Smith.] 



Islands of Southern California. 



217 



their outlines. In comparing the terraces of San Clemente, San 

 Pedro Hill, and San Nicolas, their present sharpness is seen to vary 

 with the resistance to erosion of the rocks in which they have been 

 cut. The best preserved and sharpest terraces are cut in the vol- 

 canics of San Clemente, while those cut in the sandstones of San 

 Nicolas have been most modified through general subaerial erosion. 

 The rounding of terrace forms may take place in part through the 

 falling away of the upper portion of the cliff, with deposition of 

 talus at the angle of cliff and terrace, and in part through stream 

 erosion wearing away the upper portion of the cliff, and forming 

 alluvial fans at its base. 



Terraces are better preserved where they form continuous 

 benches, on little dissected surfaces, than where they are discon- 

 tinuous, forming merely a series of notches on the sharp ridges of 

 a well-dissected surface (in hard rocks). In the latter case, erosion 

 on the slopes of the ridges, by narrowing their crests, soon tends 

 to obliterate the imperfect terraces. At the outset the dissected 

 area is in a condition (as regards the preservation of the terraces) 

 similar to that of the undissected area after a considerable period of 

 subaerial erosion. 



Where, along the coast line of well-dissected areas of hard rocks, 

 with high angle of slope, the cutting has been sufficient to produce 

 continuous terraces (which will be narrower, as has been shown, 

 than those formed under similar conditions on gentler slopes), 

 these terraces, on account of the more incisive cutting due to sub- 

 aerial erosion in these resistant rocks, will soon become discontinu- 

 ous, and therefore more readily obscured. 



With cliffs of varying height or platforms of varying width, in a 

 given rock, the higher the cliff or the broader the platform, the 

 longer will the terraced character be evident. Where a narrow 

 terrace with comparatively high cliffs at its rear is developed (in 

 hard rocks), it may soon be buried by detritus from the cliff above, 

 and its character thus be lost. Thus breadth of platform is seen to 

 be more important than height of cliff to the preservation of terrace 

 character. 



All things considered, then, in a region of mature topography 

 (accidents excepted), a rock of only a moderate degree of resist- 



