Submarine Canyons 



43 



cross almost the entire width of the main- 

 land shelf. Five others cross more than the 

 outer third of the shelf. The remainder (all 

 except San Pedro Sea Valley are unnamed 

 and poorly known) make only small inden- 

 tations at the edge of the shelves. Even 

 though not all the canyons cross the shelves, 

 there is no reason to believe that they did 

 not formerly cross them. For example, jet 

 drilling by oil companies has shown the 

 presence of a narrow sediment-filled valley 

 extending across the shelf from shore to the 

 head of San Pedro Sea Valley. This is sim- 

 ilar to the filled channel that extends from 

 the delta of the Mississippi River to the head 

 of a large submarine trough (Carsey, 1950), 

 and it also is similar to the partly filled Hud- 

 son Channel which crosses the continental 

 shelf between the mouth of the Hudson 

 River and the head of the large Hudson 

 Canyon (Veatch and Smith, 1939, Chart V). 

 In addition, records of wells on land have 

 shown the presence of sediment-filled chan- 

 nels reaching the shore at points inshore of 

 Hueneme Canyon, Santa Monica Canyon, 

 Redondo Canyon, San Pedro Sea Valley, 

 Newport Canyon, and Coronado Canyon 

 (Poland and colleagues, 1945, 1948, 1956; 

 E. J. Zielbauer and R. G. Thomas, personal 

 communications), counterparts of features 

 in New England and Australia (Flint, 1947, 

 p. 447; Hack, 1957). Since the channels 

 were cut 123 to 255 feet below present sea 

 level, they doubtlessly extend seaward of 

 the shore and may connect with the canyons. 

 Similar channels occur at Oceanside and 

 Goleta, where no canyons are known to cut 

 the shelf; however, contours of the lower 

 part of the trough slope off" Oceanside are 

 indented as though by a largely filled 

 canyon, and the slope off" Goleta is so gentle 

 that a canyon could formerly have existed 

 there and now be completely buried. Even 

 where the canyons are open across most of 

 the shelf, partial and at least temporary 

 filling by sediments is shown by photo- 

 graphs (Fig. 44) and direct examination 

 (Fig. 45) (Shepard, 1949; McAllister, 1957) 

 of the bottom in Scripps and La Jolla 

 Canyons and by observations of sliding of 

 sediment at the heads of La Jolla, Scripps, 



Newport, Redondo, and possibly Mugu 

 Canyons (Shepard and Emery, 1941, pp. 94- 

 103; Shepard, 1951a). Similar changes may 

 occur at the heads of canyons located farther 

 from shore, but the required precise naviga- 

 tion is impossible or at least impractical far 

 from shore. 



Canyons which reach close to shore have 

 been found to have numerous tributaries, 

 four for Scripps Canyon, at least seven for 

 La Jolla Canyon, and four for Hueneme 

 Canyon. Canyons such as Coronado, San 

 Pedro, and Santa Monica, which head far 

 from shore, are mapped on Chart I and else- 

 where as having simple heads. The simpli- 

 fication is almost certainly a result of the 

 difficulty and cost of obtaining accurate 

 positions for detailed surveys. To illustrate, 

 when detailed studies of the head of Santa 

 Monica Canyon were required for construc- 

 tion purposes, a survey there showed at least 

 three tributaries, although only one was 

 known before. Similar accurate surveys 

 farther along the valley walls would doubt- 

 lessly reveal the presence of many additional 

 tributaries. This question of surveying ac- 

 curacy and sounding density may have been 

 overlooked by Crowell (1952) in his discus- 

 sion of the restriction of tributaries to the 

 heads of submarine canyons. 



Profiles at the heads of the southern Cal- 

 ifornia canyons are steeper than farther 

 from shore, with mean slopes at the head, 

 middle, and end of 14.5°, 5.5°, and 4.0°, 

 respectively, according to measurements by 

 Shepard and Beard (1938). As Woodford 

 (1951) pointed out, the slope at the head is 

 far steeper than the slope at the mouth of 

 the adjoining valley on land. Such a rela- 

 tionship is reasonable, of course, in view of 

 the sediments that have filled the axis near 

 sea level, alluviating the drowned valley on 

 land and prograding into the part of the 

 valley that remains submerged. 



Some indication of the age of the canyons 

 with respect to the shelf can be provided by 

 an examination of the depth of the intersec- 

 tion of the side slope of the canyon and the 

 surface of the shelf. This feature is similar 

 to the shelf-break. If the canyons were cut 

 after the shelf had formed, the canyon shelf- 



