THE WORK OF THE OCEAN. 353 



descent. Fig. 303 represents the wave-cut terrace in its relation to 

 the sea-chff above. 



So long as wave-cut terraces are submerged, they do not appear 

 on topographic maps of the land, though they appear on the charts of 

 the coasts; but if a coastal tract with wave-cut terraces be elevated, or 

 if the sea-level be drawn down, the terraces become land. Elevated 

 sea-cliffs and wave-cut terraces are among the best evidences of change 

 of relative level between water and land (Fig. 311). 



Wave-erosion and horizontal configuration. — The structure of the 

 rock along shore has as much to do with the horizontal configuration of 

 the wave-shaped coast, as with its relief. In general, waves develop 

 reentrants in the less resistant portions of the shore, leaving the more 

 resistant parts as headlands (San Pedro Point and Devil's Slide, PI. XX^ 

 Coast of California). It is to be noted that the resistance of rock to 

 wave-erosion is not determined by its hardness alone. Every division 

 plane, whether due to bedding, to jointing, or to irregular fracture, is a 

 source of weakness to the rock, and rock of great hardness may be so 

 broken as to offer relatively little resistance. Inequalities of resistance, 

 whatever their cause, give origin to inequalities of coastal configuration 

 where wave-erosion is in progress. Given a coast of marked regularity 

 and equal exposure, but composed of unequally resistant material, the 

 waves will make it irregular by cutting most where the material is least 

 resistant. A regular coast of uniform material, but unequal exposure, 

 will be made irregular by the greater cutting at the points of greater 

 exposure. A coast of marked irregularity and homogeneous material 

 will be made more regular by the cutting off of the projecting points, 

 because they are most exposed. With a given set of conditions, waves 

 tend to develop a certain sort of shore-line which, so far as its horizontal 

 form is concerned, is relatively stable. Such a shore-line may be said to 

 be mature ^ so far as wave-erosion is concerned. Since coastal lands are, 

 in general, both heterogeneous and unequally exposed, a mature coast- 

 line is somewhat irregular. Its maturity is attained when the lesser 

 exposure in the reentrants developed in the less resistant parts, balances 

 the superior exposure of the projections of the more resistant portions. 



Since the conditions of erosion along coasts are constantly, even if 

 slowly, changing, maturity is constantly being approached, but rarely 



* Gulliver, Shore Line Topography: Proc. Am. Acad. Arts and Sci., Vol. XXXIV, 

 1899, pp. 151-258. A valuable study of shore-line topography. 



