There is no general theory of geomorphology. We cannot cast the subject in a single equation, or set of equations. As with geology, geomorphology is a tangle of physics, chemistry, biology and history. It is also geometry, as the geomorphology plays out in a complex geographic, topographic setting in which both the tectonic and climate processes responsible for driving evolution of the topography change in style and intensity. There is no grand quest for a Universal Law of Geomorphology. Our subject is often subdivided according to the geographic elements of the geomorphic system: hillslopes, rivers, eolian dunes, glaciers, coasts, karst, and so on.
There is indeed order to the natural system on and near the earth’s surface that
in turn serves to connect these subdisciplines. Features in common among many
geomorphic realms include:
• Surface materials most often move in one direction: downhill, downstream, downdrift,
or downwind. (Prior to emergence on the surface, the motion is effectively
vertical, in the reference frame of the surface, during near-surface exhumation.)
• Materials are transformed as they move through the system. Some of this is again
unidirectional, this time meaning irreversible: large grains can be broken into small
grains, but not the reverse; chemical reactions are for the most part permanent, resulting in solutional loss and change in mineralogy toward low temperature hydrous phases.
Exceptions to this general statement do exist: duricrusts form by cementation of soils, and carbonate deposits accumulate.
• Motion is concentrative. Material gathers itself into more efficient streams (Shreve’s
dichotomy of hillslopes and streams (Shreve, 1979)) resulting in spatially-branching
networks. This is one of several instances in which the surface processes lead to selforganization.
• If given enough time, the system evolves toward a state in which the material flow is
adjusted to transport that supplied to it.
These features argue for some degree of universality in our subject, some degree of connection between the elements of landscape and our treatment of those elements. The
most useful scientific principle that we will employ here is that of conservation: conservation of mass, of energy, and momentum. “mass is conserved” or “energy is conserved”. These are indeed fundamental laws of physics. And physicists lean on them heavily. So also is momentum conserved, and angular momentum. So also is baryon number, strangeness, and so on. The fact that on the earth’s surface the speeds involved are far less than the speed of light allows us to dodge the complexity introduced by Einstein; we can go right back to Newton. As we do not need to worry about the conversion of mass to energy, mass is conserved perfectly.