The glacial drift lying on bedrock can only be inferred. The coarse deposits 

 noted in the discussion of the Sonoprobe runs ore most likely glacial material 

 deposited by the last ice sheet (Cary) to occupy the preglacial Penobscot River 

 valley. The stratification of some of the deposits recorded at fix N of run I are 

 probably outwash deposits from the last ice sheet, or a late remnant or lobe trapped 

 in the valley depression north of fix N . The material below the subbottom and o- 

 bove the bedrock north of fix N to about fix L could be till and ice contact de- 

 posits which record a stand of the ice sheet during its regression. 



Following deglaciation, or perhaps contemporaneously with deglaciation, the 

 sea inundated the coastal area to a maximum of 300 feet above present sea level. 

 The sediments deposited during this time were not sampled, but probably they do not 

 differ from the silty clays described In the geology of the land areas. These are 

 the sediments below the subbottom horizon In Penobscot Bay. 



The two sequences of late-glacial marine sediments (Leavitt and Perkins, 1935) 

 are not apparent on the Sonoprobe records. The deposits noted as till or ice-contact 

 deposits on run I south of marks H and L, however, may be marine sediments deformed 

 by a later advance of the ice sheet. The subbottom records do not resolve the prob- 

 lem since either Interpretation of these deposits may be possible. 



The sediments below the subbottom horizon were deposited in shallow water by 

 sediment laden glacial meltwater in the bay and elsewhere (Bastin, 1908, and 

 Goldthwalt, 1949) during the transgresslve phase of the marine submergence. The 

 development of the subbottom horizon requires a marked change in conditions over 

 the previous depositlonal cycle. The few cores which approached or penetrated the 

 subbottom reflecting surface Indicate that at or above this level the sediments are 

 pebbly and sllty sands. This horizon Is flat north and northeast of Long Island atmax- 

 Imum depths of 70 to 80 feet below sea level . Northwest of Long Island, inside the 

 60-foot contour, this horizon drops abruptly to 90 feet and greater. Towards the 

 shores in the northern sector of the bay the subbottom horizon becomes shallower. 



In the central and southern sectors of the bay, the subbottom horizon Is deeper, 

 becomes less continuous, is probably more variable in Its degree of coarseness, and 

 has considerable relief relative to the northern part of the bay. The top of the 

 horizon also appears to begrodational with the sediments above and below as the 

 echoes are not as sharply defined as In the northern part of the bay. 



As mentioned above, the sediments below the subbottom marker horizon are 

 probably late-glacial marine clays described in the literature for the surrounding 

 land areas. The sediment -Sonoprobe correlation is not precise enough to assign the 

 coarse material found at or near this horizon to the top of the late-glacial clays or 

 to the base of the sediments above. This may not be of great significance, however. 



47 



