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soon as the slope begins to level off, and so the speed of the current 

 decreases. At the bottom of the continental slope the current adds 

 the remains of its deposit to those of earlier currents ; so we find 

 deep-sea deltas in the shape of shallow, conical fans leading up to 

 the bottom of the canyons. Often these fans are crossed with small 

 channels, rather like river beds, that have been formed by smaller 

 turbidity currents. 



A turbidity current has a remarkable capacity for retaining its 

 identity and flowing hundreds of miles over very gently sloping 

 bottoms. It will not completely die until it has reached the greatest 

 depths of the basin that confines it, and here finally with nowhere 

 to go it comes to rest and the remaining sediments settle out. It is 

 by this means that the bottoms of basins adjoining the continent 

 have become filled with sand and silt from the land and that the 

 amazingly flat floors of the vast abyssal plains have been formed. 

 We can travel for hundreds of miles over the abyssal plains and 

 find slopes no greater than one in several thousand. 



Turbidity currents are not alone responsible for the sediments 

 covering the deep-ocean floor. These currents flow only along the 

 paths of greatest depth, and then only spasmodically. A constant 

 rain of sediments showers down from the great water canopy high 

 above the floor of the sea. In many areas this rain comes primarily 

 from the minute shells of plankton that live and die in the sunlit 

 waters near the surface. Century after century the shells drift down 

 over all parts of the ocean floor and lodge in the cracks and crevices 

 of the bottom. Because these shells are sometimes pure calcium 

 carbonate, sometimes pure silica, the sediments may be the white 

 chalky oozes we find in temperate regions, or the siliceous oozes of 

 colder waters. 



Along with these organic remains there are very fine clay parti- 

 cles that rain down on the sea floor. They are washed into the sea 

 by erosion of the coasts and by the thousands of rivers that termi- 

 nate in the sea. These minute particles sink so slowly that they may 

 travel hundreds of miles before finally settling to the bottom. 

 There are other contributors to the deep-ocean sediments: wind- 

 blown sands and dust from the deserts, volcanic ash thrown high 

 into the atmosphere, and minute particles known as cosmic spher- 

 ules that come from the melting of meteorites as they enter the 

 atmosphere from outer space. 



We might expect the sediments covering the abyssal floor to be 

 perfectly smooth, like a field of new-fallen snow. Undersea photog- 

 raphy shows that the surface of the deep-ocean sediments is far 

 from being smooth. It is disturbed by animals that live in the inky 

 darkness of the bottom — sea cucumbers, sea urchins, starfish, and 

 worms, all of which- produce an array of mounds and burrows, 

 tracks and subsurface galleries that churn up the top few inches. 



We believe from seismic measurements that the sediment carpet 

 of the sea floor is nearly half a mile thick, the result of millions of 

 years of slow accumulation, an inch or so every three thousand 

 years. But even this slow rate, assuming that it remained constant 

 throughout the history of the Earth, would produce much more 

 sediment than we have found; so we have to ask : What has happen- 

 ed to the older sediments? Have they been transformed into the 

 solid rocks that Me farther down in the crust? What, in fact, are the 

 deeper rocks? The only satisfactory way to answer these questions 



- 48° 



^ 



t 



12° 



This map of the Bay of Biscay shows in relief 

 coloring typical features found on the sea 

 floor - abyssal plains, seamounts, valleys, 

 and continental slopes. All of the 

 features shown here are based on the most 

 recent detailed soundings made by Britain's 

 National Institute of Oceanography. 



196 



