7.0 Survey Results

Blossom Rock

Blossom Rock is located approximately 4,000 feet (0.75 miles) north of San Francisco's North Point, and 12,000 feet (2.25 miles) southeast of the other underwater rock masses.  Located near the center of the designated ship channel, Blossom Rock has been problematic to shipping in San Francisco Bay since the British Navy sloop Blossom struck it in 1826.  Originally, the rock rose to an approximate elevation of –5 feet MLLW.  As discussed above, its top was lowered three different times between 1869 and the 1930s, the last effort reducing its elevation to about –42 feet MLLW.  Presently, a buoy identified as "BR" marks Blossom Rock.

 Blossom Rock currently stands about 50 feet above the surrounding seafloor and has a flattened top.  Its surface area measures 100,380 square feet along the 55-foot contour.  The feature comprises exposed rock surrounded by unconsolidated sediments.  Pockets of sediment are also present on the rock itself.  Sand waves of two to three feet in amplitude are present on the seafloor to the east and west of the rock, but finer material blankets the depression on the south side of the Rock.  Some rock debris is located southeast of the rock, near the corner of the survey area (Sea Surveyor, Inc. 2001).

 The magnetometer data indicate relatively high levels of indiscriminate background noise in the range of 10 gammas (Figure 18).  A significant magnetic anomaly appears in the southeast portion of the survey grid, along survey transect 101.  It is a large bi-pole of approximately (-)100/(+)60 gammas.  To the northeast lies a smaller monopole of approximately 30 gammas.

Analysis of the side scan sonar data determined that the magnetic anomaly is associated with a rectangular acoustic target, approximately 125 feet long and 30 feet wide located approximately 100 feet south of the edge of Blossom Rock.  It projects above the bottom surface approximately 18 feet and appears to be the remains of a sunken barge (Figure 19) and (Figure 20).  It is designated as target Blossom-1.  The barge is lying upright on the seafloor, but may be listing to one side.  The small magnetic anomaly to the northeast of the sunken barge may be an anchor that is partially buried in the seabed. 

 A small debris field was identified in the acoustic data and labeled as Blossom-2.  Comprising a small, discrete scatter of linear targets, each measuring approximately 2.5 meters in length, the scatter does not appear to be associated with any of the magnetic disturbance observed in the area (Figure 21).  This suggests they are probably wood.

 A third target identified in the acoustic data may be associated with the small magnetic disturbance depicted at the western end of transect 98 (see Figure 18).  Analysis of the sonar image suggests the target, designated as Blossom-3, may be a small anchor with an associated length of chain (Figure 22).  Aside from these three targets, no evidence of maritime cultural material was identified in the survey of Blossom Rock.

 Shag Rocks

Shag Rocks consist of two flattened mounds atop a rock mass standing about 50 feet above the surrounding seafloor.  The mounds are the center of three rock masses that create a 4,000 foot-wide, crescent-shaped navigation hazard facing the Golden Gate.  Shag Rocks is the southeast end of a 0.5-mile long ridge of bedrock that extends to Harding Rock.  They are located about 1,800 feet (0.34 miles) north of Arch Rock, and about 4,500 feet (0.85 miles) northwest of Alcatraz Island (Sea Surveyor, Inc. 2001). 

 Shag Rocks have had their tops removed twice by underwater blasting; first in the early 1900's to a depth below 30 feet MLLW, and again in the 1930's to a depth of -35 feet MLLW.  The two flattened mounds on top of Shag Rocks are above elevation –40 feet MLLW, and the mounds are separated by a 5 foot-deep depression.  Numerous pinnacles from 5 to 20 feet in height are distributed on the south and west flanks of Shag Rocks.  Boulders and large rock debris surround the flattened tops.  Water depths increase to over 100 feet on the western corner of the survey area.  Shag Rocks comprise exposed bedrock rising from the unconsolidated sediment of San Francisco Bay.  Medium- to high-amplitude sand waves surround them, which is indicative of coarse-grained sediment (Sea Surveyor, Inc. 2001).    

