Monuments are smaller masses and may be but partially detached from the cliff face.  In the breaking down of sheets of horizontal strata, outliers grow smaller and smaller and are reduced to massive rectangular monuments resembling castles (Fig. 17).  The rock castle falls into ruin, leaving here and there an isolated tower; the tower crumbles to a lonely pillar, soon to be overthrown.  The various and often picturesque shapes of monuments depend on the kind of rock, the attitude of the strata, and the agent by which they are chiefly carved.  Thus pillars may have a capital formed of a resistant stratum.  Monuments may be undercut and come to rest on narrow pedestals, wherever they weather more rapidly near the ground, either because of the greater moisture there, or—­in arid climates—­because worn at their base by drifting sands.

Stony clays disintegrating under the rain often contain bowlders which protect the softer material beneath from the vertical blows of raindrops, and thus come to stand on pedestals of some height.  One may sometimes see on the ground beneath dripping eaves pebbles left in the same way, protecting tiny pedestals of sand.

Mountain peaks and ridges.  Most mountains have been carved out of great broadly uplifted folds and blocks of the earth’s crust.  Running water and glacier ice have cut these folds and blocks into masses divided by deep valleys; but it is by the weather, for the most part, that the masses thus separated have been sculptured to the present forms of the individual peaks and ridges.

Frost and heat and cold sculpture high mountains to sharp, tusklike peaks and ragged, serrate crests, where their waste is readily removed.

The Matterhorn of the Alps is a famous example of a mountain peak whose carving by the frost and other agents is in active progress.  On its face “scarcely a rock anywhere is firmly attached,” and the fall of loosened stones is incessant.  Mountain climbers who have camped at its base tell how huge rocks from time to time come leaping down its precipices, followed by trains of dislodged smaller fragments and rock dust; and how at night one may trace the course of the bowlders by the sparks which they strike from the mountain walls.  Mount Assiniboine, Canada (Fig. 20), resembles the Matterhorn in form and has been carved by the same agencies.

“The Needles” of Arizona are examples of sharp mountain peaks in a warm arid region sculptured chiefly by temperature changes.

Chemical decay, especially when carried on beneath a cover of waste and vegetation, favors the production of rounded knobs and dome-shaped mountains.

The weather curve.  We have seen that weathering reduces the angular block quarried by the frost to a rounded bowlder by chipping off its corners and smoothing away its edges.  In much the same way weathering at last reduces to rounded hills the earth blocks cut by streams or formed in any other way.  High mountains may at first be sculptured by the weather to savage peaks (Fig. 181), but toward the end of their life history they wear down to rounded hills (Fig. 182).  The weather curve, which may be seen on the summits of low hills (Fig. 21), is convex upward.