Reasons for Designation
The Isles of Scilly, the westernmost of the granite masses of south west
England, contain a remarkable abundance and variety of archaeological remains
from over 4000 years of human activity. The remote physical setting of the
islands, over 40km beyond the mainland in the approaches to the English
Channel, has lent a distinctive character to those remains, producing many
unusual features important for our broader understanding of the social
development of early communities.
Throughout the human occupation there has been a gradual submergence of the
islands' land area, providing a stimulus to change in the environment and its
exploitation. This process has produced evidence for responses to such change
against an independent time-scale, promoting integrated studies of
archaeological, environmental and linguistic aspects of the islands'
settlement.
The islands' archaeological remains demonstrate clearly the gradually
expanding size and range of contacts of their communities. By the post-
medieval period (from AD 1540), the islands occupied a nationally strategic
location, resulting in an important concentration of defensive works
reflecting the development of fortification methods and technology from the
mid 16th to the 20th centuries. An important and unusual range of post-
medieval monuments also reflects the islands' position as a formidable hazard
for the nation's shipping in the western approaches.
The exceptional preservation of the archaeological remains on the islands has
long been recognised, producing an unusually full and detailed body of
documentation, including several recent surveys.
The strategic location of the Isles of Scilly in the Western Approaches
assumed increased importance nationally by the mid-16th century. From that
point the islands' defences have reflected national considerations, from
initial works in the 1540s-1550s involving artillery castles on Tresco and
on St Mary's to counter threats from France and Spain, through to defences
of the mid-20th century designed to counter German landings. The early
recognition of The Garrison as the chief defensive focus on the islands has
resulted in 350 years of successive fortification of this promontory.
The powder magazine and prison in this monument have survived well, the
magazine in particular displaying the means by which such structures were
rendered blast-proof in the mid-18th century. The magazine and prison form
integral parts of their contemporary defences on The Garrison; although the
individual components of those defences each have considerable importance in
their own rights, even greater is their value as a surviving defensive system
inspired by considerations of national defence. Spatially, the rare survival
of such a complete defensive system demonstrates the relationships between its
components and the strategic methods by which those defences were intended to
be used. Through time, the fact that each successive stage in the Garrison's
defensive system has tended not to obliterate the previous defences is in
itself rare; amongst other considerations, this allows study of the developing
organisation of ground defence works and ancillary structures against changes
in armament technology and military strategic thinking through most of the
post-medieval period, within the controlled background of a single location.
Details
The monument includes a 17th-18th century powder magazine, known as the Rocket
House, together with an adjacent small prison cell, situated near the main
gateway through the defensive circuit of The Garrison, the south western
promontory of St Mary's in the Isles of Scilly.
The Garrison has long been the strategic focus for the islands' defence,
commanding their main deep water approach through St Mary's Sound and The
Road, and the chief harbour on Scilly, St Mary's Pool. The fortification of
The Garrison, in which this powder magazine and prison formed an integral
part, was undertaken in successive stages from the later 16th century to the
mid 20th century. The powder magazine and its blast walls in this monument are
a Grade I Listed Building; with the adjacent prison and their courtyard they
also form part of a monument in the care of the Secretary of State.
The Rocket House survives as a masonry magazine chamber with a ridged vaulted
roof, surrounded by a cobbled passage and enclosed within a tall blast wall.
The entire structure is built on a stance levelled deeply into the adjoining
hillslope which rises to the south and west. The backscarp of the levelled
stance is separated by a gap of approximately 5m from the outer face of the
blast wall. The central magazine chamber is almost square in plan, measuring
approximately 9.5m east-west by approximately 8.5m north-south externally. Its
thick walls are faced externally by neatly coursed ashlar slabs, with a single
doorway at the centre of the east wall closed by doors on both the inner and
outer faces. The walls are perforated by pressure-release vents forming
regular patterns: a central vent in each wall except the east forks into three
on reaching a small buttress at the centre of each external wall face; each
central vent is flanked by angled vents through the wall thickness. The east
doorway is also flanked by angled vents. These vents served to maintain
ventilation of the magazine, essential for the dry storage of powder, while
dissipating pressure from any explosion that might occur, containing the
effects of the blast within the magazine and its blast wall. The interior of
the magazine chamber has a modern raised floor, above which the walls rise
1.7m, faced by irregular stonework; above this is a ridged vault of coursed
slabs. Joist slots in the vault's lowest course indicate a former upper floor
within the vault. Externally the vault is faced by a steep, slate covered,
hipped roof rising to moulded granite ridge stones.
The magazine chamber is surrounded by a finely cobbled passage, approximately
1.75m wide; a drainage gully along its outer edge feeds into a slab-covered
drain at the centre of the south side. The outer side of the passage is
defined by the magazine's blast wall. This wall rises approximately 4.5m,
fully enclosing the passage and the walls of the magazine chamber. Its
entrance doorway is in its east wall by its north east corner, with a
segmental arch and projecting keystone; steps lead down from the doorway to
the cobbled passage. From the centre of the south side, a tall, masonry,
chimney-like structure rises from the blast wall top, supporting a lightning
conductor. The blast wall fabric has a clear horizontal joint at approximately
2.8m high, level with the top of its entrance arch and indicating its initial
height before later being raised.
