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XIII. Problems of Denudation. By Harold Jeffreys, 

 M.A., D.Sc, Fellow of St. John's College, Cambridge*. 



THE major phenomena of physical geology may be 

 divided into three main groups, namely crust-move- 

 ments, denudation, and sedimentation. They are closely 

 interrelated ; the occurrence of sedimentary rocks in high 

 mountains and the frequency of synclinal mountains show 

 further that they are of the same order of magnitude. Their 

 dynamical treatment has not been extensive in the past, and 

 geologists have, as a rule, been content with qualitative 

 explanations of the observed configurations of the surface 

 rocks. Such treatment is, however, highly desirable; for a 

 mathematical investigation enables us to specify accurately 

 the causes we are taking into account, and the correspondence 

 •or divergence between the effects it predicts and the actual 

 phenomena indicates the extent to which we have succeeded 

 in tracing the most important causes. The differences re- 

 vealed may then lead to the discovery of further causes, and 

 thus observed facts may gradually become understood in 

 greater completeness and detail. 



The present paper deals with problems of the flow of surface 

 water during rain. The ground is supposed completely 

 covered with a thin la} T er of water, supplied at a known rate 

 all over it. The movement of the water is found to be com- 

 pletely determinable in ordinary conditions. It must be 

 carefully distinguished from the flow of a stream ; in the 

 present problem the surface of the water may be considerably 

 inclined, for it closely follows that of the ground, whereas in 

 a stream the section of the free surface by a plane across the 

 lines of flow is always nearly horizontal however much the 

 bed may be inclined. In English conditions the frictional 

 resistance to the motion is usually mostly due to viscosity, 

 turbulence being important only in mountainous regions. 

 The form of an ideal peneplain that would sink at a uniform 

 rate all over owing to the denudation caused by such flow is 

 then determined, and its stability considered. 



I. The Flow of Surface-water during Fain. 



During rain water is supplied at a fairly uniform rate over 



wide areas, and as fast as it falls it runs away to lower ground 



under the action of gravity. The supply being practically 



■continuous, the whole surface is always covered, the depth at 



* Communicated by the Author. 



N2 



