HYDROGRAPHY 67 



ROCKS 



In a general way, rocks are classified as bare rocks, rocks awash, and sunken rocks. Bare 

 rocks are those extending above the plane of Mean High Water ; rocks awash are those exposed 

 at any stage of the tide between Mean High Water and the chart datum; sunken rocks are 

 those covered at the chart datum. 



For cartographic purposes, a more specific classification has been adopted. This classifi- 

 cation is different, as between the Atlantic and GuK Coasts on one hand, and the Pacific and 

 Alaska Coasts on the other. For a graphic representation see Page 68. 



Atlantic Coast 



Bare rocks (0-1) — extending more than 1 foot above Mean High Water. 



Rocks awash (0-2, 3) — having their peaks between 1 foot above Mean Low Water 



and 1 foot above Mean High Water. 

 Rocks awash at chart datum only (0-3) — having their peaks between 1 foot below 



and less than 1 foot above Mean Low Water. 

 Sunken rocks (0-5a, 26) — covered more than 1 foot at Mean Low Water. 

 Pacific Coast 



Bare rocks — extending more than 2 feet above Mean High Water. 



Rocks awash — having their peaks between 2 feet above Mean Lower Low Water and 



2 feet above Mean High Water. 

 Rocks awash at chart datum only — having their peaks between 2 feet below and less 



than 2 feet above Mean Lower Low Water. 

 Sunken rocks — covered more than 2 feet at Mean Lower Low Water. 



Page 69 shows rocks as they are shown on survey sheets and the equivalent symbols and 

 notes to be used on charts. 



In using the rock symbols, the cartographer should consider the character of the area, 

 whether exposed or protected; the proximity to shore; the range of tide; and the probable 

 visibility of the rock at some stage of the tide. 



Special care should be used in charting dangerous rocks. Isolated and dangerous rocks, 

 whether bare, awash, or sunken, are emphasized on the charts by a black dotted fine encircling 

 the symbol. It should be noted that, on some hydrographic survey sheets of this Bureau, a 

 rock symbol encircled by a dotted line indicates any rock that has been accurately located, 

 whether it is dangerous or not. 



SUNKEN ROCKS AND ROCKS AWASH (O) 



In areas that are generally foul, and where it is impracticable to get actual depths, the 

 symbols for sunken rocks and rocks awash may be used in such a manner as to give a general 

 picture of conditions prevailing in the area. The sunken-rock sj^mbol must not be used when 

 the depth on the rock is known, except on smaU-scale charts where there is no room for the 

 depth figure. 



To give prominence to dangerous offshore rocks, the complete note relating to them shall 

 be charted, thus: Uncovers 3 ft MLW (Mean Low Water). Survey notes of awash MHW 

 should be converted to sounding datum. 



The abbreviations MLW, MLLW, MHW, or MHHW shall be used. 



