GEOMORPHOLOGY 



a unit of magnetic field intensity, but the latter 

 quantity is now measured in oersteds. (5) 



Gaussian wave packet — An isolated group of 

 waves, with infinitely long crests in the direction 

 perpendicular to the direction of propagation 

 and with a constant wavelength in the direction 

 of propagation, of the sine wave form, which is 

 modulated mathematically according to the 

 normal probability curve (or Gaussian curve). 

 (73) 



Geiger counter — Historically, a "point counter." 

 By popular usage, a Geiger-Miiller counter 

 tube or such a tube together with its associated 

 electronic equipment. (41) 



Geiger-Mueller Counter (or tube) — See Geiger- 

 Miiller counter tube. 



Geiger-Miiller counter tube — (or Geiger-Mueller 

 tube). A gas-filled chamber usually consisting 

 of a hollow cylindrical cathode and a fine wire 

 anode along its axis. It is operated with a volt- 

 age high enough such that a discharge triggered 

 by a primary ionizing event will spread over the 

 entire anode until stopped by the reduction of 

 the field by space charge. (41 ) 



generating area — See fetch. 



generation of waves — 1. The creation of waves 

 by natural or mechanical means. (61) 



2. The creation and growth of waves caused 

 by a wind blowing over a water surface for a 

 certain period of time. The area involved is 

 called the generating area or fetch. 



genus — See classification of organisms. 



geo — An Icelandic term for a narrow inlet walled 

 in by steep clifts. (2) 



geodesic — A geodesic line. (37) 



geodesic line — A line of shortest distance between 

 any two points on any mathematically defined 

 surface. Also termed a geodesic. (37) 



geodesy — The investigation of any scientific ques- 

 tion connected with the shape and dimensions of 

 the earth. The term geodesy is often used to in- 

 clude both the science which must depend upon 

 determinations of the figure and size of the earth 

 from direct measurements made on its surface 

 (triangulation, leveling, astronomic, and gravity 

 determinations), and the art which utilizes the 

 scientific determinations in a practical way. 



geodetic datum — A datum consisting of five quan- 

 tities: the latitude and longitude of an initial 

 point, the azimuth of a line from this point, and 

 two constants necessary to define the terrestrial 

 spheroid. 



It forms the basis for the computation of 

 horizontal control surveys in which the curva- 

 ture of the earth is considered. ( 37 ) 



geodetic line — A geodesic on the reference 

 spheroid. 



geodynamic height — See dynamic height. 



geodynamic meter — See dynamic meter. 



geoid — The figure of the earth considered as a 

 mean sea level surface extended continuously 

 through the continents. The actual geoid is an 

 equipotential surface to which, at every point, 

 the plumb line (direction in which gravity 

 acts) is perpendicular. It is the geoid which is 

 obtained from observed deflections of the verti- 

 cal, and is the surface of reference for astronom- 

 ical observations and for geodetic leveling. 

 Theoretical geoids obtained with computed 

 values of deflections of the vertical include the 

 compensated geoid and the isostatic geoid. 

 (37) 



geological oceanography — The study of the floors 

 and margins of the oceans, including descrip- 

 tion of submarine relief features, chemical and 

 physical composition of bottom materials, inter- 

 action of sediments and rocks with air and sea 

 water, and action of various forms of wave 

 energy in the submarine crust of the earth. 



geologic time scale — The divisions of time, from 

 the formation of the earth to the present, into 

 eras, periods, and epochs based primarily upon 

 fossil evidence. See table on next page. 



Geomagnetic Electrokinetograph — ( abbreviated 

 and also commonly called GEK) . A shipboard 

 current measuring device used in depths greater 

 than 100 fathoms. It is dependent upon the 

 principle that an electrolyte moving through a 

 magnetic field (the earth's) will generate an 

 electric current. 



geomagnetic equator — A great circle on the 

 earth's surface that is everywhere equidistant 

 from the geomagnetic poles ; that is, the equa- 

 tor in the system of geomagnetic coordinates. 



geomagnetic pole — The point where the axis of a 

 centered dipole that most nearly duplicates the 

 earth's magnetic field would intersect the sur- 

 face of the earth. The earth's geomagnetic 

 poles are located approximately 78.5°N, 69.0° 

 Wand78.5°S,111.0°E. (36) 



geomagnetism — (or terrestrial magnetism). 1. 

 The magnetic phenomenon, collectively con- 

 sidered, exhibited by the earth and its atmos- 

 phere. 



2. The study of the magnetic field of the 

 earth. 



(5) 



geometric shadow — In wave diffraction theory, 

 the area outlined by drawing straight lines par- 

 alleling the direction of wave approach through 

 the extremities of a coastal protective structure. 

 It differs from the actual protected area to the 

 extent that the diffraction and refraction effects 

 modify the wave pattern. (61) 



geomorphology — That branch of both geography 

 and geology which deals with the form of the 

 earth, the general configuration of its surface, 

 and the changes that take place in the evolution 

 of land forms. 



69 



