Following this stage of development, the sea readvanced depositing the silty 

 clays found above the subbottom layer. These sediments, as mentioned earlier, are 

 fairly uniform throughout the area except in the northern sector and in the entrance 

 and approaches to the bay. 



The main features of the recent sediments of Penobscot Bay are the prevalence 

 of the olive-gray silty clay, lenses and laminae of sandy and silty material, and 

 scattered pebbles found in the fine grained sequences of sediments. The black 

 banding found in many of the cores as well as the hard clay masses and layers found 

 in the cores and the similar mo'-erial found on the surface of the sediments were also 

 common. Woody material also was found in many of the cores. Nearly all the 

 sediments were poorly sorted. 



In the northern sector of the bay, the cores containing coarse material grade 

 upward into finer material . The lenses and laminae of silts and sands, the clay lay- 

 ers and/or bolls and fragments, and the pebbly and woody material all suggest 

 shallow water estuarine conditions. The sea level probably rose gradually to its 

 present position or above as indicated by the finer sediments upward from the sub- 

 bottom horizon. Channeling such as that found at the beginning of run I would be ex- 

 pected over tidal flats or very shallow water. At any rate these channels must have 

 been formed at an earlier time and are still preserved. As the sea level rose, the rate 

 of sedimentation decreased to its present condition. Some fine-grained sedimentation 

 is probably taking place in the northern part of the bay and on the tidal flats and 

 marshes. The sediments are being derived largely from the reworking of the bay 

 deposits farther south . 



The usefulness of the Sonoprobe as a tool for studying the Pleistocene and Recent 

 geologic history of Penobscot Bay has been demonstrated. The principal advantage 

 of this survey method is to provide continuity and to delineate the morphology of sub- 

 bottom reflections throughout the area under study. Although direct correlation of 

 vertical lithologic changes and Sonoprobe reflections was not always possible, the 

 lack of correlation is not in itself a shortcoming of the instrument. Correlation 

 from core to core was equally difficult. Moreover, the properties of bottom deposits 

 or the magnitude of changes in the physical properties, required to produce a reflec- 

 tion of the acoustic signal are not precisely known. The Sonoprobe does define 

 changes in bottom and subbottom characteristics or type that may be only subtly re- 

 flected in the physical properties of the sediments. These changes, so well shown 

 on the Sonoprobe records, may be difficult to distinguish in cores. 



The subbottom horizon in Penobscot Bay could not have been defined in depth, 

 lateral extent, or morphology of its reflecting surface by the bottom sampling pro- 

 gram alone. In addition, the slump structures, the bedrock basin, and the subbottom 

 glacial deposits are features which could not have been defined so readily and graph- 

 ically by other means. 



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