475 
There are many canyons off the East Coast (Figure 15). They differ from 
those of the west in that they head many miles out from shore, near the 
outer edge of the continental shelf. There are numerous canyons off 
Georges Bank and they occur in an almost uninterrupted series to Norfolk 
Canyon off Chesapeake Bay. 
Off the western side of the Mississippi Delta a troughlike valley is found 
which qualifies as a special type of canyon. 
Other Submarine Canyons. At the mouth of the Congo there is an es- 
tuary with a canyonlike valley extending into the estuary for about 25km. 
It accommodates the enormous amount of sediment transported by the Congo. 
The canyon is the longest in the world, extending some 250km from the es- 
tuary to a large outer fan. Turbidity flows occur in the canyon about 50 
times per year (Heezen et al., 1957). 
There are troughlike valleys outside the deltas of the Ganges, Indus, and 
the Niger. 
CREATION OF OFFSHORE ISLANDS TO SEQUESTER VERY CONTAMINATED MATERIAL 
Description of the Technique 
In many places of the world there will be environmental advantages 
connected with the construction of artificial islands in selected offshore 
waters. One such advantage would be for the islands to serve as barriers 
against wave actions that produce serious erosion of beaches. An equally 
important use in the present context involves designing the centers of 
these islands as receptacles for extremely polluted dredged material. This 
process would entail creating one or more borrow-type pits into which the 
polluted material would be dumped and then sequestered by a cover of clean 
material. Accordingly, this is an extension of the capping process whereby 
the island's shell is constructed of clean material, whereas the central 
pit is filled with the pollutants and eventually capped. The cover can be 
left as open sand or planted to appropriate vegetation. 
