oxides, calcium carbonates, minor amounts of siderite, manganese oxides, 

 and manganese carbonate, with additional strength contributed by accumu- 

 lation of fine clay fractions after burial. 



A similar process may have affected the Plum Island sediments. The 

 possibility presents a question that could be answered by further study. 

 An undisturbed core of the indurated layer is needed. Although the ce- 

 menting agents mentioned may be present deeper in the glaciomarine clay 

 sequence, it is possible that there is a zone of greater accumulation at 

 the top of the clay. Unquestionably, carbonates exist in the blue clay 

 sequence, because calcite nodules can be seen and a positive carbonate 

 reaction to hydrochloric acid is common. 



4. Weathered Zone 



The weathered zone above the glaciomarine clay is believed to rest on 

 a surface of deposition on which clastic materials were deposited during 

 a low stand of the sea. This weathered zone is composed of coarse, 

 poorly sorted and highly oxidized sand and gravel. Figure 29 is a photo 

 of this material from Plim Island hole PIG. These elastics could come 

 from nearby drumlin tills, ice-contact drift, or stranded beach deposits 

 of earlier sea levels. 



5. Black Peat 



The black peat here is likely a correlative of the black peat found 

 by McCormick in his drill holes further north. Johnson (1925), Davis 

 (1910) and Bloom (1968) all recognized this peat in New England marshes. 

 These investigators agree that the black peat represents an accvunulation 

 of fresh- and salt-water plants deposited in the zone of transition from 

 salt marsh to upland vegetation. The peat is a highly organic compacted 

 layer. Its dark color is probably due to a reducing environment estab- 

 lished after transgression of the sea. Its silt content is variable. It 

 ranges in thickness from 1 foot to a few inches, and is totally absent 

 in some of the more seaward drill holes. The black peat does not overlie 

 clay everywhere, because this transitional situation probably could not 

 develop in lower or more seaward locations. This peat layer commonly 

 lies directly under a thick estuarine sequence of either channel, point- 

 bar, or low energy mud- flat deposits rather than a marsh deposit. It is 

 found directly on the weathered zone when all of these layers are present. 



6. Estuarine Sediments 



The sands that overlie the glaciomarine clay sequence and its capping 

 weathered zone represent various environments of deposition. Most of the 

 subdivisions grouped under the heading of estuarine sediments that are 

 only drill hole horizons, not seismic horizons. Occasionally the seismic 

 record shows a horizon within these sediments, but detailed stratigraphy 

 can only be accomplished with closely spaced drill holes. 



45 



