PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 
595 
of floating debris of one sort or another. In all this, Georges Bank, in the south of 
the gulf, agrees with the coastal belt generally in the northeast, as it does in being 
colder at the surface than is the intervening basin where “ the water moves to and fro 
in an unbroken sheet, clear of obstruction,” as Dawson (1905, p. 15) expresses it. 
Doctor Kendall’s temperatures, added to readings taken by the Grampus in 
July, 1908 (Bigelow, 1909), and from the Halcyon in the summer of 1923, show 
that the surface is correspondingly cool (12° to 16°) in August over the shallow 
broken bottom south of Nantucket, with similar fluctuations within short dis- 
tances and at different stages of the tide, due to the same disturbing influence of 
tidal mixings. Thus, the Halcyon had surface readings varying from 11.6° to 15° 
in August, 1923, as she fished at various locations within a mile or two of Round 
Shoal bouy; 13.3° to 16.4° over Rose and Crown Shoal; 15.5° over the slightly deeper 
channel between Round Shoal and Rose and Crown Shoal; and 13.8° to 15.5° on 
the Great Rip fishing ground 12 miles southeast of the island of Nantucket. Unfor- 
tunately, it is not yet known whether this cold area is separated from the equally low 
surface temperatures of Georges Bank by a band of warm surface water along the 
so-called “south channel,” as seems probable, or whether the cool surface forms an 
unbroken band, west to east, from the one shoal ground to the other. 
In 1913 the surface to the seaward of the 50-meter contour off Nantucket had 
warmed to upward of 19° by the last week in August (Bigelow, 1915, p. 350, fig. 2, 
stations 10107 to 10112). This was true also of the whole breadth of the shelf 
abreast of Marthas Vineyard on the 26th of the month in 1914, except close in to 
the land (station 10263), where a surface reading of only 17.9° probably reflected 
some tidal disturbance or other. With this same exception, Doctor Kendall likewise 
had 18° to 19° at every station off Marthas Vineyard early in September, 1897, 
paralleling Libbey’s (1891) record of surface warmer than 19° over this part of the 
continental shelf during August, 1889. 
These data locate the isotherm for 18° as following the southern and western 
edges of Nantucket Shoals around into the submarine bight west of the latter, but 
with cool pools next the southern shores of Marthas Vineyard, as just noted. 
It is probable that the surface temperature rises higher than 20° over the outer 
part of the continental shelf off southern New England every August, and Libbey’s 
(1891) extensive data show that in some years temperatures slightly higher than 20° 
are to be expected within a few miles of Marthas Vineyard. But his records also 
show that a considerable variation in surface temperature is to be expected within 
short periods of time over the inner half of the shelf, where a sudden cooling of the 
surface would be the natural accompaniment of any unusual stirring of the water or 
of the upwellings that so often follow offshore winds. 
There is also considerable variation in the surface temperature off Marthas Vine- 
yard from year to year. In 1914, for example, the isotherm for 20° included only 
the outer half of the continental shelf on August 21 at longitude 71° (fig. 46). 
In spite of these fluctuations, it is safe to say that the surface is invariably 
warmer than 20° along the edge of the continent in the offing of Marthas Vineyard 
and Nantucket Island by the end of August. To find the surface warming to 
upward of 22° to 23° it is only necessary to sail seaward a few miles farther. 
