CONTINENTAL TERRACES AND SUBMARINE VALLEYS IOI 



been established during millions of years of terrace-building, 

 was so modified is a question to be answered when we consider 

 the origin of valleys sunk in the continental slopes. 



The oceanward gradient of the whole continental slope 

 down to the 1500-fathom level averages about 1 in 15 or 350 

 feet to the statute mile. It is steeper at the top; between the 

 100-fathom and 500-fathom levels the average gradient for the 

 belt off the eastern United States is nearly 1 in 10 or, say, 500 

 feet to the mile. 



Special Topographic Features. — Whether brought by rush- 

 ing river or eroded by waves from sea-cliffs, the terrace sedi- 

 ment is carried seaward by an endlessly complex system of 

 tidal and wind-driven currents in the ocean. Undertow of 

 waves and offshore motion of other currents co-operate. A 

 vast job has been done and is now under way, without pause, 

 in all the oceans. In general we land dwellers have no direct 

 evidence of the submarine construction, but there are two in- 

 stances where the dry-land topography of our own continent 

 has been spectacularly molded during the growth of the conti- 

 nental terrace. 



The first example is found in the remarkable asymmetry of 

 the Mississippi Delta. At a point near Baton Rouge the Missis- 

 sippi turns sharply from a north-south direction to run south- 

 eastwardly for the remaining two hundred miles of its course. 

 The whole stretch of two hundred miles lies on the more 

 recently formed part of the delta. Professor R. J. Russell of the 

 State University of Louisiana has summarized the results of 

 studies by himself and colleagues regarding the evolution of 

 the delta during the last 50,000 years. 1 The Mississippi has 

 changed its course over the delta several times; the last major 

 change brought the river into its present course, from Donald- 

 sonville through New Orleans. Since then the delta has grown 

 steadily toward the southeast. Thus, the river has been turning 



