why reflections recorded on the Sonoprobe while lylng-to at a sampling location 

 often could not be correlated with the core taken at the station is lack of lateral 

 continuity of the sedimentary layers. Rapid lateral and vertical lithologic changes 

 are to be expected in estuarine environments where daily and seasonal changes in 

 the energy conditions affect the quantity and type of material deposited at any loca- 

 tion . In some cases,an apparent sedimentary layer recorded by the subbottom profiler 

 while on station at a sampling location may be due to side echoes from highly re- 

 flective strata or a point source not directly beneath the ship. 



On nearly all of the Sonoprobe runs in the survey area, penetration to bedrock 

 was achieved. The greatest depth of signal penetration to bedrock was about 110 

 feet. Nevertheless, the thickness of the sediments in the center of the bay opposite 

 Rockland may have been somewhat greater since over 100 feet of penetration was 

 achieved, yet bedrock was not encountered. 



The silty clays that make up most of the bay sediments permitted penetration 

 to over 100 feet. There are indications that 20 to 40 feet of penetration is possible 

 through a sequence of silty-cloy sediment that also contains sand layers which are 

 several feet thick. Cores 130, 137, 138, and 211 all contained several feet 



of fine to coarse sand. Although the bedrock trace was not apparent on the Sonoprobe 

 records at these stations, the acoustic signal had penetrated 15 to perhaps 40 feet be- 

 low the bottom. At locations 129, 136, 148, 201, 207, and 209, the bedrock trace 

 was faint, but the signal had passed through at least several feet of sand as shown by 

 the cores. The sandy sediments were all poorly sorted and contained an average of 

 30% silt-and clay-sized particles. The thickness of coarse material below the sampled 

 depth down to bedrock is not known, but the acoustic signal had passed through the 

 coarse material which lies in the zone of the subbottom reflecting horizon. 



C . Interpretation of Subbottom Records 



1 . General 



In an attempt to utilize the Sonoprobe as a tool in understanding and determining 

 the types and conditions of deposition of subbottom sediments, factors such as instru- 

 ment capability, geologic history, and sedimentary data must be taken into account. 

 With this knowledge of the instrument and the area under study, it is possible to 

 know what can actually be determined with the instrument, what types of sediments 

 are to be expected, and, more specifically, what the characteristic reflections and 

 penetration are of these known surface and near surface sediments. Some of these 

 factors were discussed in previous sections . 



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