 The largest magnetic anomaly found on or near Shag Rocks is a 10-20 gamma target located south of Shag Rocks in approximately 95 feet of water.  No corresponding acoustic image was identified in the side scan sonar data, suggesting that the ferrous material causing the magnetic disturbance is buried in the unconsolidated sediment (Sea Surveyor, Inc. 2001).  No other cultural material deemed to be of a potentially significant nature was identified in the side scan sonar survey. 

 Arch Rock

Arch Rock is located approximately 4,500 feet west of Alcatraz Island and 1,500 feet south of Shag Rocks.  Arch Rock is a rock mass standing about 40 feet above the surrounding seafloor, which is approximately 70–80 feet deep, except for a depression exceeding 100 feet in depth on the northwest corner of the Rock.  Most of Arch Rock's flattened top is at elevation –38 feet MLLW, but there are several small peaks or boulders on top that extend up to elevation –36 feet MLLW.  The data collected in the geophysical side scan survey clearly show a mass of exposed rock, surrounded by unconsolidated sediments.  Well-defined sandwaves are located north and west of Arch Rock, which suggests coarse-grain sediments on the seafloor.  The seafloor appears to be smooth or has small sandwaves to the east and south of Arch Rock, suggestive of fine-grained sediments.  A single 15-foot high ridge, at least 500 feet in length, leads from edge of Arch Rock towards the northwest (Sea Surveyor, Inc. 2001).

 In the magnetometer data collected during the maritime archaeology survey, two substantial magnetic anomalies were identified on Arch Rock.  Smaller magnetic targets abound throughout the survey area.  These no doubt are related to the numerous anchors and chain described by Boyd in his discussion of Arch Rock.  The source of the two large magnetic anomalies was the subject of much of the analysis of the data collected during the side scan sonar survey.  For purposes of discussion, the targets were labeled as Arch-1 and Arch-2 (Figure 23).

 Arch-1 is the northernmost magnetic anomaly.  It appears to be a monopole with an intensity of approximately +/-90 gammas, located at 37°49.7418’N/122°26.4995’W.

Analysis of the side scan sonar data at or near these coordinates determined that the probable source of this anomaly is at least two, and possibly three solid, round objects, approximately 50 to 75 centimeters in diameter, with surficial expressions ranging from 1 to 1.5 meters (Figure 24).  The magnetic signature, and the shape, size, and location of these targets suggest they may be associated with drilling activities that were conducted on behalf of the U.S. Navy in 1985 in support of a proposed magnetic degaussing facility on Arch Rock (Subsurface Consultants, Inc., 1985).  However, the coordinates provided by the Navy’s contractor indicate that the drilling took place some 40 to 50 meters to the northeast of the anomaly’s location, although the determination of these coordinates, which were calculated 16 years ago, may allow for some margin of error. 

Arch-2 is situated approximately 50 meters southeast of Arch-1.  It appears to be a tightly compacted bipole measuring +110/-330 gammas, located at 37°49.7438’N /122°26.4303’W.  The side scan sonar data indicates that the probable source of this anomaly is a sinous target, visible for approximately 7 meters of its length, that terminates in a roughly-triangular shaped object with a single projection, approximately 4.2 meters in length, and 1.3 meters wide, with a surficial expression of approximately 2 meters (Figure 25).  The target may be an anchor and chain, although its location at the listed coordinates for the degaussing station drilling that took place in 1985 also suggest it may be debris associated with that activity.

 During the side scan sonar survey, the sonar sensor’s left channel failed.  Consequently each transect was surveyed twice, once in each direction, so that the desired 100% overlap could be achieved using only the right channel.  Numerous acoustic targets were identified in the survey area.  Analysis of this data determined that for the most part, the targets were scattered pieces of shattered rock, and isolated deposits of debris, such as abandoned anchors and chain, a possible barrel or two, and other types of discarded, non-sensitive cultural materials.  

