418 FIRST YEAR SCIENCE 



of the ridges are more gullied than others, showing longer 

 exposure to erosion. 



These ridges are due to strains which have broken the 

 rock layers and elevated those on one side of the fracture 

 above those on the other side, so that a steep fracture 

 cliff has been formed with the rock layers slanting back- 

 ward from its elevated edge. (Fig. 118.) Mountains of 

 this kind are called block mountains. As is seen from the 

 fracturing of the debris slopes, the movement of elevation 

 is not yet completed. 



Fig. 118. 



As some of the ridges are more gullied than others, it 

 appears that the fracturing did not take place all at one 

 time, but that the more gullied ridges were formed first. 

 Earthquakes are not uncommon in this region. These are 

 caused by a small slipping along the fault line. 



In Oregon these ridges are little eroded. They are 

 simple in structure and young in age. The longer streams 

 flow down the gentle slopes parallel to the surface of the 

 rock layers and the shorter streams along the steeper 

 slopes across the edges of the layers. 



197. Folded Mountains. Experiment 132. To the long edge of 

 a piece of board about 10 cm. \vide and 20 cm. long tack securely one 

 of the shorter edges of a piece of rather thick rubber dam about 20 

 X 25 cm. Tack the opposite edge to a strip of board about 2 cm. 

 wide and 20 cm. long. Place the rubber dam thus arranged on a smooth 

 table and secure the wide board firmly to the table by a clamp or nail. 



