F-25 



as great as 170 m would have turned the canyon areas into 

 near coastal estuaries, thus allowing sediment to be intro- 

 duced from numerous local sources around the estuarine shore. 

 The dendritic pattern is interpreted as a morphologic develop- 

 ment of landward erosion towards numerous sources all along 

 the canyon rim, rather than focused into the modern canyon 

 head. 



The sea-level rise accompanying glacial retreat should 

 have caused the canyon rim sources to disappear. Initially 

 the canyon head would have been fed by streams eminating 

 from glacial moraines surrounding glacial lakes. Such 

 leveed channels can be recognized in the shallow subsurface 

 seismic profiles of Knott and Hoskins (1968) and Knebel et 

 al. (1979) , although they have been buried in time. The 

 present source of material to the canyon head includes migra- 

 ting sheet sands transported in fraction by oceanic circu- 

 lation on the drowned shelf. 



We have encountered relict gravel and cobble pavements 

 along the Georges Bank canyon rims and active sand flows in 

 the canyon head. The modern canyons are essentially funneling 

 systems and the dominant bypass route is the thalweg floor. 

 In Oceanographer and Lydonia Canyons the funneling is suffi- 

 ciently strong to both prevent the formation of a wide 

 thalweg surface and to keep the base of the canyon wall 

 actively scoured and undercut. Polished rock surfaces are 

 commonplace as are overhanging ledges of indurated sediments. 

 The 1977 diving program with ALVIN (Ryan et al., 1978) re- 



