F-29 



enroute to the continental rise. Such materials, if they 

 were more fine grained than the thalweg sands would winnow 

 from the sands and exit the canyon quickly in the net 

 downcanyon current transport (Keller and Shepard, 1978) . 

 One of the best analogies that we have for comparison is 

 the natural dumping of ice-rafted sediment in the Georges 

 Bank region. From all the evidence at hand, the ice-rafting 

 material resides today on the surface segregated in local 

 areas. It is mostly concentrated as gravel lag deposits 

 adjacent to the shelf edge along the canyon rim (Stanley 

 and Wear, 1978) down to depths of about 250 m, but occasion- 

 ally found within tributaries to depths in excess of 600 m. 

 The high frequencies used in side-scan sonar would be 

 expecially effective for mapping the present gravel distri- 

 bution. 



The three canyons discussed here represent different 

 depositional environments which are reflected in their 

 sedimentary characteristics. In general Oceanographer and 

 Lydonia Canyons are quite similar with considerable exposed 

 outcrop and sandy sediments. Baltimore Canyon, on the other 

 hand, has only local exposures of outcrop and more poorly 

 sorted silty sediment. The carbonate contents of Oceanographer 

 and Lydonia Canyons are also similar. Average carbonate 

 values in these canyons is consistently lower than those 

 found in Baltimore Canyon. These results indicate that the 

 Georges Bank Canyons represent a more active environment of 

 non-deposition or reduced deposition while Baltimore Canyon 

 represents an environment of active sediment deposition. 



