CHANNEL ISLAND REGION 315 



the courtesy of the Army Engineers, corroborates the presence of the 

 terraces. However, the terraces appear to be more highly dissected than 

 those on the other islands, suggesting the possibility of their having 

 been cut at an earlier date. On the other hand, this appearance may be 

 the result of their being less well developed because of the presence of 

 more resistant rock, as suggested by Smith. It is the opinion of the 

 present writer that the latter is the case. 



Another striking feature of the topography of the region is the 

 submarine canyons mentioned above and discussed by so many writers. 

 Shepard has probably done more actual research on the canyons than any 

 other person, and it is his opinion that they are of fluviatile origin, cut 

 under subaerial conditions when the landmass was emergent (Shepard, 

 in Shepard and Emery, 1941, pp. 109-158, and Shepard, 1948, pp. 207- 

 250). A number of other writers, of whom Crowell (1952) is the most 

 recent, have tried to explain the canyons by submarine erosion of one 

 type or another, calling particularly on turbidity currents as the agent. 



The chief obstacle to the acceptance of fluviatile origin seems to be 

 the amount of emergence or lowering of sea level that would have been 

 required for the canyon cutting, in some cases (as the Monterey Bay 

 canyon) amounting to several thousand feet. And yet geologists ap- 

 parently accept without question the Pleistocene age of wave-cut, marine 

 terraces as high as 1,325 feet above present sea level (Woodring, 1935). 



The principal reason for this anomaly probably is that the commonly 

 accepted figure for the lowering of sea level during the most extensive 

 glaciation is from 350 to 400 feet (Flint, 1947, p. 427). This figure 

 has been arrived at by calculations based on assumptions of the thickness, 

 areal extent, and contemporaneity of the ice sheets, as well as on the fur- 

 ther assumption that sea floor and continental platforms remained rela- 

 tively quiet during the time the ice sheets were at their maximum. The 

 figure obviously could be in error by several hundred per cent ; neverthe- 

 less, it has influenced the thinking of a great many geologists. 



Shepard's earlier concept was that the submarine canyons were cut 

 during the Pleistocene when sea level was lowered or the land uplifted 

 (or both) by a total of 2,000 to 3,000 feet or perhaps more {op. cit.). 

 More recently he has modified his original view (Shepard, 1952), and 

 now concedes that the deeper parts may have been cut by processes of 

 subaerial erosion during a time of emergence earlier than the Pleistocene, 

 and kept open by turbidity currents, and that only the upper parts were 

 cut as the result of Pleistocene lowering of sea level. Even this would 

 require more than the accepted 350 to 400 feet of lowering, and he 



