the bottom, and outcrops or very shallow subcrops occur without topo- 

 graphic expression. Therefore, a consistent relationship exists between 

 outcrops o£ the basement reflection unit and the occurrence of both inshore 

 and offshore submarine hills and ridges. 



Since few cores are available from outcrops of the basement reflection 

 unit the rock character of these submarine hills and ridges is unknown. 

 However, it is likely that most of the inshore hills and ridges are 

 essentially complex pre-Cretaceous basement rocks because they lie close 

 off rocky stretches of coast with only thin ground moraine cover, and 

 some inlets in the inshore groups are known to be composed of basement 

 rocks (LaForge, 1932). 



The offshore hills and ridges are generally larger and not as steep 

 sided as the inshore features. Scattered surficial samples indicate that 

 these features have a veener of till-like material. Many of the offshore 

 ridges are similar in size and general orientation to the drumlins that 

 form a familiar element of glaciated terrain in the Boston area. However, 

 core data for both inshore and offshore hills and ridges are inadequate 

 for reliable determination of their specific environment of deposition. 



A generally consistent relationship also exists between level bottom 

 (area A in Fig. 5) and the occurrence of the transparent reflection unit 

 directly underlying the sea floor. Internal reflections in the transparent 

 unit indicate the level bottom is composed of stratified sediment material. 

 Most cores in level bottom areas entered greenish-gray mud less than 3 feet 

 downhole and several cores indicated that this composition persisted to 

 depths of over 30 feet. The relationship between level bottom, stratified 

 subbottom sediment, and greenish-gray mud, either exposed or under thin 

 fine sand overburden, is especially consistent in area Al (Fig. 5) which 

 occupies much of the northern half of the study area. 



In places, the upper part of the transparent reflection unit contains 

 fine sand rather than the characteristic greenish-gray mud. Where this 

 occurs, the reflection records usually show the transparent unit divided 

 into upper and lower subunits (see Section II, 2c) and the upper part 

 contains strong internal reflectors. However, strong internal reflectors 

 in the upper subunit also occur where cores show the subunit to be composed 

 of mud; therefore, this characteristic reflection is not everywhere 

 indicative of a sand layer. 



2. Origin and Distribution of Sediments . 



a. Glacial Drift . Most clastic sediments in the study area are of 

 glacial origin, originating as ice-borne detritus subsequently deposited 

 in ice contact and outwash features. In a few places these sediments are 

 distributed in the original deposition patterns left by the retreating 

 glacier and associated braided outwash streams. However, in most places 

 the original character and distribution of these sediments have been 

 altered by postdepositioned reworking and redistribution in a shallow 

 marine environment. This was a result of an emergence (relative uplift of 



38 



