destructive processes such asweathering and mass wasting, and erosion. Weath-
ering loosens materials, mass wasting brings the materials downslope because of
gravity, and erosion moves the materials away (sometimes far away) from their
origins. There are constructional landforms—notably the floodplains built by
stream flooding, deposition by glaciers, and deposition by wind—but the resulting
landforms are not as widespread as the landforms of destruction.
Although previous scientists had made contributions to the corpus of knowl-
edge of geomorphology, it was the geomorphologist Davis who summarized the
science of geomorphology as rooted in structure, process, and time. This succinct
characterization has proven useful for over a century.
Structure is a term applied to an inherent quality of rock. “Structure” implies
several things. It refers to the minerals composing the rocks because minerals
can be quite varied as to the amounts and nature of possible deformations.
Although rock seems solid and immutable under normal circumstances, there is
no doubt that deformation occurs over long times when considerable force is
applied. Structure is concerned with thephysical arrangement of rock masses
and layers. Rocks can be warped, folded, or broken (seeFolding and faulting).
A horizontal set of rock layers, for instance, will produce a markedly different
set of landforms than rock, which are arranged in anticlines. An examination of
structure implies that the geomorphologist is querying about deformation of the
rocks over past time.
“Process” refers to any of the multitudinous array of internal and external fac-
tors combining to produce the observed landscape. These include erosion and dep-
osition by liquid water, ice, and wind. Also, physical and mechanical weathering
and mass wasting are important players. Internally, the tectonic processes and
related volcanism have observable impacts. Yet, these factors have definite geog-
raphies and can be more or less important in different regions. For instance, weath-
ering by solution is a dominant process in precipitation-rich regions underlain by
limestone; solution is responsible for the distinctivekarsttopography. However,
limestone weathers more slowly with less solution in desert area so that karst is
not very common.
“Time” is an exorable characteristic of the physical universe and immense
amounts of time are available in which the processes can shape landforms. Some
landscapes are rearranged in a matter of moments—the explosion of a stratovol-
cano for instance. Other landscapes are shaped bit by bit, obvious only to the keen-
est of observers. Geomorphologists have traditionally writtenabout “stages” of
landscape development in which sequences of events take place over thousands
or millions of years. Davis, for instance, invoked the notion of peneplains, which
are low-lying surfaces of modest topography developed over extremely long time
sequences (presumably millions of years). Modern rock dating techniques146 Geomorphology