SECT. 3] 



ORGANIC TRANSPORTATION OF MARINE SEDIMENTS 



777 



first, the holdfast can break off and hft part of the bottom and the plant be set 

 adrift. If one plant drifts against another, the combined pull of both may break 

 the second one loose, finally tearing open a wide swath through a kelp bed. 

 The anchor, buoyed by the plant, may be floated away from the kelp bed 

 (Fig. 1), occasionally touching or dragging on the bottom to form markings 

 which in geological strata sometimes have mistakenly been identified as plant 



Fig. 1. Two kelp holdfasts {Macrocystis pyrifera) afloat in 25 m depth off Coronado 

 Islands, Mexico. Largest is about 40 cm long and both consist of probable Middle 

 Miocene sandstone and conglomerate. Note that intermittent touching of the bottom 

 can produce holes and other markings on the sand. (U.S. Navj^ photograph by 

 R. F. Dill.) 



impressions. Most of the rocks are drifted inshore, as shown by the fact that 

 beaches near kelp beds usually contain many rocks having remnants of hold- 

 fasts and encrustations of calcareous red algae, bryozoans, rock oysters, and 

 other organisms which live below the low tide level (Emery and Tschudy, 1941 ), 

 Single rocks rafted to beaches by kelp weigh as much as 10 kg, and one holdfast 

 was found to contain an estimated 25,000 pebbles and granules coarser than 

 one millimeter in diameter. 



Some plants also float seaward where masses of the larger ones have been 