In the analysis of the data obtained during the geophysical side scan sonar survey, a length of cable was identified leading from the top of Arch Rock to an object on the seafloor near the base of the rock.  The object was determined to be 25 feet x 30 feet in size, and did not generate a magnetic signature.  The object was not seen in the acoustic data collected during the archaeological side scan sonar survey.

 No evidence of the Autocrat or the Flying Dragon was observed in either the magnetic or acoustic data.  This is consistent with the possibility that the Autocrat did not actually sink after grounding on the rock, and that the Flying Dragon is recorded as lying 270 yards east of Arch Rock, which is outside the survey area.  It is also consistent with the lack of evidence observed by the diver sent down to locate the wrecks in 1953, and the hypothesis that if either wreck once lay on the bottom adjacent to the rock, its remains would have been destroyed when the rock was blown up in 1901.

 Harding Rock

Harding Rock is the northern-most rock investigated.  It is located approximately mid-way between three land features: Alcatraz Island, Angel Island, and the Marin County coastline.  All deep draft vessels passing through the Golden Gate must traverse the Bay through the ship channel around the north tip of Harding Rock.  Because of its proximity to the deep draft ship channel, Harding Rock is considered a hazard to navigation (Sea Surveyor, Inc. 2001).

 In 1932, Harding Rock had its top lowered by blasting to an elevation –35 feet MLLW.  Presently, a buoy marked "HR" is anchored on the northern tip of Harding Rock, marking the edge of the designated shipping channel. 

 Standing about 50 feet above the surrounding seafloor, Harding Rock is the northwest end of a ridge of bedrock that extends 0.5 miles southeast to Shag Rocks.  The rock has a flattened top, with two high spots standing four to six feet higher than the surrounding rock.  Most of Harding Rock is below the 40 foot MLLW depth contour, but there is a 250 foot x 75 foot area on top of Harding Rock that is above elevation –40 feet MLLW.  The feature designated as Harding Rock is approximately 600 feet x 1,200 feet of exposed rock that rises from the overlying sediment in San Francisco Bay.  Coarse-grained sediments create high-amplitude sand waves on the seafloor north and south of the rock, but fine-grained sediments leave a smooth bottom to the northwest and northeast. (Sea Surveyor, Inc. 2001).

 With one exception, the magnetometer data revealed only slight variations in the earth’s magnetic field throughout the survey area (Figure 26).  A significant monopole of approximately 100 gammas was identified in the northwest corner of the grid, but this proved to be associated with the anchoring chain of the buoy that marks the northern extent of Harding Rock.  A few small anomalies of less than 10 gammas are scattered throughout the survey area.  One of these appears to be associated with one of two acoustic targets identified in the side scan sonar data.

 Aside from the typical scatter of debris and the widespread remains of shattered rock, only two acoustic targets suggestive of cultural material were observed in the side scan sonar data.  The first, designated as Harding-1, appears to be a length of cable or chain, measuring approximately 14 meters in length (Figure 27).  As mentioned, it is also the probable source of the small, (-)10 gamma anomaly depicted on line 7 in (Figure 26).  A second acoustic target, designated as Harding-2 is located at the southeast corner of Harding Rock and appears to be a sunken buoy.  The target had no magnetic signature, which would be consistent with an object fabricated from a non-ferrous material such as aluminum  (Figure 28).

 Golden Gate Mound

As mentioned previously, the side scan sonar data obtained in the geophysical survey and the magnetometer data collected for this project were analyzed to determine the potential for the presence of cultural material.  The magnetometer data depict a fairly uniform magnetic field, punctuated with a small number of minor magnetic disturbances in the range of 10 gammas (Figure 29).  The data obtained in the geophysical side scan sonar survey depict a relatively sterile, culturally featureless bottom.  Based on the analysis of these two data sets, WSA determined that it was unnecessary to conduct an additional remote sensing survey in the area of Golden Gate Mound.