CONTINENTAL TERRACES AND SUBMARINE VALLEYS IO3 



about 200 feet. After the flood season the 'longshore current 

 erodes the bar and especially at its eastern end. At that end 

 the depth of water on the bar is correspondingly increased. 

 With the rush of the next flood of the river, the following year, 

 the Mississippi finds its way least obstructed at the eastern end 

 of the bar and therefore extends the channel to the left of the 

 river's axis. Through thousands of years the step-by-step diver- 

 sion has proceeded, the river more or less steadily turning 

 eastward, that is, toward the direction from which the marine 

 current comes. 



Deflection of the river's axis by the marine current is pos- 

 sible because the Gulf of Mexico is not affected by strong tides. 

 Manifestly the complex, reversing currents of the tidal sort 

 would tend to annul the systematic effect of the wind-driven 

 current on the ground-plan of the delta. For this reason strong 

 asymmetry for outgrowing deltas should not be expected in 

 the case of rivers that empty directly into the open ocean, 

 where tidal disturbances are great. However, we should look 

 for parallels to the Mississippi deflection in other nearly tideless 

 seas. The Mediterranean of Europe furnishes examples. There 

 the Ebro delta, like that of the Rhone River, has grown toward 

 the river's left because a dominant marine current runs from 

 left to right. See the arrows in Figure 55. 



As shown by soundings, the asymmetry of the dry-land part 

 of each of the three deltas matches an asymmetry in the river- 

 made embankment covered by the sea. The forward growth of 

 the continental terraces is correspondingly uneven. 



And there is a second way in which the shore contour of a 

 continent reflects the hidden process of shelf-building. Its prin- 

 ciple is illustrated along our eastern coast, from Virginia to 

 Florida. Since school-days we have been familiar with the 

 exquisitely molded shapes of the sandy forelands at Cape Hat- 

 teras, Cape Lookout, Cape Fear, and Cape Canaveral. Each of 



