322 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 60 



REGIONAL CLASSIFICATION 



The nearest and most accessible region of the sea floor is that of 

 the Continental Shelf, where the water depth seldom exceeds 100 

 fathoms. The coastal part of the shelf has been frequently photo- 

 graphed by divers and so will not be considered further here. It is 

 difficult to generalize about the nature of the bottom on the shelf, 

 since there is as much variety here as there is in the geology of the 

 land. Apart from rocky outcrops and beds of seaweed in the shallower 

 water, a lot of the shelf off the coast of Europe consists of sand, shells, 

 gravel, and mud. The presence of abundant mollusks and other large 

 shells and of coarse gravel is typical of shallow-water shelf deposits. 

 Frequently, one finds the sand patterned by ripplemarks, the product 

 of the strong tidal streams and currents around the continents. 



As we proceed toward the deep ocean, we come to the continental 

 slope which begins quite suddenly, the depth increasing to 2,000 fath- 

 oms or more in 20 miles. The slopes are often cut with deep canyons 

 carrying the shallow-water sediments out to the deep sea. Apart from 

 these canyons, the slope is a region of deposition of the fine silt 

 and clay material that has been carried out from the coast. Photo- 

 graphs show a smooth, featureless bottom suggesting fairly rapid 

 sedimentation. 



In deep water, the bottom of the great ocean basins can be divided 

 phj^siographically into several regions. Apart from seamounts, ocean 

 trenches, and mountain ranges, the bottom is more or less completely 

 covered with a blanket of sediment many hundreds of feet thick. Some 

 areas reflect the buried topography in an midulating bottom, and 

 others have an extraordinary flatness associated with sedimentation 

 by turbidity currents. These are currents of water containing sedi- 

 ment in turbulent suspensions that can flow along gentle gradients 

 on the sea floor for many hundreds of miles. At close quarters, how- 

 ever, both the abyssal plains and the undulating bottom are very 

 similar. The ooze is soft enough for extensive reworking by bottom- 

 living organisms which leave tracks, burrows, mounds, and other evi- 

 dence of their existence. In some cases there are indications of the 

 originator of these features, such as the impression of a five-armed 

 star on the top of many of the small humps, but more often one is 

 left to guess when and how they were made. Occasionally, one is 

 lucky enough to see a sea cucumber or a brittlestar crawling along 

 the bottom, or some tube worms or a sponge sticking out of the ooze, 

 but the density of life in abyssal depths is very low. Typical photo- 

 graphs of this region are shown in plate 2, figures 1 and 2, and plate 

 7, figure 1. 



The great mid-Atlantic ridge divides the Atlantic Ocean into east- 

 ern and western basins. This vast chain of mountains has an obscure 



