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Shaded relief map of Boston Harbor


     This shaded relief image was created by vertically exaggerating the topography four times and then artificially illuminating the relief by a light source positioned 45 degrees above the horizon from an azimuth of 350 degrees. In the resulting image, topographic features are enhanced by strong illumination on the northward-facing slopes and by shadows cast on southern slopes. The shaded relief image accentuates small features that could not be effectively shown by contours alone at this scale. Some features in the image are artifacts of data collection and environmental conditions. They include small highs and lows and unnatural looking features or patterns oriented parallel or perpendicular to survey tracklines. The orientation of the tracklines are identified by the faint parallel stripes in the image. White areas are areas of no data.

     The image illustrates the complex and wide variety of sedimentary environments in this region of the coastal ocean. The transitions between sediment types is often very sharp. Topographic features observed here were formed for the most part by glacial processes. Ice containing rock debris moved across the region, sculpting its surface and depositing sediment to form basins, knoll, banks and other features. Today, the sea floor is mainly modified by storm currents and waves from the northeast. These currents erode sand and mud from the shallow banks and transport them into the basins. Stellwagen Bank and Jeffreys Ledge are shallow banks (20-40 m water depth) covered with sand and gravel. Stellwagen Basin (100 m water depth) is floored with mud. In deeper water to the northeast (water depth 85-140 m), the spaghetti-like pattern on the sea floor are gouges (of order 5 m deep and up to 120 m wide) in the muddy sand caused by icebergs that grounded here at the close of the last period of glaciation. Present and past disposal sites are typically identified by a low mound having an unnatural-appearing roughness resulting from numerous individual dumps of material. The sites are also characterized by high backscatter material and are especially distinct when the background material is fine grained, such as in Stellwagen Basin (yellow arrow indicates Massachusetts Bay Disposal Site (MBDS)). The yellow line is the location of the new ocean outfall that began discharging treated sewage effluent from the Boston metropolitan area into Massachusetts Bay on September 6, 2000. The red line outlines the Stellwagen Bank National Marine Sanctuary.