6 BULLETIN 142, U. S. DEPARTMENT OP AGRICULTURE. 



PHYSICAL FEATURES. 



The soils of the Miami series occur in the northeastern part of 

 the great Central Plain, which extends from the region of the 

 Great Lakes southward beyond the Ohio River and westward be- 

 yond the Mississippi. The greater part of this area, especially 

 the extensive tracts in western Ohio and in central Indiana, is 

 drained by streams belonging to the Mississippi drainage system. 

 Large areas in the northern region are drained by the small tribu- 

 taries of the Great Lakes. In general, the region consists mainly 

 of extensive plains which range from about 600 feet in altitude to 

 extreme elevations of 1,500 feet above sea level. 



The broader features of topographic relief within this region are 

 primarily due to the elevation of the rock floor which underlies the 

 surface deposits. The eastern border of the region is indistinctly 

 separated from the more elevated Appalachian Plateau by a transi- 

 tion from the more rugged topography of the plateau to the gently 

 undulating plains to the west. Along a large part of this border the 

 difference in relief is not so pronounced as to form a distinct bound- 

 ary, there being only a gentle gradation from hilly and dissected 

 country into a region whose interstream areas are but gently undu- 

 lating and within which the major streams occupy narrow or broad 

 trenched valleys of no great depth. Along this eastern border the 

 elevation of the plain ranges from about TOO feet, east of Chillicothe, 

 Ohio, to approximately 900 feet immediately east of Columbus and 

 about 1,000 feet to the east of Bucyrus. Near its eastern border 

 that part of the plain occupied by the soils of the Miami series sinks 

 gently toward the basin of Lake Erie, the northeastern border of 

 the section following the ancient shore lines of the glacial lake which 

 occupied the Maumee Basin. This shore line has an elevation of 

 about 800 feet above tide level throughout its extent, from Tiffin, 

 Ohio, to Fort Wayne, Ind. The eastern boundary of the area, ex- 

 tending from Fort Wayne to Bad Axe, Mich., has approximately 

 the same elevation. 



From the eastern border of this region in central Ohio the plains 

 undulate westward, gradually increasing in elevation until a maxi- 

 mum altitude of 1,500 feet is attained over a small area in the vicin- 

 ity of Belief ontaine. This marks the extreme altitude in an elevated 

 ridge which extends in an almost due north and south direction from 

 the vicinity of Bellefontaine to that of Hillsboro. This ridge varies 

 from 25 to 40 miles in width and has an elevation of more than 1,000 

 feet. It constitutes a gently rolling watershed separating the drain- 

 age of the Scioto River from that of the Mad River and the Little 

 Miami. 



