NO. 3 dunkle: plant ecology, channel islands 271 



recent time. The smaller an island the greater is the amount of shore 

 line in proportion to surface area. The evidence of this wave attack is 

 strikingly evident upon all of the islands and typically so on Santa 

 Barbara. The height and slope angle of the bluffs have been largely 

 determined by their relative exposure to vs^ave and surf action. A long, 

 powerful swell with recurring periods of intensification, probably caused 

 by distant, tropical storms, comes from the south. The prevailing west 

 to northwest winds cause shorter waves to pound almost continuously 

 upon exposed flanks of the island. An occasional northeaster sets up 

 violent onslaught upon the northeastern coast. 



This wave erosion is particularly active because of the very narrow 

 beaches. Since the island is essentially a mountain ridge rising from a 

 submerged base, narrow beaches might well be expected. This condition 

 has been greatly intensified by the rising level of the ocean since the 

 Wisconsin stage of the Quarternary. It has been estimated that the 

 melting of the last glacial stage has caused, for some 25,000 years, the 

 gradual rise of the ocean level to a total of at least 76 meters (250 

 feet), (Schuchert and Dunbar, 1937). Disregarding any epeirogenic 

 activity, this must have meant a more or less continuous submergence 

 of detrital beaches, which would facilitate renewed wave attacks upon 

 the main land mass of the island. That such a condition has actually 

 existed has been shown by study of the ocean bottom about Santa Cata- 

 lina (Shepard and Wrath, 1937). 



This condition of rapid shore erosion occasions recurrent slides 

 which set up xeroseres in the plant life of the bluffs. Xeroseres may be 

 found in various stages at different localities, being more noticeable 

 upon the islands with softer or more disintegrated rock layers than on 

 Santa Barbara. All successional stages do not normally occur, since 

 the bare areas are usually first invaded by xeric rock-crevice plants which 

 form on the sea-bluffs a community of many facies. On benches where 

 soil has accumulated, many shrubs, forbs, and grasses of the insular 

 uplands establish themselves. In this rapidly changing environment plants 

 might be expected to undergo considerable modification. That they do 

 so is indicated by the large number of insular endemics to be found in 

 such locations. 



Soils 



The soils appear to have been derived almost entirely from the 

 volcanic rocks. They seem to be deepest on the upper part of the main 

 terraces where they have been formed from the material transported 



