Available surface samples alone are insufficient to establish surface 

 sediment distribution. However, by supplementing surface sample informa- 

 tion with side-scan sonar and seismic reflection data, the MCMI study group 

 was able to construct a surface sediment distribution map (Willett, et al., 

 1972); a part of the map is shown in Figure 13. 



All types of surficial sediment deposits tend to be thin. Of the 

 69 cores in the study area for which data are available, only 12 contain 

 similar material throughout their length. Distinct changes in sediment 

 character occur within 5 feet of the surface in 34 of 60 cores longer than 

 5 feet. In 25 of these 60 cores the change in sediment character lies 

 within 3 feet of the surface. 



Willett, et al . (1972) stated that, in general, cobble and boulder- 

 bearing surface sediments are more characteristic of the study area off 

 Boston and southward while fine surficial sands are dominant to the north. 

 They believe most of the coarse gravelly sediments are part of the thin 

 skin of reworked glacial drift spread across the sea floor by shallow 

 water reworking; locally this layer grades into the parent till body. The 

 fine sand characteristic of the northern reconnaissance area is ascribed to 

 modern progradation of sand from the shore and littoral zone. 



Most of the fine sand in the study area seems to be a surface veneer 

 in offshore areas. In places near the coast the fine sand is thicker, 

 as indicated by cores from the relatively shallow inshore areas off 

 Nantasket Beach and in the reconnaissance areas. 



All sediment types encountered below the surficial layer are exposed 

 in places within the study area. Thus, there appears to be no sediments 

 within the upper 10 feet of the sea floor that are found only in the sub- 

 surface. The most common sediment underlying surficial deposits is 

 greenish-gray silt-clay; also present are sand, gravelly sand, and till- 

 like mixtures of sand, gravel, silt, and clay. All of these sediments 

 are widely distributed throughout the study area. Silt and clay layers 

 tend to be thicker than gravel or sand layers which are often less than 2 

 feet thick. 



Subbottom distribution patterns of the greenish-gray mud and the 

 bedrock-till components of the acoustic basement are more predictable than 

 surficial sediment distribution because of their patterned relationship 

 to the relict disrupted drainage system underlying the study area (Fig. 8) . 



IV. DISCUSSION 



1. Relationship Between Subbottom Strata, Topography, and Sediments . 



As previously discussed, the surface of the basement unit is highly 

 irregular throughout the study area. Stratified deposits (transparent 

 unit) completely cover this surface in many places; elsewhere, the strat- 

 ified material only partially fills the lows and the higher peaks outcrop. 

 At times, the basement surface appears to be planed off at the level of 



36 



