HMS Colossus - debris field survey

In 2004 CISMAS started a survey of the debris field of the Colossus wreck site in the Isles of Scilly. It has been clear since the discovery and designation of the stern section of Colossus that parts of the wreck were spread over a wide area. There was also some doubt as to exactly where the original discoveries made by Roland Morris were located. Although material has been recovered from the area around the wreck of Colossus, in most cases the precise location from which it was recovered is not known. For this reason the CISMAS debris field survey set out to characterise and map the nature and extent of the surviving debris.This was undertaken in three stages.

Firstly, the 2004 debris field survey concentrated on investigating the magnetic anomalies identified by the 2002 ADU magnetometer survey. Secondly, a geophysical survey of the debris field was undertaken by CISMAS in 2005. This consisted of a magnetometer survey of the areas not covered by the 2002 ADU survey, a sidescan sonar survey and a bathymetric survey. Lastly, the targets identified in the geophysical survey were investigated and mapped in the 2005 debris field survey.

Geophysical survey

A magnetometer survey of the debris field was carried out by the Archaeological Diving Unit in 2002 but this only covered part of the designated area. CISMAS carried out a geophysical survey of the remainder of the designated area in April 2005. The survey was carried out by six members of CISMAS and took six days; it was successfully completed despite less than perfect weather conditions.

The area covered by the ADU (left) and CISMAS (right) magnetometer surveys.

The geophysical equipment employed consisted of a WAAS/EGNOS enabled GPS unit for position fixing. Bathymetric data was collected using a dual frequency echo sounder. The magnetic data was acquired using a caesium vapour marine magnetometer, and the sidescan sonar used was an Imagenix Sportscan.

The distribution of magnetic anomalies recorded during the 2005 geophysical survey

A total of 291 magnetic anomalies and 59 sidescan sonar targets were identified during the 2005 geophysical survey. A number of the magnetic anomalies had a known cause, for example the stern site itself or known iron wreckage within the designation such as the Little Western stern section. In addition, numerous anomalies were plainly registering the same target from adjacent track lines. The targets had estimated weights varying between 22,000kg (Colossus stern section) and 2kg (uninvestigated target).

A sidescan sonar image and target	The results of the bathymetric survey

The bathymetric data was collected at the same time as the magnetometer survey. As well as being used in the interpretation of the magnetometer data, the bathymetric data collected around Colossus was used to produce detailed contour maps of the seabed. The seabed contour maps were useful in determining possible grounding positions for Colossus when she was originally wrecked.

Debris field survey

CISMAS undertook two searches of the Colossus debris field, one in 2004 and the other in 2005. In both cases fairly small teams of divers were involved, eleven in the first year and thirteen in the second year. The principal aim of the CISMAS debris field survey was to characterise and map the nature and extent of the surviving debris. This was achieved by investigating the magnetic anomalies identified in the ADU and CISMAS geophysical surveys.

Targets were investigated by deploying a heavy shot at the position of a geophysical target, around which a pair of divers could perform a thorough circular search. The dive briefings always included the credo that it was better to have searched a 3m circle thoroughly than a 20m circle incompletely.

Divers conducting a circular search

The position of any artefacts located was recorded by the measurement indicated on the distance line and a compass bearing taken with a hand-held compass back along the distance line to the shot. Any artefacts were sketched, photographed (using small digital underwater cameras), measured and described.

The distribution of circular searches conducted during the 2004 (left) and 2005 (right) debris field surveys

In all, 129 targets were investigated during the 2004-5 CISMAS debris field survey. A total of over 100,000 square meters of seabed have been searched resulting in 102 artefacts being mapped and recorded. Below are just a few examples of the artefacts located over the course of the project.

CM0052

At 10 nT, the predicted mass of target CM0052 was 730kg. The anomaly was caused by a broken angle crown anchor, which lies 1.5m from the position indicated by the magnetic anomaly.

CM0145

The predicted mass of target CM0145 was 370kg. The anomaly was caused by debris including iron concretions and sections of heavilly gribbled timber. The closest of these debris was 3m from the position indicated by the magnetic anomaly.

CM0221 The predicted mass of target CM0221 was 1750kg. The anomaly was caused by a 19th century iron anchor, which was found 7m from the position indicated by the magnetic anomaly.

The debris field is fairly widespread to the SW of the stern section. This area of debris also extends beyond the limit of the designated area. There is relatively little debris around the surviving stern timbers of the wreck. This may be the result of the more comprehensive breaking-up of the bows of the wreck while the stern has remained relatively intact. An area of wreckage some 170m SW of the stern wreck was located. This appears to be a small section of articulate wreckage.

The distribuion of artefacts identified during the 2004 and 2005 debris field surveys

Unfortunately, a great deal of material has been removed from the seabed around Colossus over the years. The distribution of material mapped by CISMAS represents the objects other divers have missed or chosen to leave on the seabed. The large quantity of Colossus material on display in St Marys museum is testament to the information on the wrecking of Colossus which is now sadly lost.

Garrison guns

Colossus should have been armed with 28 32lb guns on the lower gun deck, 28 18lb guns on the upper gun deck and 18 9lb guns. The upper deck guns in position on the stern of Colossus are 18lb guns of the Armstrong pattern. Of these 74 guns, only 10 are now visible on the sea bed.

Roland Morris excavated on the site from 1967 to 1983. The plan of his excavations shows 22 guns on the seabed. None of these guns were detected on either the CISMAS or ADU magnetic surveys. It is probably reasonable to assume that Mr Morris followed his usual practice of removing all nautifacts’ from the seabed. This accounts for a further 22 of Colossus' 74 gun armament.

There is a local tradition on St Marys that many of the guns on the Garrison came from the wreck of Colossus. During the 2005 debris field survey CISMAS undertook a survey of the guns on the Garrison and on the quay in order to determine how many of these guns could be from Colossus.

32lb Blomefield pattern sea-service guns on the Garrison (left) and the wreck of Colossus (right)

This survey has shown that of the 17 guns located, five are consistent with what we know about the Colossus guns. A further four guns could have been from Colossus if she had both Armstrong and Blomefield 32lb guns on her main gun deck.

For a full account of the survey methodologies employed, artefacts identified and interpretaions drawn during the CISMAS debris field survey, download the project reports from the Download Centre.

The CISMAS deris field survey was made possible by a Local Heritage Initiative grant.