366 



top of the escarpment. Farther north we have a true cuesta formed of 

 the Knobstone, as the vale or back-slope is composed of the upper part of 

 the Knobstone rock and this back-slope has a dip comparable to that of tlie 

 strata of which it is coiiipdscd. At tlie extreme north this cuesta is much 

 dissectc(l and destroyed by the short streams flowing north into the Musca- 

 tatuck KivtT. Trobably more than half of the mapped area has the Mitchell 

 limestone as the surface rock. The gi'eat 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 tlie 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 tlieir general upland outcrop, and often extend along the 

 streams into the general outcrop region of the succeeding units. Tlius, 

 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 rocl^s. 



TOPOGRAPHIC CONDITIOX OF THE KA'OBSTONE CUESTA REGION. 

 Factor's Involved in the Development of Topographic Condition. 



The explanation of the topographic condition of a region brings in an 

 inventory of the resjionsible active and conditioning factors. The active 

 factois arc siniiily tlie ]iliysiographic processes, viz., weathering and erosion. 

 The conditidiiiiig factors cdiisist of both material and time elements. The 

 material elements are the various lithologic units exposed to the physi- 

 ographic processes. The expression "time ck-ments" as used here ueeds 

 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 valle.v 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 i>bysiogiapbic development of 

 many regions. Climatic changes may give rise to impoiiaiit changes in 

 stream regimen, and should lie included here. Kcgional elevation, regional 

 depression, warping and tilting, glaciation, iuid 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 physiographic analysis of the area 

 here under discussion would certainly involve all of them. A full discus- 



