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.
Fig. 20. Mount Assiniboine, Canada
Fig. 21. Big Round Top and Little Round Top, Gettysburg, Pennsylvania
In [Figure 22], representing a cubic block of stone whose faces are a yard square, how many square feet of surface are exposed to the weather by a cubic foot at a corner a; by one situated in the middle of an edge b; by one in the center of a side c? How much faster will a and b weather than c, and what will be the effect on the shape of the block?