THE REIGN OF ICE. 215 



margins of the ice-field. The northern limit was chained 

 by eternal frost to its rocky bed. The southern only was 

 free to move, and the whole expansion would be developed 

 along the southern border. The sliding movement of in- 

 calculable tons of ice would plow the soil beneath. Rock- 

 fragments, pebbles, and gravel, frozen in the under surface, 

 were carried forward by the moving mass, while the under- 

 lying rocky surfaces were ground away, or polished, or 

 scored in parallel furrows by the irresistible agency of the 

 glacier (Fig. 80). These phenomena are noticeable all over 

 the Northern States wherever the " bed rock" is exposed 

 to view. The bold shore of the north side of Lake Supe- 

 rior has been extensively carved and modified by this re- 

 sistless action. At Marquette, upon the south shore, are 

 some striking and instructive illustrations. A low dome 

 of metamorphic talcoze schist rises a few feet above the 

 surface of the water at the shore, nearly in front of the 

 Jackson house, which bears the imperishable tracery of its 

 conflict with the continental glacier. The w T hole surface is 

 smoothed as with a carpenter's plane and sand-paper. The 

 undulations in the surface are scoured as neatly as the level 

 and more prominent portions. Rising from beneath the 

 water on the northern side can be seen numerous grooves 

 and scratches, which glide up the smoothed northern slope, 

 and extend continuously across the summit to the southern 

 side. There are two principal sets of these striae. One of 

 them extends nearly north and south, the other northeast 

 and southwest. Near this place, and close by the main 

 street as it passes out of town, is an isolated outlying mass 

 of the same kind of rock, which has been left standing out 

 boldly after the destructive agencies that have passed over 

 the surface had plowed away all the surrounding portions 

 of the formation. This stubborn mass stands like a sullen 

 bulwark, defying the most desperate attacks of ice, or 



