Page 303 hydrography 3843 



such areas must be sufficient to show tlie character of the bottom throughout the entire 

 area, inchiding a demarcation of the hmits where one general type of bottom changes 

 to another. In depths suitable for anchoring, a knowledge of the consistency of the 

 bottom is more useful than the descriptive characteristics or the color of the bottom 

 material. 



In offshore areas surveyed by echo sounding, controlled otherwise than by visual 

 fixes, as many well-distributed bottom characteristics shall be determined as practi- 

 cable without additional cost or additional stops during actual sounding. Bottom 

 characteristics shall be determined whenever the vessel stops for any reason whatsoever 

 within the project area if the information can be obtained during the period of time the 

 vessel is to be stopped. Bottom characteristic determinations shall be made in con- 

 jimction with each serial temperature and each vertical cast. They shall be made at 

 each anchorage of the vessel on the working ground and at the position of each survey 

 buoy. Such bottom characteristics shall be recorded in the Sounding Record, accom- 

 panied by the position data and by a short statement of the vessel's activities at the time. 

 If a fixed position is impracticable, dead-reckoning data from a known position should 

 be given. 



An attempt shall be made to cover the area as uniformly as practicable with bottom 

 characteristic determinations, as outlined above, and the determinations which are 

 made at times other than during sounding shall be indexed on page 2 of volume 1 of the 

 Sounding Records. 



During echo sounding, bottom specimens can frequently be obtained from a vessel 

 underway bj^ the use of a power-driven sounding machine and stranded wire where the 

 depths do not exceed 50 fathoms. 



To ensure a determination of the bottom characteristic at each stop in an echo- 

 sounding survey, a snapper or other adequate bottom sampler should be used if there 

 is the slightest evidence that the bottom material is such that it may be washed from 

 the sounding lead. 



In submarine valleys and canyons and other unusual submarine relief of interest 

 to scientists as well as mariners, a special effort shall be made to determine bottom 

 characteristics more frequently than would be considered necessary in other areas of 

 similar depth. 



3843. Classification of Bottom Characteristics 



Descriptions of bottom types are of most value to the mariner and the scientist if 

 they follow a standard classification and utilize standard abbreviations. The latter 

 are furnished in part " S" of the Symbols and Abbreviations chart (see fig. 189) . Descrip- 

 tive terms needed, which are not included in the chart, should be written out in full. 



A complete description of a bottom characteristic consists of three parts: One or 

 more adjectives descriptive as to size or consistency; one or more adjectives designat- 

 ing color; and one or more nouns naming the class of bottom material, the abbreviations 

 of which are always capitalized. These are to be arranged in the following order: 

 Descriptive adjective, color, noun. (See also 783.) 



3844. Types of Bottom Material 



The marine deposits commonly encountered by hydrographers in coastal surveys 

 are sediments composed chiefly of terrigenous material with which may be mixed one 

 or two other constituents. The several constituents may be briefly described as follows: 



a. Terrigenous material.- — These are derived from erosion of the land, and consist of rock fragments 

 and mineral particles from the size of boulders down to colloidal dimensions. Terrigenous material 



