Page:Popular Science Monthly Volume 53.djvu/505

Rh 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 rounding 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



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 illustrated at Lookout Mountain, is typically and characteristically displayed at hundreds of other localities in the same general region, but is not confined to the basin of the Columbia. With minor modifications 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 approximately 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 floors. 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 hardened about them, in much the same manner that nunalakas 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