Clam-flat sequences overlying tke clays are often thick Csee log for 

 drill hole PID in the Appendix). The clam-flat facies are identified by 

 a large accumulation of clam shells and by an extremely silty to muddy 

 nature . 



Figure 30 is a sketch of the study area near Castle Neck as it might 

 have looked before the readvance of the sea over the glaciomarine de- 

 posits . Stranded beach ridges probably contributed the material to the 

 weathered zone that appears over the glaciomarine clay. Glacial material 

 provided topographic highs on which to anchor the modem barrier sands. 



It is suggested that the Essex Bay flowed in nearly the same channels 

 as it does at present. This suggestion is supported by the fact that the 

 Essex flood- tidal delta is anchored on bedrock. The bedrock topography 

 is glacial or preglacial, and this tidal delta has apparently been stable 

 throughout the sea-level rise. The existence of a deep bedrock low be- 

 tween the Essex delta and the modem Castle Neck suggests that a deeper 

 relict Castle Neck River might have flowed in this preglacial-bedrock 

 low, and built the thick sequence of sediments presently filling it. Gla- 

 ciomarine clay is found in the bedrock low at the margins of the modem 

 Castle Neck. Its depth varies between 50 feet (log of CBF) and 70 feet 

 Clog of CBG) . An extensive clam-flat sequence overlies the clay horizon 

 in drill hole CBF, which suggests a fringing clam-flat environment behind 

 a topography of glacial material during the low stand of sea level. 



Other drill holes support the scheme in Figure 30. For example, 

 drill hole CBB bottomed in till at 60 feet. Directly on top of the till 

 is a brown clay that grades upward into a blue-green clay before becoming 

 a brown clay again. It is presumed that some weathering changes have 

 been preserved in this record. A highly organic, perhaps fresh-water, 

 black peat overlies the clays. Coarse Seind to fine gravel predominates 

 in the section upward from this point. 



Drill hole CBB is on the Ipswich River west of Castle Hill. It is 

 one of the more inland sites, and its record suggests that much geologic 

 history could be discovered by a comprehensive deep drilling program in 

 some of the more protected or inland marshes. Drill holes at such areas 

 could sample the black-peat horizons for age dating purposes and deter- 

 mine more closely the sea-level curve for this coastline. 



V. CASE HISTORIES FROM PLUM ISLAND 



Extensive sand deposits underlie the Parker River estuary. For ex- 

 ample, the log of wash-boring MGA (Middle Ground) shows more than 55 feet 

 of sand on top of the clay horizon. Only the lower 10 to 15 feet of sand 

 is thought to be clam- flat deposits. The upper part of the deposit, fine 

 quartzose sand interspersed with organic layers, is thought to have been 

 deposited on sand flats dominated by strong currents and rapid sediment 

 transport. Shellfish do not thrive in such environments on the modem 



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