GULF OF MEXICO 



59 



for shoreline work. This supposition is justified 

 by (1) the consideration that erosion at the shore- 

 line has a vertical as well as a horizontal com- 

 ponent, (2) comparison of variations in the form 

 and offshore gradients of the bottom of the 

 continental shelf on various sectors of tlie Gulf, 

 and (3) inspection of the charts of resultant winds 

 along: the shorelines of the Gulf (U. S. Weather 

 Bureau, 1938). These factors indicate that it 

 may be feasible, from the partly quantitative, 

 partly qualitative data presented or referred to 

 here, to set up a preliminary energy classification 

 of the coasts and shorelines of the Gulf of Mexico. 

 This is attempted in the tabulation. 



Extensive Marine Modification of Coasts of 

 Gulf. — A summarj' of prominent shoreline condi- 

 tions that indicate the degree of coastal modifica- 

 tion is shown in tabular form below. The simpli- 

 fied coasts (the secondary or mature coasts of 

 Shepard) greatly dominate in linear distribution, 

 indicating that the sea has been at about the same 

 level for a substantial period of time in relation to 

 the resistance of most of the coastal materials to 

 shoreline modification. 



Percenfage 

 Approximate of marine 

 length in shoreline 



/-, ,r J . . statute miles length 



(juIi and major parts: 



Marine shoreline 3,000 100 



Coastal plains 2,500 *3l 



Volcanic and other sectors 500 17J 



Secondary shorelines: 



Simplified (smooth) 2,250 75] 



Moderately smooth 250 SflOO 



Little modified 500 17] 



Sandy beach 1,553 52 



Barrier islands and bay bar- 

 riers 1,370 46 



Inactive and elevated beach 



plains 810-1- 27 + 



Beach ridges, average of 10 (?) 



ridges per beach 20,000+ 667 + 



EQUILIBRIUM PROFILE OF CONTINENTAL SHELF 

 BOTTOM 



Definitions. — Figure 15 shows bottom profiles for 

 sectors of the continental shelf having different 

 steepness of curvature. Only for the broad shelves 

 off the alluvial and limestone plateau coasts (fig. 

 14, Sectors 1 and 2) of the Gulf of Mexico are there 

 enough data for analysis. On the shelf sectors 

 studied, the profile of the bottom is concave in the 

 first mile or two, this section being the shoreface, 

 an extension of the beach or other shore. The 



shoreface grades into a nearly smooth plain, here 

 called the ramp, the gradient of which flattens 

 slowly in an offshore direction for varj'ing dis- 

 tances, commonly to 30 fathoms or more. The 

 profiles drawn on this section of the shelf are math- 

 ematically of the hyperbolic or asymptotic type, 

 the so-called logarithmic or exponential curves. 



The ramp grades, usually far offshore, into a 

 usually smooth convex section, here called the 

 "camber," the gradient of which usually increases 

 rapidly to the top of the irregular, steep, conti- 

 nental shelf slope. The sparse soundings available 

 for the shelf of the young orogenic coast of Mexico 

 (3, fig. 14), suggest that, except where a beach or 

 barrier is present, this coast may lack a ramp, the 

 camber beginning at or near the base of whatever 

 shore cliff or shoreface is present. The so-called 

 shelf break (Dietz and Menard, 1951) should be 

 the junction between ramp and camber. 



Data showing the locations and ramp slopes of 

 the profiles (curves) of figure 15 and the sectors 

 on which the curves are located are given in a 

 tabulation following the illustration. 



Location of profiles in figure 15. — All profiles 

 measured perpendicular to shoreline from naviga- 

 tion charts U. S. Coast and Geodetic Survey. 



(1) Off old Corpus Christi Pass and Padre 

 Island barrier island 27°35' N. Lat., 97°13' W. 

 Long. Chart 1286, 1922 edition. A profile at 

 same place from original survey sheet (1880) 

 shows only minor irregularities and smoothly 

 asymptotic curvature to 90-foot depth. Beach. 

 Sand and clay bottom. 



(2) Off Padre Island at Baffin Bay mouth, 

 27°I8' and 97°20'. Chart 1286, beach sector: 

 "Little Shell." Beach. Sand and clay bottom. 



(3) Off Matagorda Peninsula barrier island, off 

 mouth Trespalacious Bay, 28°00', 96°10'. Chart 

 1284, 1945 edition. Beach. Sand and clay bottom. 

 Fathogram off Galveston shows ramp as smooth 

 as curves 1-3. 



(4) Off barrier island on Florida peninsula 10 

 miles north of Cape Romano, 26°03' and 81°48'. 

 Chart 1254, 1931 edition. Beach. Sand inshore. 

 Rock bottom (limestone) with some sand and 

 shells. 



(5) Off Pine Islands-Key West shoals (Miami 

 oolite with mangrove swamp deposits above), 

 Florida, at Johnson Keys, 24°42', 81°36'. Chart 

 1251, 1940 edition. Profile begins at -8 feet. 

 Add 8 to all depths for this curve in figure 15. 



