CKUSTAL MO\ LMLNTS Ol THL LARTH 



Lake Erie, could cut but a narrow gorge — the part beginning 

 about 2,000 feet above the mouth of the gorge and extending 

 to the bend just norili of Niagara University, a distance of 

 about one and one-eighth miles — known as the Old Narrow 

 Gorge. 



Crustal Movements of the Earth 



Up to this time the story of the lakes has been a tale of the 

 carving of valleys by pre-glacial rivers, of the coming of the 

 ice and the consequent deepening and widening of these val- 

 leys, of melting ice and filling of the basins with glacial waters. 

 Now another force, which all the time had been gently active, 

 becomes most important in the story. 



The rock crust of the earth rests upon a more-or-less plas- 

 tic, yielding interior. Thus, it is probable that so great a mass 

 of ice covering the northern half of the continent and weigh- 

 ing millions of tons must have burdened the land surface and 

 depressed it far below the level it had attained in the highlands 

 where the glaciers first developed; also, it is plausible that as 

 the ice melted the land would gradually rise again when re- 

 lieved of this weight. From the evidence that the lakes have 

 left us, this seems to be exactly what happened. 



As lake surfaces are level, the beaches and strand lines must 

 be essentially horizontal when made. However, studies of the 

 glacial lake beaches show that, although their southern parts 

 are horizontal, as we follow them northward they gradually 

 rise in elevation. Only an uplift of the continent after the 

 beaches had been made could so disturb their horizontality. 

 The Whittlesey and Warren beaches are tilted upward toward 

 the north, showing that the depressed portion of the continent 

 started to be re-elevated when the ice had retreated half-way 

 across the lake region. It is notable that the places of initial 

 tilt of the beaches of each of the lakes, which are well defined 

 and readily observed on opposite sides of the lakes, lie on prac- 

 tically straight connecting linos. These arc known as hinge 



J9 



