234 MODIFICATION OF THE GREAT LAKES 



water surfaces, they must originally have been level, and their 

 present lack of horizontality is due to unequal uplift of the land. 

 The region has been tilted toward the south-southwest. The dif- 

 ferent shore-lines are not strictl}^ parallel, and their gradients 

 vary from place to place, ranging from a few inches to three or 

 four feet to the mile. 



The epoch of glacial lakes, or lakes partly bounded by ice. 

 ended with the disappearance of the ice-field, and there remained 

 only lakes of the modern type, wholly surrounded b)'- land. 

 These were formed one at a time, and the first to appear was in 

 the Erie basin. It was much smaller than the modern lake, 

 because the basin was then comparatively low at the northeast. 

 Its outline is approximately shown by the inner dotted line 

 of the accompanying map. Instead of reaching from the site 



FIG. I — ANCIENT AND MODERN OUTLINES OF LAKE ERIE 



The broken lines show the positions of the shores at two epochs of the lake's history 



of Buffalo to the site of Toledo, it extended onl}' to a point oppo- 

 site the present city of Erie, and it was but one-sixth as large 

 as the modern lake. Since that time the land has gradually 

 risen at the north, canting the basin toward the south, and the 

 lake has gradually encroacht upon the lowlands of its valley. 

 At a date to be presently mentioned as the Nipissing, the west- 

 ern end of the lake was opposite the site of Cleveland, as indi- 

 cated by another dotted line. 



The next great lake to be releast from the domination of the 

 ice was probably Ontario, though the order of precedence is here 

 not equally clear. Before the Ontario valley held a land-bound 

 lake it was occupied by a gulf of the ocean. Owing to the dif- 

 ferent attitude of the land, the water surface of this gulf was not 



