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The north shore of Yucatan is smoothed by beaches of shell sand and barrier islands 

 which, however, are not well aligned, probably due to irregularities of the limestone. The 

 mapping is poor and only a few aerial photographs have been examined. 



Before I had any numerical figures for energy, I considered first, the two wind systems 

 of the Gulf, the southeast trades and the cold fronts of winter; and second, the slopes of the 

 ramps or hard-rock shelf surfaces (Fig. 15). The trades (a) blow at right angles against the 

 west coast of the Gulf, delivering their full energy, (b) blow alongshore or diagonally at the 

 north coast, giving less energy there— I call it medium energy, and (c) blow offshore at the 

 east, which I call a low-energy coast with zero energy where the hard- rock shelf is both broad 

 and very shoal. 



The cold fronts come from the north. Their winds, and the induced wave fronts, blow 

 alongshore at the east and west, and do not do much work. The northern Yucatan shoreline lies 

 across their path, and these "northers" produce high energy at the edge of the shelf and medium 

 energy at the shore. The very high energy figure for Vera Cruz shown on Fig. 13 (736 horse- 

 power-days) is due to the long fetch of the northers and the work done by cold air on a warm 

 ocean. Apparently the north Yucatan coast must have some similar figure, perhaps three- 

 fourths at least of the Vera Cruz value. This would account for that fairly high energy north- 

 facing coast. The appreciable slope of the bottom and the considerable distance off shore and 

 depth (20 fathoms) of the coral platform allow a fairly large percent of the energy ("medium 

 net energy") to reach the shore. 



On the low-energy coast, wave activity is low and breakers absent. Along this type of 

 coast on the northwest Florida peninsula the U. S. Army Engineers, much to their surprise, 

 could work fer out on the shelf in small 2-spud coring boats even in winds up to 25 or 30 miles 

 per hour. The same thing is true behind the barrier reef along the Florida Keys. There is 

 only choppy water, even with high winds. 



These relative energy concepts were applied before we had any numerical energy values. 

 Now we have the Thompson figures for the wind- wave energy of onshore winds on a deep water 

 basis. You will see that in the north half of the Gulf the values range from about 400 at the 

 west to 200 at the extreme east where the broad shallow shelf absorbs nearly all the energy of 

 the onshore wave systems. 



In the energy study there are two situations: the gross-energy values at the shelf edge 

 determined either by a relative scale, or numerically from the regional winds on the deep 

 water basis; and the net-energy values of waves that reach the shallow water zones near the 

 shoreline. The inshore net energy largely determines the individual type of modification of 

 the shoreline, but the total or gross values are involved in shaping the continental shelf. Ac- 

 tually there is a continuous energy spectrum and a connected series of bottom conditions across 

 the shelf. 



I have drawn on the maps of Figs. 13 and 15 the obstacles which stand above the ideal 

 continuous smooth profile of the ramp and camber. These obstacles absorb more energy than 

 would be absorbed by a smoothly graded shelf. On relatively flat ramp slopes there is a large 

 energy loss by bottom friction, while on steeper slopes there is only a small frictional loss. 



Off the Mississippi-Red- Brazos deltaic zone there is a great submerged deltaic plain 

 which is quite rough, rising above the ramp-camber curve. Off Yucatan a coral platform 

 standing above the drowned land surface has gross features which probably have not been very 

 much modified by the waves. The two shelf regions are basically similar, although of different 

 origin, lithology, and energy relations. Each has a ramp-camber plain encumbered by an un- 

 reduced elevation extending to the camber. 



Along the north Gulf coast (Fig. 15) the energy is low and the ramp is relatively flat be- 

 hind an obstacle. Where there is no obstacle, the energy is greater and the ramp is steeper. 



We are now ready to understand and interpret the maps of the continental shelf. Figures 

 16, 17, and 18 show the first mapping of the physical environments of the continental shelf of 

 the Gulf of Mexico that I know, other than sediment maps. The environments mapped are 



