526 THE ICE AGE IN NORTH AMERICA. 



glacial front was at the sea-level, and the ocean itself may 

 have kept it from advancing farther. In Pennsylvania the 

 boundary-line crosses the Alleghany Mountains diagonally, 

 being nearly as high on Pocono Mountain, in the 'eastern part 

 of the State (about two thousand feet), as in the southwestern 

 part of Kew York, where it is sixty or seventy miles farther 

 north. In Ohio the highest portion of the State is in Logan 

 county, almost directly north of Ripley, in Brown county, 

 the most southern point reached in that State. The unglaci- 

 ated portion in southern Indiana, projecting about seventy 

 miles northward into the glaciated region, is indeed some- 

 what higher than the land on either side, but nowhere is its 

 elevation greater than that of the larger portion of Ohio. 

 The farthest extension of the ice in Illinois is closely coinci- 

 dent with the trough of the Mississippi Valley, and westward 

 of the Mississippi Piver the edge of the ice withdrew farther 

 and farther north pretty nearly in proportion to the increas- 

 ing elevation of the country, until, at Sim's Station, in the 

 vicinity of Bismarck, Dakota, it is nineteen hundred and sixty 

 feet above tide, and continues thence to ascend northward to 

 a height of three or four thousand feet in the upper valley of 

 the Saskatchewan. 



There is, thus, a general conformity in its southern exten- 

 sion to the valley of the Mississippi. The ice of the Glacial 

 period as a whole did, indeed, move down that great valley, 

 and its most southern point is in the middle of it, where it 

 it is not more than five hundred feet above the sea ; but it is 

 evident that the total width of the southern portion of this 

 ice-sheet is so great, and the slope itself so slight, that this 

 depression could not have been the main cause of the great 

 extension to the south in Illinois. The width of the glaci- 

 ated area from southern Ohio to eastern Kansas, on the thirty- 

 ninth parallel, not far from the extreme limit of glaciation, 

 is nearly a thousand miles. The cause, therefore, of the lo- 

 bate character of the southern boundary must, in all proba- 

 bility, be sought in the irregular areas of excessive snow-fall 

 existing to the north during the advance and continuance of 



