SECT. 2] TOPOGRAPHY OF THE DEEP-SEA FLOOR 235 



records was exciting and absolutely new, it soon became obvious that much 

 critical detail was being lost due to the small size of the record. In addition, 

 precision of timing was not guaranteed and large errors in depth were common. 



Between 1946 and 1952 several oceanographic institutions modified sounders 

 to give larger records and attached constant frequency units. Suitable for 

 more advanced studies of physiography than previously possible, these 

 makesliift models were still inadequate. 



The Precision Depth Recorder and the Precision Graphic Recorder de- 

 veloped in 1953 and 1954 now achieve an accuracy better than 1 fm in 3000 

 when operated at recording speeds of 18 inches = 1 sec (400 fm)s. ' '>"-ic.. 



In summary, then, the systematic study of the physiography of the deep-sea 

 floor was not made possible until the continuously recording echo-sounder was 

 put into widespread use following World War II ; and it has been possible to 

 study the smaller details of topography only since the development of the first 

 precision sounder in 1953 (Luskin d al., 1954). 



The study of the geomorphology of the deep-sea floor is in a very exciting 

 stage. So few areas have been sounded adequately that it is impossible to make 

 a cruise with a precision sounder and not discover entirely new physiographic 

 features. The next decade should be one of the most exciting in geomorphology, 

 for in this period enough data will have been coUected to produce a fairly com- 

 jDrehensive geomorphology of the world oceans. 



Although it is the genetic side of physiography that separates it from "topo- 

 graphy" and lends it an absorbing interest, physiographic classifications need 

 not be purely genetic ones. Indeed, the state of knowledge of sea-floor topo- 

 graphy rather prohibits the use of an entirely genetic classification. Thus, the 

 general classification used in this chapter is largely a descriptive one. 



In writing this chapter, the authors have drawn heavily upon papers written 

 in collaboration with their co-workers. Particular reference in this regard should 

 be made to the following papers: Heezen, Tharp and Ewing (1959), Menard 

 (1959), Menard and Dietz (1951), Dietz and Menard (1953), Menard (1955), 

 Heezen, Ericson and Ewing (1954), and Ewing and Heezen (1955). 



2. Outline of Submarine Topography 



The relief of the Earth lies at two dominant levels : one, within a few hundred 

 meters of sea-level, represents the normal surface of the continental blocks ; the 

 other, between 4000 and 5000 m below sea-level, comprising over 50% of 

 the Earth's surface, represents the deep-sea floor. The topographic provinces 

 beneath the sea can be included under three major morphologic divisions: 

 continental margin, ocean-basin floor, and mid-oceanic ridge. These are 

 indicated on a typical trans-oceanic profile taken from the North Atlantic in 

 Fig. 2. Each of these major divisions can be further divided into categories of 

 provinces and the categories divided into individual physiographic provinces 

 (Fig. 3). 



