The surface features of the Earth are commonly split into two categories, the first of which comprises those features that are due to processes occurring inside the solid Earth (endogenic features) and the second those that are due to processes occurring outside the solid Earth (exogenic features). Specifically, the endogenic features are treated in the science of geodynamics, the exogenic features in the science of geomorphology. I have treated the theoretical aspects of the endogenic features in my Principles of Geodynamics, and it is my aim to supplement my earlier book with a discussion of the theory of the exogenic features, the taxonomy of the latter having been discussed in my Systematic Geomorphology. It is my hope that the three books will together pre­ sent a reasonably coherent, if necessarily incomplete, account of theoretical geology. Contrary to endogenic phenomena, exogenic processes can often be directly observed as they occur: the action of a river, the develop­ ment of a slope, and the evolution of a shore platform are all suffi­ ciently rapid so that they can be seen as they take place. This has the result that in geomorphology one is generally on much less specula­ tive ground regarding the mechanics of the processes at work than one is in geodynamics.