II. GEOMORPHOLOGY AND SHALLOW SUBBOTTOM STRUCTURE 

 1. Geomorphology . 



a. General . Analysis of submarine bottom topography in ICONS study 

 areas has proven useful in extrapolating core and seismic data between 

 data points where interrelationships exist between bottom topography and 

 other elements of the geological environment. In areas of consistent 

 relationships, it is possible to use available bathymetric data which in 

 most shelf areas are available in more detail than information on any 

 other aspect of the environment. 



In the Massachusetts Bay ICONS study area, relationships between 

 bottom morphology and subsurface structure are fairly consistent. Most 

 topographic highs on the sea floor are associated with outcrops of the 

 irregular bedrock surface or glacial till, while topographically flat 

 areas occur where the bedrock and till surfaces are buried. In contrast, 

 relationships between bottom morphology and sediment distribution patterns 

 are ambiguous in most of the area, especially for surficial sediments. 



A generalized bathymetric chart of the primary study area, compiled 

 from National Ocean Survey (NOS) smooth sheets at 1:10,000 and 1:20,000 

 scale-, is shown in Figure 4. Since chart control is approximate the 

 figure is intended only to depict bottom morphology and not to provide 

 accurate geographic location of features. A contour interval smaller than 

 the 10-foot interval used in Figure 4 would be required to detail the 

 complex and often subtle bottom topography of the study area. 



Although a wide variety of submarine topographic features occur in 

 western Massachusetts Bay, most can be classified under three broad 

 categories: (a) level bottom; (b) submarine hills and ridges; and (c) 

 connecting slopes. Areas in which these terrain elements are predomi- 

 nant are outlined in Figure 5; each area is identified by an alpha- 

 numeric to facilitate description. Submarine hills and ridges are 

 divided into two types (Fig- 5) because of differences in form, com- 

 position, or genesis. 



b. Level Bottom . Areas labeled A in Figure 5 have a gently sloping 

 or level bottom and occur throughout the study area. The most extensive 

 occurrence is in the northern part, generally to the east, southeast, and 

 south of Nahant (Al in Fig. 5). In the southern part of the study area 

 there is a large area of level bottom extending southeastward from the 

 prominent group of islets and ledges in the near approach to Boston Harbor 

 (A2 in Fig. 5). Smaller areas occur off Nantasket Beach (A3 and A4 in 

 Fig. 5) and minor occurrences, not delineated, exist throughout the area 

 in places where other topographic elements predominate. 



Most level bottom areas lie in water deeper than 80 feet but some 

 occur in shallower water. Relief features in the level bottom areas 

 are rare; they consist mostly of a few isolated hills and smooth-surfaced 

 "mounds" of low relief rising generally less than 20 feet above the sur- 

 rounding sea floor. The smooth surface of these mounds is in contrast 



19 



