Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 227

The nature of the rock type itself has a major impact on landscape development.
Massive and resistant crystalline rocks, such as granite or quartzite, have the potential
to produce ridges, whereas landforms formed in weak rocks, such as shale, often form
valleys. The exact outcomes depend on local circumstances, but the more resistant the
rock, the more likely it is to produce peaks and ridges. Joints, fractures, and fault lines
are zones of weakness and can experience more weathering. Streams exploit such fea-
tures and develop preferentially along them, providing a good indicator of an area's
structural framework.
While structure and rock type exert significant control on landform development,
they are also largely innate and unique in their character and distribution, depending
upon internal processes. External processes operating at the surface of the Earth, such
as precipitation, temperature, or other climatic variables, display a very different distri-
bution. Climate provides the basic framework into which all natural phenomena must
it. It is obvious that environmental conditions (temperature and precipitation) vary with
climate type. Therefore, despite widespread regional differences in the character of
mountains, many pervasive and overriding similarities suggest that similar geomorphic
processes operate under comparable environmental conditions and give rise to similar
types of landforms.
Landscape Development
As early as 1850, scientists began to question how flat erosional surfaces formed or why
unconformities—breaks in the building structure—existed. The ideas of William Morris
Davis coincide with these early attempts to reconstruct the denudational history of the
Earth, the weathering and erosional processes that contribute to lowering land. Charles
Darwin influenced the popular trend of thought during this period; as a result, Davis
applied evolutionary ideas and terminology to describe landforms and the landscape. In
1889, Davis first presented his explanation of the erosion cycle, which explained land-
form development by investigating the rivers and valleys of Pennsylvania. He assumed
that the landscape experienced an initial rapid tectonic uplift. Throughout the youth-
ful, mature, and old stages, erosional forces reduced the landscape to its original form:
a peneplain, a low surface with little relief. The cycle could then renew itself as more
uplift or changes in climate occurred. Base level, the lowest point to which a river can
flow, may have temporarily been met, but stream gradient can increase with uplift until
a stream matures and weathers relief. German geomorphologists Albrecht and Walther
Penck offered an alternative view on landscape evolution. Rather than mountain sum-
mits consisting of peneplains from the past, they were simply upper limits to which
mountains could grow, because uplift balanced denudation. The Pencks' ideas about
landscape evolution were different from those of Davis, because they were rooted in
crustal tectonic processes rather than strictly erosion.
Their ideas were simplistic, but their effect on mountain geomorphology was monu-
mental. Each saw the evolutionary development of the landscape as a whole. This re-
gionalized approach led to climatic geomorphology, according to which climatic regimes
control process, and process controls landform development (Thorn 1992). There was,
however, much discontent, since Davis's ideas were over-generalized. Those seeking a