Page 210 - Vol_2_Archaeology of Manila Galleon Seaport Trade

10 Searching for the San Francisco (1609), a Manila Galleon … 181

composed of several reefs in shallow waters, potentially hazardous for sailing,
selected by examining the seabed topography data. The rocks of the valleys were
extensively covered by large kelp (Ecklonia cava). The low visibility in the waters
as well as the strong surface current and surge at the bottom restricted the effec-
tiveness of the diving survey with a metal detector. The kelp and water conditions
prevented us from conducting ef!cient circular and straight-line searches. The
coarse sand !lls the valleys, in which the underwater metal detector helped us to
locate corroded ferrous objects beneath the surface of the sand. Later these were
identi!ed as a metal pipe, plates and rivets associated with the British-built ship MV
Rubens, which sank in the area in the 1950s.
In 2017, following the completion of the seabed topography recording, a remote
sensing survey with a cesium-vapor marine magnetometer was conducted to detect
possible ferrous remains on the seabed. It has been presumed that in the shallow
waters near the shore there would be more anomalies related to the wreckage of the
MV Rubens and other modern iron objects, such as arti!cial reefs. The 2017–2018
survey season was prior to an offshore reef approximately six kilometers from the
coast, known as Mashione (Fig. 10.4). The detailed seabed topography data have
revealed that two large standing rocks compose the offshore reef. The linear tran-
sects of the marine magnetometer surveys consisted of forty lines in a north-south
direction and forty lines perpendicular to these in an east-west direction, twenty
2
meters apart, covering 1.4 1.2 km . A few anomalies were detected that appear
not to be related to seabed topography changes and sediments. A diving search was
conducted on some of the anomalies, but apart from the detection of modern debris,
clear evidence of ferrous remains related to the San Francisco was not identi!ed on
the seabed. At the area of one of the anomalies on the south side of the reef at a
depth of about forty meters, a volcanic stone, has an oval shape, was found. The
surface of the stone is covered by marine organisms such as barnacles. Its weight is
about 2.8 kg and the diameter measures 13 mm. It is a volcanic stone identi!ed as
andesite, with inclusions of pyroxene, peridotite, and feldspar. The !ne-grained
nature and volcanic origin of the stone does not match the local geology of Onjuku,
but it is highly disputable if the stone is related to the San Francisco (Fig. 10.5).

Fig. 10.5 Oval shaped
volcanic stone from the study
area at Onjuku in Chiba of
Japan