44 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Figure 13. — Major geologic structures exposing uplifted rock masses surrounding Gulf of Mexico. Cross-hatched, folded 

 sedimentaries, granitic areas, volcanic belts. Stippled, uplifted arches or horsts. Stipple and dash, emerged parts 

 of arches form limestone plateaus at south and east. Under-water contours, 100 and 1,900 fathoms, the former 

 out-lining the continental shelf, the latter, the Mexican Basin (Sigsbee Deep) . Long broken lines, axes of arcuate 

 Caribbean folding (axis of Gulf coast geosyncline, supposedly along northwest shore, not yet located. Scale: Hun- 

 dreds of miles. 



REGIONAL COASTAL TYPES 



YOUNG OROGENIC COAST 



Where geologically young mountains (Tertiary 

 to Quarternary) closely border the coast (Umb- 

 grove 1947, pi. 5), as in Cuba and the south- 

 western Gulf coast in Mexico (figs. 12, 13; fig. 14, 

 Sector 3), coastal plains and the continental shelf 

 are absent, narrow or of irregular width, and the 

 shelf tends to be rocky with shoals and irregular 

 elevations (Fleming and Elliott 1950) as well 

 as somewhat steep (slope greater than about 5 



feet per statute mile). Sand and mud* occur on 

 the shelf and mud along the outer margin and in 

 shelf deeps. The coast may have alternating 

 narrower erosional and wider depositional sectors, 

 the latter with smooth shorelines and bottoms, 

 the former with uneven surfaces. These coasts 

 and shelves are unstable and subject at any time 

 to earthquakes, fracturing and warping of the crust. 



s Sediment terminology used is that of the coast charts. "Mud" is a field 

 term implying no accurate knowledge of the clay fraction. 



