10 THE RELATION OF PLANTS TO TIDE-LEVELS 
wooded swamp. From the lateral banks, especially the western, the ponds are 
fed by numerous rivulets of cold spring-water, all of them first coming to the 
surface within a few yards of the margin of the pond. 
The Inner Harbor, as shown on the map (plate 1) is, at high water, about 
3,500 feet long. It has a width of about 400 feet near the south end, and 
widens to an extreme width of 2,200 feet at the north end. The total area of 
the water-surface at the 8-foot tide-level is about 5,000,000 square feet, or 
110 acres. At mean low water (see plate 1) the water-surface measures only 
1,400 feet from north to south and 1,700 feet from east to west, the total area 
at this level being about 2,000,000 square feet, or 45.5 acres. Ata level of 1 foot 
below mean low water the water-surface measures less than 700 feet from north 
to south, and a little more than this from east to west. At this level, however, 
there is a long tide-basin near the west side of the harbor, which is connected 
with the center of the harbor by a long tide-stream that sweeps around near the 
northwest corner of the harbor. The total area of water-surface when the tide 
is at this level is reduced to about 750,000 square feet, or 1% acres. That is, 
at such an extreme low tide, less than one-sixth of the bottom of the harbor is 
covered by water. 
The deepest part of the harbor has a bottom at but 7 or 7.5 feet below mean 
low water, and even this depth is found only over an area little more than 100 
feet in diameter. This area lies between 1,340 and 1,470 north and 500 and 
610 east. The bottom of the harbor is covered over most of its area by a thick 
layer of fine silt, which is slightly grayish on the surface, but dense black 
below the upper millimeter or two. The thickness of this black mud differs in 
different parts of the harbor. Near the shores there may be only a few inches, 
or at most a foot, while near the center of the harbor (e. g., at 1,800 north by 
0 east) a steel sounding-rod may be pushed down 9 or 10 feet below the surface 
of the mud before reaching the hard gravel bottom. Since the surface of the 
mud at the point referred to is but little below mean low-water level, the gravel 
bottom is here 9 or 10 feet below mean low water. 
It is worthy of note that at that part of the harbor where the water is 
deepest (eé. g., near 1,400 north by 600 east) the bottom is of sand, gravel, or 
shells, and is only about 7 feet below mean low water. This is evidently due 
to the fact that this part of the harbor is in the line of the swift current which 
carries on with it all but the coarsest particles. On the contrary, all the parts 
of the harbor bottom about this depression, except on the line joining it with the 
main stream and with the Inlet, 7. ¢., all parts outside the strongest current, are 
being filled up with the finer silt carried by the more slowly moving water. It 
is evident that Zostera plays an important part in retarding the tidal currents 
flowing over the harbor bottom to and from the Inlet. 
The silting up of the harbor is due partly to the remains of organisms 
growing in it, partly to material brought down by the fresh-water streams 
running into it. A relatively considerable amount of material is also brought 
in by the flood tides from the Outer Harbor. Part of this may evidently be 
material carried out by the last ebb tide, but part of it may come from other 
streams emptying into the Outer Harbor or may be eroded from the edges of the 
tide channel. The filling up of the Inner Harbor is going on at a rather rapid 
rate. In some localities, where the presence of Zostera or of the very numerous 
