be overlain by younger sand derived from Nan- 

 tucket Shoals. They believed that the silt accumu- 

 lated in a topographic depression during the Holo- 

 cene rise^ of sea level. Furthermore, they suggested 

 silt beds under Nantucket Shoals as the source and 

 placed the age as late Holocene (after tlie sea had 

 risen to about .3.5 fm. below present sea level) be- 

 cause the surface appears smooth and uneroded. 



The smooth appearance in this area may be a 

 data artifact, resulting from a rather wide spacing 

 of survey tracklines. Just to the east of the Block 

 Delta is a small well-surveyed area (USCGS Hy- 

 drographic Survey No. 6659) that shows the bot- 

 tom finely dissected by many small channels and 

 covered with a few small mounds and depressions. 

 Although this sui^vey may indicate what the sur- 

 rounding region would look like if surveyed in 

 comparable detail, there is some doubt that it does." 

 The average standard deviation of the isobath posi- 

 tion error in the area of USCGS Hydrographic 

 Survey No. 6659 is 0.1.3 nautical mile and, because 

 the principal tracklines run parallel to the trend 

 of the small channels (i.e., up and down slope), 

 lengthwise line shifts of one or two times this 

 amount would account for much of the fine detail 

 shown. vSome of the crosslines run in this survey, 

 however, give evidence, of shallow channels, and it 

 seems probable that the true appearance of the 

 bottom lies somewhere between the two extremes 

 indicated. 



Nantuc'kfif f^hoah. — For a distance of 30 to 50 

 nautical miles to the south and southeast of Nan- 

 tucket Island is a vast expanse of sand shoals and 

 a tangle of many smaller ridges and depressions 

 (charts 0708N-5i and -52). Collectively, this area 

 is called Nantucket Shoals and has been known 

 since the earliest explorations of the east coast (see 

 Kich, 1929). Two old maps showing Nantucket 

 Shoals, one made about 1656 and the other about 

 1730, are reproduced in Gottmann (1961). 



From geologic mapping of Nantucket Island, 

 Martha's Vineyard, and Cape Cod, it appears that 

 Nantucket Shoals are relict glacial deposits laid 

 down when the sea was 25 fm. below its present 

 level. The large shoals abutting on the Great South 

 Channel contain a few patches of gravel and prob- 

 ably constitute a much modified glacial moraine 

 formed by a late Wisconsin ice-lobe in that chan- 



- Personal communication from 

 logical Survey, Washington. D.C. 



John S. Schlee, U.S. Geo- 



nel. Farther to the west, Nantucket Shoals prob- 

 ably were derived by reworking outwash from the 

 west side of this South Channel moraine, or from 

 an interlobate outwash deposit formed between the 

 South Channel ice-lobe and another ice-lobe ex- 

 tending through Cape Cod Bay (Zeigler et al., 

 1964, suggested that outer Ca^ie Cod is an inter- 

 lobate deposit formed between these two lobes), 

 or from end moraines of the Cape Cod Bay ice- 

 lobe. Whatever their exact source, these Shoals 

 have been much altered by early Holocene stream 

 erosion and by late Holocene and modem tidal cur- 

 I'ent and surf-zone action (Lindenkohl, 1883; 

 Curtis, 1913). 



Old silt beds occur under the Shoals, and Living- 

 stone (1964) considered them to be of Sangamon 

 age. (Athearn (1957) came to the same conclusion 

 for a similar silt layer about 43 fm. below sea level 

 some 60 nautical miles south of Moriches Bay, 

 Long Island.) Groot and Groot (1964) found that 

 samples of the upper 5 feet (1.5 m.) of the silt 

 near Fishing Eip contained a mixture of Creta- 

 ceous, Tertiary, and Pleistocene pollen and spores, 

 as well as a marine shell about 11,500 years old. 

 It, thus, seems that the silt layer, at least near 

 Fishing Rip, has been covered by the Shoal sands 

 only in the late Holocene — probably by material 

 washed southwestward from the lateral moraine 

 of the South Channel ice-lobe. This type of win- 

 nowing has been invoked by Garrison and Mc- 

 Master (1966) to account for the band of fine sand 

 covering the silty area to the west of Nantucket 

 Shoals (see also Shaler, 1893). Uchupi (1968) sug- 

 gested, however, that some of this sand may have 

 come from the erosion of the outer arm of Cape 

 Cod. 



The Great South Channel. — The existence of the 

 Great South Channel was inferred from local sur- 

 face currents by Captain John Smith as early as 

 1614 (Rich, 1929) . It sejiarates Georges Bank from 

 Nantucket Shoals and is a broad and flat but 

 rough-bottomed valley with a sill at about 40 fm. 

 (lat. 40°36' N. on chart 0708N-52). It is divided 

 into a number of sliallow basins by low sills. This 

 Channel was probalily occupied by a lobe of the 

 late-Wisconsin ice-sheet, from which outwash and 

 moraines contributed to Iwth Little Georges Shoal 

 to the ea.st and Nantucket Shoals to the west (see 

 Zeigler et al., 1964) . In pre-Pleistocene time Great 

 South Channel may ha^•e been a stream valley 



BATHYMETRIC MAPS AND GEOMORPHOLOGY OF MIDDLE ATLAMTIC CONTINENTAL SHELF 



57 



