The subbottom (or Sb horizon as shown on the records in this report) is defined 

 for the purpose of interpreting the Sonoprobe records as any laterally continuous 

 reflecting horizon below the bottom, except bedrock or coarse glacial material . 

 This definition causes no ambiguity in the Penobscot Bay area since only one such 

 reflection was found beneath the bottom, and it is physically and genetically related 

 throughout the area. Cores 136, 137, 201, and 211, although not precisely defining 

 the nature of this horizon, do indicate that at the depth of this horizon the sediments 

 are considerably coarser than those above. 



2 . Sonoprobe Runs 



a . Run I 



Run I (Figs. 4, 5, and 6), starting near Searsport and extending through the 

 eastern portion 'of the bay, illustrates nearly all of the bottom and subbottom features 

 found throughout the surveyed area. From marks A to D, the bedrock trace is very 

 distinct and shows characteristic reflections and topography. In this area, the sub- 

 bottom reflecting horizon and the bottom are essentially flat. The subbottom horizon 

 truncates against the higher peaks of bedrock at depths of about 80 feet . The top of 

 the sediments represented by this horizon are probably the late glacial clays 

 described in the geology of the land areas. At this horizon, poorly sorted sands 

 and pebbly sands were sampled, but since there was considerable penetration until 

 bedrock was encountered, it is reasonable to assume that the sediments below the 

 subbottom reflecting surface decrease in coarseness with depth. The sediments above 

 the subbottom horizon are silty clays and sandy silts. 



Between fixes A and B, several channels from 2 to 18 feet deep and up to 90 

 feet wide are cut into the surface layer of sediments. Channels also appear on 

 several of the records over the coring sites in this general area. The channels are 

 all in the northwestern portion of the northern sector of the bay in depths of 70 to 

 80 feet. In the vicinity of core locations 203 through 206 (Figs. 15A to 15E), there 

 are several channels extending through the subbottom layer which is found at depths 

 of 90 to 115 feet. 



Between fixes B and C/faint stratification of the sediments below the subbottom 

 horizon was recorded. Similar features were found at core locations 138, 139, and 

 210. It is possible that this stratification represents old channel deposits of the 

 Penobscot River. The corer did not penetrate these layers. 



As the subbottom goes deeper, south of mark D, the top of this layer becomes 

 less sharply defined, and the contact seems to be gradational with the sediments 

 above. The relief of this horizon and of the bottom becomes greater beyond this 

 point. South of mark D^ the bedrock reflection becomes less distinct and is lost in 



43 



