The Rocket House 17th-18th century powder magazine and adjacent prison on The Garrison, St Mary's


Heritage Category: Scheduled Monument

List Entry Number: 1014553

Date first listed: 21-May-1963

Date of most recent amendment: 19-Mar-1998


Ordnance survey map of The Rocket House 17th-18th century powder magazine and adjacent prison on The Garrison, St Mary's
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The building or site itself may lie within the boundary of more than one authority.

District: Isles of Scilly (Unitary Authority)

Parish: St. Mary's

National Grid Reference: SV 90033 10639


Legacy Record - This information may be included in the List Entry Details.

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.


Legacy Record - This information may be included in the List Entry 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.


The contents of this record have been generated from a legacy data system.

Legacy System number: 15435

Legacy System: RSM


Books and journals
Arlott, J, Island Camera, (1983)
Heath, R, A Natural and Historical Account of the Isles of Scilly, (1750)
Heath, R, A Natural and Historical Account of the Isles of Scilly, (1750)
Heath, R, A Natural and Historical Account of the Isles of Scilly, (1750)
Ratcliffe, J, Scilly's Archaeological Heritage, (1992)
Ratcliffe, J, Scilly's Archaeological Heritage, (1992)
Laws, P, 'Isles of Scilly Museum Publications' in The Buildings of Scilly, , Vol. 12, (1980)
1358-0/8/83; Garrison Powder Magazine, DNH, List of Buildings of Special Architectural or Historic Interest, (1992)
DoE/EH, Ancient Monuments Terrier AA 72982/1-3; Garrison Walls, Scilly, (1984)
DoE/EH, Ancient Monuments Terrier for AA 72982/1-3, (1984)
Keystone Historic Buildings Consultants, Star Castle, St Mary's, Isles of Scilly, 1993, Unpublished report for EH
Keystone Historic Buildings Consultants, Star Castle, St Mary's, Isles of Scilly, 1993, Unpublished report for EH
Keystone Historic Buildings Consultants, Star Castle, St Mary's, Isles of Scilly, 1993, Unpublished report for EH
Keystone Historic Buildings Consultants, Star Castle, St Mary's, Isles of Scilly, 1993, Unpublished report for EH
Parkes, C/CAU, Scilly SMR entry for PRN 7901.03, (1994)
Parkes, C/CAU, Scilly SMR entry for PRN 7901.04, (1994)
Taken from record/deed they had read, Spoken data to MPPA from Mr & Mrs McPherson of Gatehouse Cottage, (1994)
Title: 1:2500 Ordnance Survey Maps; SV 8909; SV 8910; SV 9009; SV 9010 Source Date: 1981 Author: Publisher: Surveyor:

End of official listing