F-8 

 butaries are dusted with a thin (1 - 8 m) drape of acousti- 

 cally homogeneous sediment. The photo sled observations 

 showed that most of the tributary walls do not have out- 

 cropping strata in contrast to the abundant outcrops along 

 deeper reaches of the thalweg walls. We believe that much 

 of the dendritic drainage network is relict, and thus in- 

 active at present. We also note that the numerous cross- 

 canyon profiles (Fig. F-2) show a steepening of the lower 

 canyon walls when compared to the upper canyon walls. 



All of the echograms obtained in our field program were 

 studied for their reflectivity character. Areas of inferred 

 outcrop or shallow subcrop were mapped on the basis of 

 reflectivity. These studies are essentially distributions 

 of various acoustic patterns that we have related to local 

 canyon bedforms. In all canyon studies, the outcrops recog- 

 nized by acoustic mapping occur along narrow sinuous belts 

 adjacent to or at a short distance from the central thalweg, 

 in some instances 50 m or more above the edge of the thalweg 

 floor. The thalweg and the outcrops are separated by gently 

 dipping sediment ramps and fans. The outcrops form cliffs 

 100-200 m in height. They are exposed along the edges of 

 all the major slope tributaries, but seldom occur along the 

 second or third order tributaries. Large talus blocks are 

 very rare in the thalweg area, although some relatively 

 fresh avalancije debris was observed on the canyon wall. 

 Our mapping indicated that the central area has experienced 

 a relatively recent phase of axial excavation which seems 



