206 N. M. FENNEMAN — EFFECT OF CLIFF EROSION ON FORM 



erosion by the advancing waters has removed a part of the original land 

 surface and altered, or rather erased, the details of relief. It was for 

 the more exact interpretation of the forms as seen at this place that 

 the following elementary formulation of certain general principles was 

 undertaken. 



Effect of preexisting Topography 



When an eroded land surface is submerged and covered by later sedi- 

 ments the surface of contact between the two formations may be expected 

 to have some dependence on the topography of the preexisting land. 

 Other factors being equal, a topography of steep hills and valleys will 

 show itself after burial in a corresponding unevenness of the surface of 

 unconformity. The surface of contact is, of course, not visible, but a 

 section of it appears when the formations at the line of contact are up- 

 lifted and eroded. If the outcrop be on a plane surface and the line of 

 contact straight, the surface of contact is supposably a plane. There is, 

 then, a suggestion, at least, that the land surface of the lower formation 

 before submergence and burial was one of faint relief. The more the 

 line of contact deviates from straightness (ignoring subsequent folding) 

 the greater the presumption in favor of strong relief before submergence. 

 These inferences very properly take into account the effect of the topog- 

 raphy of a submerging land on the form of the surface of contact with 

 later formations, but they leave out of consideration the important change 

 which may accompany the advance of the sea. 



Factors and Conditions of the Problem 



There are two factors which determine the deviation of the surface of 

 contact from a plane. These factors are (1) the form of the preexisting 

 land surface, and (2) the strength of shore erosion during submergence. 

 These elements may combine in any proportion. Hypothetically, two 

 extreme suppositions may be made. The extreme of rapid subsidence 

 and small wave erosion would result in leaving the original subaerial 

 topography unaltered and retaining it as the surface of unconformable 

 contact. At the other extreme is the complete dominance of shore ero- 

 sion, whose tendency is to plane off the hills and to make the contact 

 with the newer sediments a horizontal plane. 



It should be stated here, though somewhat parenthetically, that shore 

 erosion may, under certain circumstances, increase instead of destroy 

 the relief of a submerging land surface, but such conditions are excep- 

 tional. This is apparent when it is remembered that the shoreline is a 

 contour line; that whatever straightens the contours is bringing the 



