392 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



or it may have been comparatively imbroken, but traversed by tunnels in 

 which the terraces were built up. About 5 miles north of the glacial 

 boundary a remnant of the hig-h terrace was noted, which was bordered 

 by a low sag on the side next the bluff, resembling somewhat the sag which 

 so often accompanies the esker ridges. Phenomena such as these may 

 serve as a connecting link between the eskers, which are probably sub- 

 glacial, and moraine-headed terraces which, having their origin at the ice 

 margin, are extraglacial. The morainic features and the high, level-topped 

 terraces both disappear somewhat abruptly near the north border of the 

 moraine, and the valley for some miles above has a low, nearly smooth, 

 plain but little higher than the stream. From this it appears that the ice 

 sheet at the late Wisconsin invasion deposited but little material in this 

 valley except at its margin, a feature not uncommon in other valleys of 

 northeastern Ohio and northwestern Pennsylvania. 



On the uplands bordering Lake Fork the sharpest knolls and ridges 

 are found near the southern border, there being in the northern portion 

 gentle till swells but 1 to 20 feet high. 



The moraine is exceptionally strong from Lake Fork westward past 

 Loudonville and Perrysville to the -sacinity of Lucas. In a lowland tract 

 south and west of Perrysville numerous knolls and ridges rise abruptly to 

 a height of 30 to 50 feet, and among them landlocked basins occur. It is 

 probable, as indicated in Chapter III, that Clear Fork and Black Fork 

 had, in preglacial times, a course north of the present one, from the lowland 

 tract near Perrysville eastward, near the line of the Pittsburg, Fort Wayne 

 and Chicago Railroad, past Loudonville to Lake Fork, as a lowland tract 

 there occurs a mile or more in width, deeply filled with drift, while the 

 present streams are in narrow, rock-bound valleys for several miles above 

 their junction with Lake Fork. This lowland tract is now occupied by a 

 strong morainic belt. 



East from the reentrant angle near Mansfield the moraine for several 

 miles is weak compared with its strength on the west side of the reentrant 

 angle, or compared with its strength a few miles farther east. It is repre- 

 sented by scattering drift knolls and occasional low ridges whose height 

 seldom exceeds 20 feet. The striae east of this reenti-ant angle, near Wind- 

 sor and Mifflin, instead of bearing west of south toward the moraine, bear 

 southeastward, approaching it at an oblique angle, a fact which strengthens 



