42 



GEOIvOGY OF LA SALLE COUNTY. 



south of the Potomac — the}^ nowhere reach this 

 stream, nor do they approach within several miles 

 of it — the Ohio and Lower Missouri. 



A glacier is a vast body of ice filling moun- 

 tain valleys and covering- plateaus, which by the 

 pressure of its thickest and central portion tends 

 to flow^ toward lower levels, for we must not look 

 on ice as a solid like limestone, but rather as a 

 viscous body like pitch. As these arms of the gla- 

 cier — true ice rivers — descend through the valleys 

 they plane down the rocks over w^hich they slide, 

 scratching and grooving them, and forming a large 

 amount of fine material which the w^ater that 

 forms a stream under the ice bears on down the 

 valley to be deposited in pools and lakes according 

 to its fineness and the force of the current. Be- 

 sides, it bears on its surface masses of rock and 

 earth which fall from the sides of the valley upon 

 it, and other pieces sink into and become buried in 

 its substance. The point to which the glacier de- 

 scendsis determined b}^ the temperature and moisture. 

 It usually presents a precipitous face to the coun- 

 try below — a veritable wall of ice in cold seasons, 

 advancing, it may be, many rods- in warm ones 

 retreating. In front is a vast, rugged wall com- 

 posed of blocks of stone of all forms and sizes, 

 some smooth and scratched, others angular and 

 rough. This wall is called the terminal moraine, 

 while the piles of material lying on its face, form- 

 ing, usually, three lines, one on each side and one 

 near the middle, are called the lateral or side, and 

 medial or middle moraines. 



It is not uncommon to find several terminal 

 moraines, one behind the other, and when the gla- 

 cier advances one or more of these may be pushed 



