i6o 
E, R. Scheffel 
2. Glaciation^^ This has been considered the principle factor i 
in changing the drainage over considerable areas. Tight^^ ascribes 
the reversals in the drainage of Ohio to this cause. Leverett i 
inclines to the same explanation for this and other areas. Leverett i 
has shown a tendency, however, to admit the possibility of another i 
explanation for changes in glaciated areas. Carney, particu- 
larly, has suggested a theory of preglacial diversion for certain 
Ohio streams. I 
Glaciation may effect drainage in various ways, i. e., by planing 
topography and by eroding divides. I: 
Planing Topography. This may be accomplished bytheeros- jl 
ive action of a glacier combined with its later passivity with [ 
resultant heavy aggradation. This may effect a changed drainage 1 
having the same general course as the preglacial, or the debris | 
filling may take a slope at variance to the original valley bottoms | 
necessitating a very different and perhaps reverse course. i 
Eroding Divides. Cols may be cut directly by the corrasive | 
action of glaciers. Again, a valley may be dammed by a morainal I 
deposit^^ necessitating outflow of the drainage in a new direction, 
sometimes over rock divides. Perhaps the most commonly recog- I 
nized cause is damming^® of the headwater areas by the ice-front. 
In such instances the water is ponded between the ice and divide, 
and is forced to seek an outlet over the lowest point in the latter, i 
Eventually a deep channel or channels may be cut through it. j 
The deposition of drift in such a lake would normally be heaviest I 
near the ice, with the possible result of a change in the slope of 
the bottom, downward toward the col, leaving on the retreat of | 
the ice a reversed drainage. Sometimes glacially formed lakes fi 
f 
jl 
R. S. Tarr: Physical Geography of New T ork, pp. 154-184, 1902. G. D. Hub- | 
bard: Ohio Naturalist^voX. viii,pp. 349-355, 1908. W.G. Tight, J. A. Bownocker, jj 
J. H. Todd, and Gerard Fowke: Special Paper No. 3, “The Preglacial Drainage 
of O.,” O. State Acad, of Sc. i; 
“ W. G. Tight: Bull Sci. Lab., Denison Univ., vol. viii, part ii, pp. 35“6l, 1894. j 
Bull. Sc. Lab., Denison Univ., vol. ix, part ii, p. 2i, 1897. Monograph ;! 
199. G. C. Matson: Jour, of GeoL, vol. xii, p. 139, 1904. | 
Bull Sc. Lab., Denison Univ., vol. xiii, p. 151, 1907. 
F. Carney: American Journal of Science, vol. xxv, pp. 217-223, 1908. T. L. 
Watson: Untv. of State of N. T., State Museum Report (No. 51), vol. i, p. r7l“2, j 
1899. H. L. Fairchild: Bull. Geol. Soc. Am., vol. x, pp. 27-68, 1899. 
vol. vi, pp. 354-5, 1895. :i 
G. K. Gilbert: Bull. Geol. Soc. Am., vol. iii, p. 286, 1897. 'i 
