5. Hydrography . 



Tidal ranges are relatively uniform throughout the study area. At 12 

 stations within the study limits they vary less than 1 foot--from 8.7 to 

 9.5 feet mean range and 10.1 to 11 feet spring range (National Ocean Survey, 

 1974a) . Tidal currents in the near approaches to Boston Harbor reach velo- 

 cities of 1.5 knots. At other inshore stations, tidal current velocities 

 are generally less than 0.5 knot (National Ocean Survey, 1974b). 



Schlee, Folger, and O'Hara (1971) reported that bottom flow in the area 

 west of Stellwagen Bank is mostly southerly; however, inshore tide- 

 dominated currents generally set east-west. They also report that bottom 

 drifters released within 15 kilometers of shore often ground at points on 

 shore nearest the release point, indicating a net shoreward bottom drift 

 in the zone encompassing most of the study area. 



Generalized ocean wave data for the Gulf of Maine show that waves over 

 5 feet high occur 12 percent of the time from October through March and 

 from 2 to 10 percent of the time in other months with the minimum fre- 

 quency occurring from June through August. Waves exceeding 15 feet in 

 height occur less, than 1.5 percent of the time in all months; the maximum 

 frequency occurs in December (from unpublished wave data held at CERC) . 



Surface water temperatures in the study area average 2° to 6° Celsius 

 from January through April, 18° Celsius in August, then decrease to 6° 

 Celsius in December. At a depth of 20 meters, the water temperature 

 averages 2° to 5° Celsius from January through April, risfes to 12° Celsius 

 by August, and maintains this temperature through October. The temperature 

 declines after October and reaches 6° Celsius in December (Colton and 

 Stoddard, 1972). 



6. Geologic Setting . 



a. Topography . The land area adjacent to the primary study area is 

 comprised of three large geomorphic units (Fig. 3). These units are: (a) 

 the Boston Lowland, occupying a large central segment of the area between 

 study limits; (b) the Fells Upland to the north; and (c) the Sharon Up- 

 land to the south. Most of the Boston LoWland consists of marsh and valley 

 flats less than 50 feet above sea level and the remainder contains low 

 hills cresting at less than 150 feet elevation. The original character of 

 this landscape has been considerably altered by urban development. The 

 highlands which flank the Boston Lowland to the north and south consist of 

 rugged hills in places over 300 feet high. 



b. Basement Rocks . Most of the Boston Lowland is underlain by a 

 fault-bounded structural low in rocks of pre-Devonian age (Boston Basin) 

 filled with late Paleozoic continental sediments and interbedded igneous 

 rocks known collectively as the Boston Bay Group. Postdepositional 

 erosion carved a highly irregular surface into the Boston Bay Group rocks 

 before the onset of Quaternary glaciation (Upson and Spencer, 1964) . This 

 old topography, which now forms the bedrock surface under the Boston 



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