A small external side chamber opens off and projects from the west side of the
blast wall north of its midpoint. The chamber is 2.2m square internally, with
a window in its north wall and angled pressure-release vents on each side of
its doorway through the blast wall. The vents may be precautionary at this
potential weak point in the blast wall to dissipate the force of a blast from
the central magazine chamber, but they may indicate that the chamber itself
could contain explosive hazards, possibly as a fuse store.
The blast wall's entrance faces a small subrectangular courtyard, defined on
the west by an extension north of the blast wall's east wall and to the south
by a wall running east from the blast wall entrance, revetting the hillslope
behind and containing the entrance to the prison. Low edging slabs define its
other sides and curving north east corner against the roads beyond. The
courtyard's east and west walls meet the blast wall at the level of its
original height. However the west wall was originally slightly lower and
sloped down to approximately 0.5m high at its north end; a joint in the fabric
shows it was later raised to its present gently sloping profile.
The south wall is also a composite of builds. In its western end beside the
blast wall entrance is a doorway, 1.7m high with chamfered jambs and lintel,
giving access to the prison. The prison extends south from the doorway,
against the outer face of the blast wall and beneath the present hillslope
surface. It is a single-roomed cell measuring 2.45m north-south by 1.15m
east-west internally, reached by steps down from the doorway in its north
wall. The doorway itself is closed by a modern wooden door. Around the prison
doorway the courtyard wall fabric is of finely jointed ashlar but from c.1m
east of the doorway this is replaced by a poorly jointed fabric indicating a
later extension to revet the slope behind.
Historical sources amplify our knowledge of this monument and the context in
which it was built. In the 1590s, after the Spanish Armada, a review of the
islands' defences identified The Garrison as the prime focus for fortification
and an artillery castle, the Star Castle, was built on its northern crest in
1593-4. A major programme of works from approximately 1601 enhanced the
controlling position of The Garrison by building a bastioned curtain wall from
coast to coast across its landward approach from the sandy isthmus linking it
to the main body of St Mary's. A quay wall was also built into St Mary's Pool
from The Garrison's north east coast, encouraging the growth of Hughtown which
rapidly became the Scillies' main settlement under the protection of the new
fortifications.
Construction of a powder magazine on the site of this monument was part of
these early 17th century works, built into the slope 40m WSW of the main
gateway through the curtain wall. It appears on 17th and early 18th century
plans which, where sufficently detailed, consistently show a rectangular
building with an east-west long axis and a short projection from its east
wall.
By 1742 the magazine was described as in a `very bad' condition, but it still
appears with its rectangular plan on a 1746 plan. An account in 1750 by Robert
Heath, a former officer of The Garrison troops, comments that the magazine
formerly suffered from dampness because its walls had been in direct contact
with the earth of the slope. Heath also notes that it had been `lately
improved by Mr Tovey', describing his actions as quarrying away the slope
behind the magazine to separate it from contact with the soil, and creating a
`square paved way' around the magazine's bomb-proof walls and roof. These
works by Master Gunner Abraham Tovey were part of a massive refurbishment
which he undertook on The Garrison defences between 1715 and 1750. Heath also
mentions the prison, separated from the magazine as today, and described as
the `Hole, or Military Prison', also suffering from severe damp.
Although not mentioning the blast wall, Heath describes an arrangement of
structures otherwise similar to those visible today; his account provides an
indication of how the 17th century-early 18th century arrangement became
transformed to the present one. While the magazine was simply a building
levelled into the slope, it would matter little if a small prison was levelled
in beside it on the roadside, giving the long rectangular magazine and the
small projecting prison on the east. However quarrying away the slope behind
the magazine to create the paved way and blast wall would force a decision
whether to include the prison within the new blast-proof bounds of the
magazine or to exclude it. There would be no reason why the prison would need
to remain within the blast-proof cordon and good security reasons why it would
be undesirable to keep prisoners confined within the powder store cordon. The
present structural arrangement reflects a decision to exclude the prison by
driving the eastern side of the paved way and blast wall across the eastern
end of the earlier long rectangular magazine building, shortening it to its
present plan and leaving the prison outside the blast wall entrance. This
implies that the system of pressure-release vents in the wall of the magazine
chamber reflect Tovey's work, as does the neat ashlar facing on the chamber's
outer walls and buttresses; the walling around the prison doorway may derive
from the 17th century fabric of the combined magazine and prison.
Subsequent alterations raising the blast wall and the courtyard's west wall
are not closely dated but had occurred by approximately 1870 when they were
first photographed. The masking of the prison beneath extended hillslope
deposits was achieved by the 1830s, possibly forming part of Tovey's
refurbishment. That change would also have required the present eastward
extension of the prison's north wall to revet the hillslope against the
courtyard.
All English Heritage signs, displays, fixtures and fittings, including service
cables, conduits and control boards, are excluded from the scheduling but the
ground beneath them is included.
MAP EXTRACT
The site of the monument is shown on the attached map extract.
