TOPOGRAPHIC FEATURES DUE TO LANDSLIDES. 487 



shown below. The section crosses one of the lake basins at the base of 

 the mountain, and is continued northwestward through the belt of 

 hills and basins to the plain into which they merge. In this diagram 

 an attempt has been made to indicate the breaking down and round- 

 ing of the fallen blocks, and their gradual change to an undulating 

 plain. 



In some instances a landslide plows its way out into a valley 

 for a mile or more from the base of the cliffs from which it came, and 



Lookout Mountain. 



Wheat lands. 



N. W. S. E. 



YiQ. 3. — Ideal Section thkouoh I.,ookout Mountain, WA^HI^OToN, showino Landslides. 



forces Up a series of ridges and mounds about its margin. These 

 ridges have a striking resemblance to terminal moraines left by the 

 recession of glaciers, but the scars on the adjacent escarpment or 

 mountain sides and the associated hills and basins plainly show their 

 origin. 



The sequence of topographic changes described above, so well illus- 

 trated at Lookout Mountain, is typically and characteristically dis- 

 played at hundreds of other localities in the same general region, but 

 is not confined to the basin of the Columbia. With minor modifica- 

 tions due to local conditions, it may be recognized in many lands 

 where bold escarpments occur. Where a humid climate prevails, 

 however, and streams occupy the valleys, the old age of landslide 

 topography is seldom reached. 



The Columbia lava, it will be remembered, was spread out during 

 a series of inundations of molten rock and has an area of approximate- 

 ly two hundred and fifty thousand square miles. Previous to the 

 opening of the tens of thousands of fissures through which the molten 

 rock reached the surface, the country had a rugged topography due to 

 erosion. The lava covered the plains and entered the valleys in 

 the mountains so as to give them level fioors. Hills, ridges, and 

 mountains were in some instances partially or wholly surrounded by 

 the fiery flood and became capes and islands. Isolated eminences of 

 the old land rise through the sheets of lava which cooled and hard- 

 ened about them, in much the same manner that iiunalakas break the 

 monotony of the borders of the Greenland ice fields. When these 

 islands in the sea of lava are of resistant rocks, like quartzite, which 

 withstand the attacks of the destructive agencies of the air better 



