Block Island, and the islands south of Massachu- 

 setts (Schafer and Hartshorn, 1965). Long Island 

 has two moraines: the Harbor Hill Moraine ex- 

 tends along the north shore to Orient Point and 

 then across Long Island Sound through Plum, 

 Great Gull, and Fishers Islands to Watch Hill 

 Point in Rhode Island and along the slioi*e (where 

 it is called the Charlestown Moraine) to Point 

 Judith; the Ronkonkoma Moraine runs through 

 central Long Island to Montauk Point. These two 

 moraines are considered to have formed during the 

 last advance of the late Wisconsin ice-sheet some 

 20,000 years ago (Flint, 1957; and Domier, 1964). 

 Tliey mei'ge to the west, south of Hempstead Har- 

 bor, and continue across Brooklyn to Staten Island 

 and New Jersey. 



Wliere the Harbor Hill Moraine crosses Long 

 Island Sound there is a ridge of coarse rocky sedi- 

 ments (chart 0808N-5.3 of Stearns and Garrison, 

 1967). Between the high points on this ridge are 

 elongated depressions and channels, some as deep 

 as 55 fm., containing finer sediments. Some of 

 these depressions may be kettles formed by the 

 melting of buried blocks of ice (Elliott et al., 

 1955), or they may have been cut by either ice- 

 scour or subglacial drainage streams (Dana, 1870, 

 1875, 1883, 1890; Loring and Nota, 1966, suggested 

 this origin for similar features in the Gulf of St. 

 Lawrence) . These pre-Holocene depressions would 

 have become fresh-water lakes shortly after being- 

 uncovered by the melting ice-sheet (Antevs, 1922, 

 1928; Lougee, 1953) ; evidence for these lakes, in 

 the form of fresh-water clay concretions, has been 

 found in one depression south of Fishers Island 

 (Frankel and Thomas, 1966). Some other depres- 

 sions may have been formed or at least modified, 

 by river erosion, during the period which followed 

 the retreat of the ice-sheet. All the depressions 

 probably have been scoured by tidal currents which 

 became effective in this area when the sea had 

 risen to 10 to 15 fm. below j^resent sea level. 



The Ronkonkoma Moraine extends beyond Long 

 Island, from Montauk Point to Block Island, and 

 its crossing is marked by a broad ridge of coarse 

 rocky and bouldery sediments (charts 0808N-51 

 and -53). Near its center this band is breached by 

 a channel, which contains several 25- and 30-fm. 

 holes. This breach probably represents one of the 

 ancient channels for the rivers of Connecticut and 



western Rhode Island. It was later eroded by tidal 

 currents when sea level rose to within 10 or 15 fm. 

 of its present level. 



East of Point Judith, R.I., the Harbor Hill (or 

 Charlestown) Moraine appears to bend south- 

 ward around Narragansett Bay and to join with 

 the Buzzards Bay Moraine of western Cape Cod 

 by way of Browns Ledge and the Elizabeth Is- 

 lands (chart 0808N-51). This bend is marked by 

 a submerged ridge of coarse gravelly sediments. 

 East of Block Island the Ronkonkoma ^Moraine 

 also appears to bend to the south and to join with 

 moraines on the north shores of Martha's Vine- 

 yard and Nantucket Island by way of Nomans 

 Land, the Southwest Shoal, and Cox Ledge. The 

 bottom in this area is marked by a broad rocky 

 and gravelly ridge (Schafer, 1961; and Kaye, 

 1964) . 



Between Block Island and Martha's Vineyard 

 this ridge is breached by a chamiel with depths as 

 great as 35 fm. The sea bottom between the two 

 moraines in this area contains an east-west chan- 

 nel with depths that approach 30 fm. to the north of 

 Block Island. This east-west channel continues 

 westward through Block Island Sound and east- 

 ward as far as the entrance to Vineyard Sound. 

 This channel probably represents an early Holo- 

 cene drainage system for much of southern New 

 England. Presumably, late Holocene drainage 

 from Connecticut broke tlirough the moraine east 

 of Montauk Point, while Rhode Island and Mas- 

 sachusetts drainage continued down the channel 

 east of Block Island. It appears that both these 

 systems entered the Block Channel across the shelf. 

 The channels have been modified by tidal scour and 

 tidal delta deposition wliich would have started 

 when the sea rose to about 25 fm. below present 

 sea level. 



The moraines on Martha's Vineyard, Nantucket 

 Island, and Cape Cod cannot be traced to the east 

 with certainty (see chart 0708N-51). They prob- 

 ably merge with tlie lateral moraine of an ice- 

 sheet lobe that once extended soutliward through 

 the Great South Channel (see Zeigler, Tuttle, 

 Tasha, and Giese, 1964) . Between Martha's Vine- 

 yard and Nantucket Island is a double tidal delta 

 which appears to have been built in an old tribu- 

 tary of the Block Channel that once ran between 

 the Islands. 



BATHYMETRIC MAPS AND GEOMORPHOLOGY OF MIDDLE ATLANTIC CONTINENTAL SHELF 



53 



