GULF OF MEXICO 



41 



altered the shoreline so that it may no longer con- 

 form to a simple structural classification. Johnson 

 (1919) assembled and extended previous ideas 

 of coastal development and classification to pro- 

 duce a detailed genetic-geomorphic system that 

 has since been followed by most writers. How- 

 ever, it seems not to have been applied by its 

 users to the detailed mapping of the coasts of a 

 large, diversified region such as the Gulf of 

 Mexico, although Johnson (1925) applied it to 

 the drowned and largely discordant shoreline of 

 the New England-Acadian region of northeastern 

 North America. 



Shepard (1937a, 1948) modified and extended 

 Johnson's system, giving a tabulation in which 

 shoreline and coastal types then described were 

 inserted. His major divisions differ from John- 

 son's and seem not to have been accepted by all 

 of Johnson's followers, although the scarcity of 

 papers on the classification of shorelines indicates 

 that this may be due to inertia rather than to a 

 working appraisal of the usefulness of Shepard 's 

 revised system. Johnson's text is out of print 

 and has not been supplemented by a similarly 

 detailed work. 



Regional variations in the known physical 

 oceanographic conditions in the "inshore" zone* of 

 the coasts of the United States and Mexico were 

 discussed by R. H. Fleming and F. E. Elliott 

 (1950) in lectures. They regarded the scarcity of 

 such information too great for elaboration of their 

 method at that time. It, however, classifies coastal 

 sectors into glacial, alluvial, young orogenic and 

 biogenous types, with erosional and depositional 

 sub-types for the first three. The continental 

 coasts of the Gulf of Mexico were included in the 

 maps and discussion. The Fleming-Elliott system 

 has been modified and extended in some of its 

 aspects for use in the present study as the geo- 

 oceanographic classification system. Changes in 

 their mapping of the Gulf coasts include the intro- 

 duction here of young orogenic sectors and the 

 relegation of biogenous coasts to a secondary 

 condition imposed on a framework of regional 

 geologic and geomorphic types. In the latter 

 instance, the suggestion made by Shepard (1948, 

 pp. 78-79) is followed that a regional classification 

 could be made by using large subdivisions such as 

 coasts with young mountains, old mountain ranges, 



* Shallow water or nearshore zone. Some writers use "inshore" for la- 

 goonal and estuarine environments. 



broad coastal plains, glaciated coasts, and such 

 specific but less common items as volcanic coasts 

 and tableland coasts. 



Space does not permit including here an elabora- 

 tion of the detailed genetic-geomorphic classifica- 

 tion systems. As detailed knowledge of many 

 coasts accumulates, including coastal plains such 

 as those of the Gulf, the list of the distinctive 

 small-unit features becomes encylopedic and the 

 classification headings numerous, beyond the 

 simplicity desired (Lucke 1938) for text-book and 

 lecture purposes. 



Definitions. — The shoreline is the line where 

 land and water meet. It moves back and forth 

 over the shore or shore zone. The shore on a 

 beach has been defined (Beach Erosion Board, 

 Corps of Engineers, U. S. Army) as the zone 

 between mean low tide (or lower low tide) and the 

 inner edge of the wave-transported sand. The 

 lagoonal shore is that of the tidal bays and lagoons. 

 Estuaries are tidal stream courses. Their shores 

 are not studied here except where they are em- 

 bayed. On some coasts there are extensive, 

 muddy shore-flats. Tidal flats are properly those 

 within the range of normal gravitational tides. 

 In some places winds blow the water across broad, 

 gently sloping wind-tide flats * that extend inland 

 from the ti-ue shore, hence, beyond the high tide 

 limits for gravitational tides, and have been floored 

 by deposits left by the water. 



The coast is a zone of indefinite width back of 

 the shoreline that is affected by or closely affects 

 offshore or shoreline processes and forms. The 

 waters lying near the coast where the effect of a 

 shallow bottom is felt may be called coastal waters. 

 The continental shelf (fig. 13) is a submerged, gently 

 sloping plain that extends the continent ocean- 

 ward to varying depths ranging, generally, between 

 40 and 100 fathoms. The shelf is terminated sea- 

 ward by the steeper shelf slope that descends, in 

 places precipitously, to the depths. Additional 

 definitions will be given in later paragraphs when 

 the barrier island, the shelf and its equilibrium 

 profile, and the mangrove coastal ridge are 

 discussed. 



New and undescribed types. — New types recog- 

 nized on the shorelines of the Gulf of Mexico which 

 will be readily understood from previous geomor- 

 phological knowledge are (1) the drowned karst 

 (sub-aerial limestone solution topography) of parts 



*New term. 



