Rising out of the snow-fields and rock ridges and isolated peaks, nunataks, as they are called in Greenland, and as one ascends above the glaciers a new type of scenery shows itself, no longer smoothed and rounded surfaces of rock with here and there a moraine, but rugged forms where weathering and frost have rudely done the shaping. In this higher region the character of the rock has much to do with its forms. Hard quartzite or solid limestone resist best and stand out as cliffs and ridges, while softer slates and sandstones crumble and slide, giving long slopes of loose scree into which the foot sinks, the whole surface often slipping with the climber.
In these upper regions the jointage of the rocks plays a large part, those with numerous joints, into which the water from thawing snow may sink by day, only to freeze at night and pry asunder the blocks, are quickly shattered even if of hard materials; while rocks with few open fissures stand their ground far better and rise amidst the slopes of debris as walls or pinnacles.
From the higher levels one sees, too, how the glaciers eat back their U-shaped valleys into the solid rocks of the central mountain blocks, even little "cliff glaciers" carving for themselves nests shaped like a half kettle, cirques, as they are called in the Alps. When two of these cirques have been gnawed inwards toward each other very narrow ridges of rock with knife edges may result. From the lips of empty cirques or hanging valleys hollowed during the Ice Age bridal-veil falls now spring hundreds or thousands of feet over precipices into some deeply cut main valley carved by a glacier of the first rank.
The highest of our Rockies were probably never covered by the ice sheets of the glacial period, but rose above them, so that their rugged forms are due to the tilt of the strata, their relative resistance to weathering, and their lack of joints in which frost could work.
