7. REPORTS AND MISCELLANEOUS 



249 



island shelf. A zone adjacent to an island 

 and extending from the low waterline to 

 the depth at which there is a marked in- 

 crease of slope to greater depth. 



island slope. A declivity from the outer 

 edge of an island shelf into greater depths. 



knoll. A seamount rising less than 500 

 fathoms from the sea floor and having a 

 pointed or rounded top. 



peak. A seamount rising more than 500 

 fathoms from the sea floor and having a 

 pointed or rounded top. 



pinnacle. Any characteristic rocky col- 

 umn which is dangerous to surface naviga- 

 tion. 



plain. A flat, gentle sloping or nearly 

 level region of the sea floor. 



plateau. A comparatively flat-topped ele- 

 vation of the sea floor greater than 60 nau- 

 tical miles across the summit and normally 

 rising more than 100 fathoms on all sides. 



province. A region composed of a group 

 of similar bathymetric features whose char- 

 acteristics are markedly in contrast with 

 surrounding areas. 



reef. Any coral elevation, or a detached 

 rocky elevation that is dangerous to surface 

 navigation and may uncover. A rocky reef 

 is detached from shore but a coral reef may 

 or may not extend from the shore. 



ridge. A long, narrow elevation of the 

 sea floor, with steep sides and more irregu- 

 lar topography than a rise. 



rise. A long, broad elevation that rises 

 gently and smoothly from the sea floor. 



saddle. A low point on a ridge or be- 

 tween seamounts. 



sea channel. A long, narrow, U-shaped or 

 V-shaped shallow depression of the sea floor, 

 usually occurring on a gently sloping plain 

 or fan. 



seamount. An elevation of the sea floor 

 having a nearly equidimensional plan less 

 than 60 nautical miles across the summit. 



shelf edge. A line along which there is 

 a marked increase of slope at the outer mar- 

 gin of a continental shelf or island shelf. 

 For charting purposes the 100-fathom depth 

 contour is nominally accepted as the shelf 



edge; the actual depth usually is less but 

 may be more. 



shoal. A detached non-coral or non-rocky 

 area which is a menace to surface naviga- 

 tion and may change in shape or shift in 

 position. 



sill. A ridge or rise separating partially 

 closed basins from one another or from the 

 adjacent sea floor. 



spur. A subordinate ridge or rise pro- 

 jecting outward from a larger feature of 

 elevation. 



terrace. A bench-like feature bordering 

 an elevation of the sea floor. A terrace does 

 not include the continental shelf or island 

 shelf but may include bench-like features on 

 the shelf. 



trench. A long, narrow depression of the 

 sea floor, having relatively steep sides. 



trough. A long depression of the sea 

 floor, having relatively gentle slopes, nor- 

 mally wider and shallower than a trench. 

 To distinguish between trench and trough, 

 it is useful to have a limiting characteristic 

 such as total relief. It is suggested that a 

 feature with a total relief of less than 500 

 fathoms be called a trough while one with 

 greater total relief be called a Trench. 



valley. A relatively shallow, wide depres- 

 sion with gentle slopes the bottom of which 

 grades continuously downward. This term is 

 used for features that do not have canyon 

 proportions in any significant part of their 

 extent. 



7-18 Reports on landmarks. — The report 

 on landmarks shall ordinarily consist of three 

 parts: (a) landmarks to be charted; (b) 

 charted landmarks to be deleted ; (c) fixed 

 aids to navigation. 



The hydrographer shall examine the charts 

 of the area to determine which charted land- 

 marks are adequate for the purpose intended 

 and to discover which charted landmarks 

 no longer exist. If there are more conspicuous 

 objects in the area which should be charted, 

 their position shall be determined and listed 

 as in (a) above. 



Reports on landmarks shall, if possible, be 

 complete in themselves in order that further 

 reference to triangulation or photogram- 



