Source 



237 



cm. If an equal fall occurs on each of 5 

 days per year (a typical number), the an- 

 nual dust fall would be about 1.5 mg/sq 

 cm/yr. Owing to an offshore decrease in 

 wind velocity, this dust fall diminishes in a 

 seaward direction but at a rate unknown at 

 present. Assuming that the rate dropped 

 steadily from shore to zero at the conti- 

 nental slope, the contribution of eolian 

 dust would total about 0.6 million tons an- 

 nually. However, X-ray and optical studies 

 of pelagic red clays by Rex and Goldberg 

 (1958) revealed the presence of considerable 

 numbers of chips and shards of quartz be- 

 tween 1 and 20 microns in diameter; they 

 are thus coarser than the accompanying 

 clay minerals. The percentage is greatest 

 on the floor of the northeastern Pacific 

 Ocean between latitudes 25 and 45° where 

 the average is about 16 per cent quartz. 

 The similarity of this distribution with the 

 latitudes of desert areas and of the high- 

 altitude jet stream gave rise to the sugges- 

 tion that the quartz is of eolian origin. Ac- 

 companying wind-blown feldspars and other 

 minerals yield a total of about 40 per cent 

 wind-blown grains in the deep-sea sediments 

 west of southern California. At an average 

 rate of deposition of nonbiogenous sedi- 

 ment in pelagic clays of 0.073 grams/sq 

 cm/ 1000 yr (Revelle, Bramlette, Arrhenius, 

 and Goldberg, 1955), the rate of deposition 

 of eolian sediments in the deep sea is about 

 0.03 mg/sq cm/yr. Thus the estimated rate 

 near the coast of southern California is 

 about 50 times the estimated rate for the 

 deep-sea region. 



The waves striking rocky shores are so 

 spectacular during storms that there is a 

 natural tendency to consider wave erosion 

 of cliff's as providing a major proportion of 

 the sediment available for distribution on 

 the sea floor. Although a precise measure 

 of the contribution of sediments by cliff" 

 erosion cannot be provided, a rough esti- 

 mate can be formed. In the chapter on 

 physiography it was estimated that cliffs 

 averaging 25 meters in height line about 

 360 km of the mainland coast and cliffs 

 averaging 55 meters in height line 500 km 

 of island coasts. A comparison of old and 



new photographs by Shepard and Grant 

 (1947) shows that erosion of cliffs formed of 

 consolidated rocks was inappreciable dur- 

 ing past periods of 30 to 50 years. On this 

 basis we may guess that the average present- 

 day rate of cUff" retreat is about 30 cm/500 

 yr. Almost certainly it cannot have been 

 faster. At this rate about 22,000 cu meters 

 of cliff is removed in the region each year, 

 an annual average of about 0.054 million 

 tons, a rate that is only about 10 per cent of 

 that estimated for wind-contributed sedi- 

 ment. If we deal with larger units and as- 

 sume that an average thickness of 30 meters 

 was stripped from the 14,800 sq km area of 

 shelves and bank tops during the Pleistocene 

 Epoch (1 million years), an average annual 

 contribution of about 1.1 million tons is ob- 

 tained. Although this Pleistocene rate is 

 also small, it is about 20 times greater than 

 the rate computed from present-day cliff 

 erosion, a comparison that supports physi- 

 ographic data in suggesting that the present 

 rate of cliff erosion is less than that obtain- 

 ing during the past. 



Figures for the grain size and quantity of 

 sediments annuaUy contributed to the ocean 

 by streams are very unsatisfactorily known, 

 primarily because of the brevity of periods 

 of violent stream ffow and their separation 

 by very long intervals of no or only slight 

 ffow. Many analyses have been made of 

 the grain size of sediments that cover the 

 floors of dry stream beds; as summarized 

 by Table 12, the average median diameter 

 of many samples is 610 microns, or coarse 

 sand. These sediments, however, represent 

 material that was left by the stream and was 

 not contributed to the ocean. To be sure, 

 every flood moves some of it to the stream 

 mouth as a sort of plug, but the percentage 

 of this material that reaches the ocean is 

 probably very small compared to that carried 

 in suspension. For example, Gould (1953) 

 found that bed load constitutes less than 2 

 per cent of the total load carried by the 

 Colorado River. The grain size of material 

 carried in suspension by streams is much 

 less well known. ReveUe and Shepard (1939) 

 reported that the median diameters of sedi- 

 ments from aU streams (about 20) that were 



