abundant glauconite (black grains) in some o£ the cores also provides 

 some insight into locating the base of Pleistocene deposits. Evidence 

 for identifying the Pleistocene-Holocene contact is based on core data, 

 except in certain areas where the seismic records show the Pleistocene 

 surface is irregular or the contact is acoustically different from over- 

 lying Holocene materials. Sediments at the Pleistocene surface are 

 generally oxidized medium to coarse sand with varying amounts of gravel 

 and may show some degree of compaction. Pleistocene units generally 

 appear stratified and either flat lying or with a low-angle seaward dip. 

 Pleistocene sediments along the Long Island south shore inner shelf vary 

 considerably in thickness depending on both proximity to source and the 

 relief of the pre-Pleistocene surface. However, the sediment descrip- 

 tions from the cores show the predominant sediments presently covering 

 the shelf are relict or palimpsest Pleistocene-age sand and gravel, spread 

 across the shelf in the form of outwash plains south of the Ronkonkoma and 

 Harbor Hill Moraines and river deltas. During Pleistocene time when sea 

 level was depressed several hundred feet, several southward- flowing melt- 

 water rivers were the primary agents in eroding, transporting, and spread- 

 ing outwash detritus south of the Long Island ice margin moraines. As the 

 glaciers melted and sea level rose to its present position, the moraines 

 on the Long Island and New England inner shelves were greatly influenced 

 by direct action from waves and marine processes of erosion and deposition. 

 The interaction of direct marine erosion on morainal materials is still 

 evident at Montauk Point on eastern Long Island and on Block Island, 

 Nantucket, and Martha's Vineyard. The sand fraction of the morainal cliff 

 material eroded at Montauk Point is thought to be a major source of lit- 

 toral material for the Long Island longshore transport system (Taney, 

 1961; Panuzio, 1968) . 



Maximum thicknesses of Pleistocene sediment occur in the buried ances- 

 tral river channels which, at least on the northern parts of Long Island, 

 were occupied and scoured directly by glaciers. These same channels later 

 acted as conduits for melt-water runoff. The nature, distribution, and 

 ages of these channels are discussed later. The nature of the Pleistocene 

 sediment in many of the channels is unknown; however, several cores which 

 penetrate the channels along the eastern Long Island south shore (Fig. 10) 

 indicate that the upper part of the fill is composed of fine to medium 

 sand with rounded pebbles. Additional information on deeper stratigraphy 

 in a channel in eastern Long Island is provided by the log for the 240-foot 

 (73.2 meters) boring at Orient Point (Table 3). The boring, made for an 

 engineering study for a proposed bridge, is on the extreme northeast prong 

 of Long Island (Fig. 2) and was drilled fortuitously into the Orient Point 

 buried channel which bisects Long Island and Plum Island (Fig. 6). The 

 descriptive log in Table 3 shows the top 41 feet (12.5 meters) is typical 

 Pleistocene outwash or moraine- derived sand and gravel which is yellowish 

 brown and poorly sorted. The remainder of the boring log indicates a 

 general fining downward with typical varve sequences (proglacial lacustrine 

 seasonal deposits) present from 146 feet (44.5 meters) to the bottom of the 

 hole at 240 feet (73.2 meters) (Fig. 11). Figure 6 shows the Orient Point 

 channel thalweg depth exceeds -500 feet (-152.4 meters) MSL but record reso- 

 lution makes it impossible to tell whether the varve sequence in Table 3 



34 



