THE PLEISTOCENE OR GLACIAL PERIOD. 



351 



sions in any given locality, show the greater wear, and indicate the 

 direction of ice movement. Other markings, such as chatter marks, 

 distinctive of ice work, are also found on the bed-rock, though less 

 commonly than striae, grooves, etc, 



(9) The shapes of rock hills. — The rock knolls which were left 'bare 

 when the ice retreated, often show peculiarities of form and surface 

 which are distinctive. Like the minute protuberances of surface just 



^ ryj g grg L 





Fig. 489. — Diagrammatic representation of a hill unworn by the ice. The diagram 

 also shows the irregular contact between the surface earths and the rock below. 



referred to, the rock hills of many localities over which the ice passed 

 were systematically worn more on the side from which the ice approached 

 (the stoss-side), than on the other (Fig. 490). Bosses of rock 



Fig. 490. 



-Diagram to show the effect of glacial wear on such a hill as that shown in 

 Fig. 488. 



which do not show pronouncedly unequal wear often show dis- 

 tinctive smoothing (Fig. 491). Projecting glaciated knolls of rock, 

 whether large or small, which show the characters seen in Fig. 492, 

 are known as roches montonnees. A succession of roches moutonnees 

 generally give fairly accurate information as to the direction of ice 

 movement, even though stria? be not preserved. 



Summary. — The characteristics of the drift, as set forth in the 

 preceding paragraphs, leave little room for random speculation 

 concerning its origin. From its variable thickness we know that the 

 force or forces which produced it must have been such as could leave 

 the drift now in thick bodies and now in thin, over either limited or 

 extensive areas. From its distribution we know that the force or 

 forces which produced it were largely independent both of underlying 



