8 



Physiography 



the isthmus which nearly separates the island 

 into two parts. The recent contour maps of 

 the island and examination from shipboard, 

 however, suggest the presence of highly 

 eroded terraces, mostly as ridges having 

 sharp but nearly horizontal crests, particu- 

 larly northwest of the isthmus. It is possible 

 that terraces were cut on the island, but for 

 some reason, such as greater age, are not as 

 well preserved as elsewhere; if so, Santa 

 Catalina Island may have had a history not 

 greatly different from that of the other is- 

 lands and coastal areas, a conclusion reached 

 earlier by Smith (1933). Support for simi- 

 larity of its late history is given by the pres- 

 ence of well-developed submerged terraces 

 like those found elsewhere in the region; 

 these will be described in a later section. 



Terraces of adjacent coastal sections or of 

 islands are commonly at similar elevations, 

 but correlations cannot be made reliably for 

 long distances on the basis of elevation alone, 

 owing to known variations in the thickness 

 of alluvial cover, in regional warping, and 

 in local faulting. Because all or most of the 

 terraces are of post-Pliocene age, a time too 

 short for much evolution to have occurred, 

 paleontology has proved of little or no value 

 in the correlation of terraces. Radiocarbon 

 measurements have yielded only minimum 

 ages of cutting of the youngest terraces: 

 greater than 30,000 years for the 75-foot ter- 

 race at Palos Verdes Hills (Kulp, Tryon, 

 Eckelman, and Snell, 1952), greater than 

 29,650 years for the 25-foot terrace at Santa 

 Rosa Island (Orr, 1956), and greater than 

 39,000 years for the 100-foot terrace at Santa 

 Cruz north of Point Conception (Bradley, 

 1956). 



If all the water now locked up in glaciers 

 were to be released, the sea level would rise 

 no more than about 200 feet, according to 

 Fhnt (1947, p. 437). Most of the terraces 

 thus require more than raised sea level to 

 explain them; diastrophic uplift is necessary, 

 and it is not difficult to believe this possible 

 in such an unstable region. Some of the ter- 

 races lower than about 200 feet may be cor- 

 relative in elevation with terraces elsewhere 

 in the world if the rate of Late Pleistocene 

 and Recent diastrophic movement has been 



slow compared to the rate of sea level 

 change. When more precise data are avail- 

 able, interesting comparisons might be made 

 with the terraces lower than about 200 feet 

 in Maryland (Shattuck, 1906), in Louisiana 

 (Fisk, 1939), in Florida and Georgia (Mac- 

 Neil, 1949), in the Hawaiian Islands (Stearns, 

 1935, 1945), and elsewhere in the world 

 (Charlesworth, 1957). 



Shoreline Classification 



The total length of the shoreline in south- 

 ern California is about 615 miles (990 km), 

 as measured on the recent 7y2-minute topo- 

 graphic sheets. More than half this length 

 belongs to the islands where most shorelines 

 are rocky and irregular. Along the main- 

 land there are also many rocky shorelines, 

 but straight smooth beaches are about five 

 times as common. Close behind most of the 

 sandy beaches are rocky cliffs; other beaches 

 are backed by mud flats or broad lowlands. 

 Because of the diverse nature of the shore- 

 lines, a concise summary of their character- 

 istics can be formed only by grouping the 

 various forms within the framework of gen- 

 eral shoreline classifications. Only then can 

 effective comparisons with other shorelines 

 of the world be made. The classification 

 outlined above and summarized in Table 2 is 

 descriptive but gives little information on the 

 dominant shore processes or on the past and 

 probable future form of the shorelines. 



Table 2 



Characteristics of Shorelines Facing 

 Open Ocean (statute miles) 



Sandy 



Rocky 



Backed by 

 Cliffs 



Backed by 

 Lowlands 



Mainland 

 Islands 



50 



255 



173 

 57 



80 

 0.3 



More information about shore processes 

 is given when we translate the figures of 

 Table 2 into energy characteristics — whether 

 the shoreline is dominantly of erosional or 

 depositional origin. All rocky shorelines 

 (Fig. 4) are obviously erosional. Sandy 



