GREAT MIAMI DRAINAGE SYSTEM. 183 



work they have fallen far short of reaching the old rock floors, which lie 

 100 to 200 feet below their beds. The depth of this reexcavation is but 

 50 to 100 feet, and the width but a small fraction of that of the old valleys, 

 seldom so much as one-fourth as great. The contrast between the southern 

 and northern portions of this drainage basin, therefore, is not found in 

 the work of the present streams, but is due to the less complete conceal- 

 ment of the old drainage lines by glacial deposits. 



CHANGES IN DRAINAGE. 



J. A. Bownocker has recently presented a partial restoration of the 

 old drainage in the headwater portion of this drainage basin.-' The course 

 of the old drainage line has been made known by borings for oil and gas, 

 which are numerous in that region. It is not perceptible on the surface 

 except for a few miles in eastern Indiana, where a sag or sliallow valley 

 marks its course. The present systems of drainage show a nearly complete 

 disregard of the old drainage lines. The course of the old line, as noted by 

 Bownocker, is northwestward from near the Great Miami, in Shelby County, 

 Ohio, past Anna and the Grand reservoir, to Rockford, Ohio, on the St. Marys 

 River. It there turns southwestward, crosses the Wabash River at Geneva, 

 Ind., and continues past Pennville into Blackford County, Ind., where the 

 tracing was discontinued. The length of the line thus traced is about 90 

 miles. This old line received a southern tributary at the Grand reservoir, 

 with a head probably near Xenia, but no other well-defined tributary was 

 recognized between that point and Blackford County. Two channels were 

 there found, one of which leads northward and the other westward, but 

 the data are insufficient to show which was the main channel. 



The width of this old valley appears to be about 1 mile, with a possible 

 range from tlu-ee-fourths of a mile to 1^ miles. The filling of drift is found 

 to range from 320 feet up to 514 feet, the vai'iation being principally due 

 to the different altitudes of the present surface. The rock floor is not far 

 from 500 feet above tide in tlie eastern portion, but falls to scarcely more 

 than 400 feet in eastern Indiana. It is markedly higher than the Ohio 

 at the mouth of the Great Miami, whose rock floor is less than 400 feet 

 above tide. The course seems to show that it was a tributary of the 



'A deep preglacial channel in western Ohio and eastern Indiana, by J. A. Bownocker: Am. 

 Geologist, Vol. XXIII, 1899, pp. 178-182. Also Ohio Acad. Sci., Special Papers No. 3, pp. 32-45, 

 with map. 



