 
muny regions. Climatic changes may give rise to important changes in 
stream regimen, and should be included here. Regional elevation, regional 
depression, warping and tilting. glaciation, and climatic changes are in- 
fluential conditioning factors. and are here called time factors, since they 
are occurrences which may belong to any period of time without regard to 
any other factors concerned in topographic development. 
A full explanation of the topographic condition of a region may involve 
all of the above factors. <A detailed physiographieé analysis of the area 
here under discussion would certainly involve all of them. A full discus- 
