154 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



extent that the tributaries leading in from unglaciated portions of the basin 

 have become silted np to a marked degree. In these respects a striking 

 contrast is found between this drainage system and that of the Upper Ohio, 

 which lies immediately east of it. 



Notwithstanding the low altitude of the gradation plains and conse- 

 quent absence of trenches, the present system of drainage departs greatly 

 from the ancient system. Not only have changes occurred in the glaciated 

 district, but also outside, notably in the line of discharge for the main river. 

 The filling of the valleys was sufficient to raise the streams above the level 

 of the low cols that separated the ancient di-ainage lines without the neces- 

 sity of much excavation at the cols. The present drainage lines are almost 

 entirely in old valleys, for the divides crossed b}^ them constitute but a 

 small portion of the length of the streams. The position of the old divides 

 is usually shown by a constriction in the valleys combined with a higher 

 altitude of the rock floor. The changes are numerous in the headwater 

 portions that lie within the glacial boundary, but onl}' a few have been 

 sufficiently examined to justify an interpretation. In some cases where 

 constrictions occur in the valleys there have been no borings sufficiently 

 deep to throw light upon the altitude of the rock floor, thus leaving an 

 element of uncertainty concerning the significance of the constriction. The 

 changes of drainage in the central and western portions of this watershed, 

 and also along the lower course of the valley, have been investigated by 

 W. G. Tight in some detail, but his latest results have not yet been pub- 

 lished. Tlie writer has given the region only a hasty reconnaissance. 

 The changes in the northern portion have also been investigated by J. H. 

 Todd, of Wooster, Ohio.^ In the eastern part of the watershed no changes 

 of importance appear to have taken place. M. C. Read,^ of the Ohio survey, 

 has outlined on a map, and to some extent discussed, the position of many 

 preglacial lines, some of which are not followed by the present streams, but 

 he did not attempt such a full interpretation of the connections of the old 

 lines of di-ainage as has been made by Tight and other later students. 



iQhio Acad. Sci., Special Papers No. 3, 1900, pp. 46-67. 



^Geologj' of Huron, Richland, Knox, and Licking counties, Ohio, hy M. C. Read: Geology of 

 Ohio, Vol. Ill, 1878, pp. 289-361. Also Geology of Ashland, AVayne, and Holmes counties, Ohio, by 

 the same author: Ibid., pp. 519-561. 



