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A
divided products of chemical weathering. The landforms in dry
climates, on the other hand, tend to have sharp, angular outlines
from the mass movement of rock material from vertical cliffs.
Lacking as much chemical weathering, the landscapes in dry
climate regions tend to have sharper outlines.

STAGE OF DEVELOPMENT
The stage of landform development describes how effective the
processes have been in attacking the rock structure. Stage de-
scribes the extent to which the processes have completed their
work, that is, the amount of the original surface that remains.
Mountains, for example, are said to be youthful when the processes
of weathering and erosion have not had time to do much of their
work. Youthful mountains are characterized by prominent relief
of steep peaks and narrow, steep valleys. The steep peaks may be
from cirques produced by glaciers, and the steep valleys have been
cut by streams, but neither process has yet greatly altered the origi-
B nal structure (Figure 20.16). By maturity, the original structure has
been worn down to rounded forms and slopes. Eventually, even
FIGURE 20.16 This melting glacier (A) is the source for a
stream (B) that flows through a valley in the youth stage. the mightiest mountain is worn down to nearly flat, rolling plains
during old age. The nearly flat surface is called a peneplain, which
means a region that is “almost” a plain. Often hills of resistant rock
WEATHERING AND EROSION PROCESSES called monadnocks exist on the peneplain during the last stages
The processes of weathering and erosion that attack the rock of old age. Theoretically, the monadnocks and peneplain will be
structure are influenced and controlled by other factors such reduced to the lowest level possible, which approximates sea level.
as climate and elevation. Chemical weathering, for example, More than likely, however, the land will be uplifted before this hap-
is more dominant in warm, moist climates, and mechanical pens, causing rejuvenation of the erosion processes. Rejuvenation
weathering is more dominant in dry climates. Thus, landforms renews the effectiveness of the weathering and erosion processes,
in warm, moist climates tend to have softer, rounded outlines and the cycle begins again with youthful landform structures be-
from the accumulations of clay minerals, sand, and other finely ing superimposed on the old age structures.

20-15 CHAPTER 20 Shaping Earth’s Surface 515