by weight of these three constituents average approximately 90, 8 and 2 

 per cent respectively. 



Of the three types, mica, because of its pronounced platey, or 

 disk -like, shape has by far the lowest settling velocity for its size. 

 This means that it is easily maintained in suspension, and therefore re- 

 sponds readily to both advection and diffusion processes. Thus in general 

 there is a higher percentage of mica in the bottom sediments near the 

 canyon heads where the water is less turbulent, and a lower amount in the 

 bottom sediments in the surf zone where turbulence is high (see the 

 accompanying figure). There i," also a band of mica-rich sediments on the 

 shelf between depths of about 20 and 50 feet. The presence of this band 

 is probably the result of the nearshore circulation of water, which for 

 long-period waves is towards shore near the wave convergence zone and 

 offshore in the rip zones to either side. The shoreward movement of water 

 in the region of vrave convergence is indicated by the tendency for mica 

 rich sediments to extend shoreward through the surf zone at the convergence, 

 This tendency is most pronounced in the 9-11 May 1950 samples near beach 

 station "D" (see Figure), 



Source of Beach Sand in Northern Oahu 



During May, Shepard examined a number of beaches in Northern 

 Oahu for ttie Compbell Estate and made a brief study of samples collected 

 from these beaches. This reconnaissance indicated that the chief source 

 of sand for the beaches is the Foraminifera and the shells which are 

 accumulating on the shallow sea floor directly outside the beaches. This 

 conclusion is based on the following observations: 



1. The sand is almost 100 per cent calcium carbonate, showing 

 that streams draining the volcanic areas inshore from the 

 beaches have contributed a negligible proportion of the 

 sediment, 



2. The high lime content is found even in one beach where 



all of the neighboring shore rocks are volcanic, indicating 

 the failure of coastal wave erosion to supply much sand. 



3. The Foraminifera of the sands are most commonly of the type 

 living on the sand bottoms rather than those characteristic 

 of reef environments, which suggests that the erosion of the 

 reefs does not provide an important source. 



Changes in Submarine Canyon Heads 



In the last progress report (No. 11, SIO Ref. 52-32) a recent 

 deepening in a shallow landslide valley called "New Valley" at the head 

 of Scripps submarine canyon was mentioned. The nature of this deepening 

 is shown by comparing contours of the old and new survey (see figure 2). 

 It will be seen that the valleys are discontinuous and the deepening ex- 

 tremely variable along the length of any one valley. 



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