clam flats existed in the vicinity of the drumlins, channel sands were 

 being deposited elsewhere. As sea level rose and the barrier island 

 transgressed, the glaciomarine and glacial deposits were covered with 

 barrier and estuarine sands. However, the accretion was slow enough to 

 allow continuation of the shellfish population on the clam flats. 



As the waves began to attack the drumlins just seaward of the relict 

 estuary, a sand spit built across the mouth of the channel behind the 

 drumlins and closed off this estuarine circulation. With estuarine cir- 

 culation cut off from the basin, marsh peat began to develop. The fresh- 

 water peat, identified in drill-hole log PIC, occurred during a brief 

 fluctuation in sea level or during the complete closure of the basin away 

 from the estuary. The result is a modem brackish pond which is now 

 maintained by a manmade dike. 



The logs of PIC and PIG show unusually high elevations for blue clay 

 in a Plum Island drill hole. PIC was drilled 60 feet to refusal, which 

 was determined by seismic profile to be the surface of bedrock. This is 

 an example of the marine clay being draped over a preglacial bedrock 

 high with little later erosion. However, the log of PIG shows silt at 

 22 feet. On top of the uppeinnost silt horizon is a gravelly layer with 

 angular rock fragments , typical of the weathered zone over the glacio- 

 marine clay. Glaciomarine clay does not appear until 65 feet. Between 

 these depths is a sequence of alternating clay-silt and sand layers with 

 a semi-indurated layer at 32 feet. The sand and silt are highly quartz- 

 ose. The blue clay, where it grades into silt-sized material, is usually 

 also highly quartzose. Therefore a sharp discrimination between the silt 

 and the clay is often difficult. 



Drill hole PIH is the most southerly hole on Plum Island. At 25 feet, 

 the drill bit passed into poorly sorted, coarse, angular gravel with a silt 

 cap on the pebbles. However, seismic returns indicate a discontinuous "till' 

 velocity beneath this area. If there is a till sequence here, it is likely 

 a thick one on which the southern end of Plum Island is anchored. In con- 

 trast, the presence of the mouth of a large inlet also suggests that there 

 could be a large accumulation of coarse channel deposits. A compromise 

 between the two ideas might be that the surface till of southern Plum 

 Island extends southward beneath a veneer of sand deposited by spit accre- 

 tion. This glacial material has been greatly reworked by wave and current 

 action, but remains near its source, the southern Plum Island drumlin field. 



Fitting undated drill-hole logs to sea- level curves can be arbitrary. 

 However, the presence here of such environmental determinants as clam- 

 flat facies, fresh-water peat, weathered gravel zones, and glaciomarine 

 clays permits a more certain three-dimensional space correlation with 

 time. To aid this correlation, the sea- level curves proposed by Mclntire 

 and Morgan C1963) and Kaye and Barghoorn (1964) , are presented in Figures 

 32 and 33. Kaye and Barghoorn assume a high stand of the sea before 

 14,000 B.P.; Mclntire and Morgan suggest that the high stand was before 



51 



