Ancient Monuments

History on the Ground

This site is entirely user-supported. See how you can help.

Two entrance graves, a prehistoric field system, and Civil War fieldworks and blockhouse on Innisidgen Hill and Helvear Down, St Mary's

A Scheduled Monument in St. Mary's, Isles of Scilly

Approximate Location Map
Large Map »
Street or Overhead View
Contributor Photos »

If Google Street View is available, the image is from the best available vantage point looking, if possible, towards the location of the monument. Where it is not available, the satellite view is shown instead.


Latitude: 49.9345 / 49°56'4"N

Longitude: -6.291 / 6°17'27"W

OS Eastings: 92191.756157

OS Northings: 12645.994325

OS Grid: SV921126

Mapcode National: GBR BXVV.P6P

Mapcode Global: VGYBY.XX11

Entry Name: Two entrance graves, a prehistoric field system, and Civil War fieldworks and blockhouse on Innisidgen Hill and Helvear Down, St Mary's

Scheduled Date: 7 October 1976

Last Amended: 16 November 1998

Source: Historic England

Source ID: 1013271

English Heritage Legacy ID: 15400

County: Isles of Scilly

Civil Parish: St. Mary's

Traditional County: Cornwall

Lieutenancy Area (Ceremonial County): Cornwall

Church of England Parish: Isles of Scilly

Church of England Diocese: Truro


The monument, on north eastern St Mary's in the Isles of Scilly, includes two
prehistoric entrance graves, one near Innisidgen Carn on top of the spur
forming Innisidgen Hill, the other on the lower northern slope of the hill,
together with a prehistoric regular field system which occupies much of the
northern flank of the hill. The monument also includes a group of fieldworks
dating to the English Civil War. The fieldworks include a length of defensive
bank and ditch, called a breastwork, which runs behind the northern and
north east coastal cliff of Innisidgen Hill and extends south eastwards along
the foot of Helvear Down to cross the landward side of Block House Point.
Three small Civil War gun batteries are included at intervals in the line of
the breastwork. At Block House Point the breastwork includes a defended
approach to a broadly contemporary strongpoint called a blockhouse situated on
the Point itself. The two prehistoric entrance graves and their immediately
surrounding areas are monuments in the care of the Secretary of State.
The south eastern entrance grave, on the spine of the Innisidgen Hill spur,
occupies a prominent position on the narrow ridge top with the ground sloping
away steeply to the north and south east. The entrance grave survives with a
slightly ovoid mound of earth and rubble measuring 9m north-south by 8m east-
west, set within traces of a flattened outer platform, up to 2m wide, whose
outer edge is visible as a slight break in slope around all sides except the
south and beside the entrance. The mound rises up to 1.8m high, with a
shallow-domed, turf-covered upper surface, uneven due to an unrecorded
antiquarian excavation and highest over the northern half of the mound. The
mound has near vertical edges defined by a kerb, generally 0.7m high, built of
both contiguous edge-set slabs and coursed slab walling, two to three courses
high, but absent due to later robbing along parts of the southern side.
The mound contains a well preserved chamber aligned on an ESE-WNW axis, with
its entrance situated at the eastern side of the mound. The chamber measures
4.6m long by up to 1.5m wide and 1.2m high. The chamber walls are built of
large edge-set slabs overlain where necessary, and all along the NNE wall, by
coursed smaller slabs and rubble to remove the irregularities and produce a
consistent chamber height. The WNW end of the chamber is closed by a single
large edge-set slab. The walling gives a slightly asymmetrical chamber plan,
with a straight NNE wall and curving, concave, SSW wall. The chamber is roofed
by five large slabs, called capstones, up to 3m long, 1.1m wide and 0.6m
thick, each resting on the chamber walls, spanning the chamber width, and laid
side by side along the chamber from its WNW end-slab, leaving uncovered the
eastern 1.25m of the chamber at the entrance. The capstones' upper surfaces
are exposed in the mound surface along their southern sides and at the
The other entrance grave in the monument is located 90m to the north west,
near the foot of the northern flank of Innisidgen Hill. This entrance grave
survives with a circular earth and rubble mound measuring 8.5m in diameter and
rising up to 1.7m high from its downslope, northern edge. Parts of a
peripheral kerb of single laid and edge-set slabs survive along the north,
north east and east sides of the mound and flanking the chamber entrance on
the SSW and SSE sides. The kerbing is up to 0.5m high and corresponds with a
marked step in the mound's profile in those sectors. Except on the north, that
step defines the edges of a shallow-domed central platform, 5.5m in diameter,
with the mound's gentle outer slope extending a further 0.5m - 1.5m beyond the
The mound contains a central chamber on a north south long axis with the
entrance at the southern side of the mound's kerb line. The chamber measures
5.4m long by 1.3m wide and 1m high. Its parallel side walls are built mainly
of coursed rubble, with an edge-set slab closing the northern end and large
slabs incorporated into the central and northern parts of the east wall. The
slabs forming the coursed walling over the southern 1.5m of the chamber are
markedly smaller than those deployed elsewhere. Because the chamber floor
is level despite the hillslope, near the southern entrance it is
considerably below ground level and rises steeply to the southern edge of the
mound. Two capstones survive, spanning the central and southern parts of the
chamber, the largest measuring 2m long by 1m wide and 0.5m thick. The chamber
of this entrance grave formerly contained an earth and rubble fill that was
cleared during an unrecorded and unauthorised excavation in 1950.
The prehistoric regular field system survives over 0.6ha of the northern
slope of Innisidgen Hill adjoining the two entrance graves, extending over an
area measuring at least 130m WNW-ESE by 60m NNE-SSW. The field system is
defined by a network of earth and rubble banks, up to 2.5m wide and 1m high
though commonly much slighter, aligned on two axes at approximate right angles
to each other. At least six banks are visible on a WNW-ESE axis, almost along
the contour but descending gradually to the WNW, while at least four banks
with some incorporated slabs run uphill on a NNE-SSW axis, linking various of
the other banks. The WNW-ESE `contour' banks are emphasised by a marked
build-up of soil, called a lynchet, against their uphill, SSW, sides due to
the effects of early cultivation on the steep slope. The southernmost
`contour' bank merges with the northern edge of the outer platform around the
entrance grave on the spine of the spur. Near the WNW edge of the field
system, another such lynchetted bank, 0.5m high, extends ESE from the
north east edge of the lower entrance grave. After several metres, this bank
reaches a corner with one of the banks running uphill; to the east of this
corner is a distinct original gateway gap before another contour bank appears,
running to the ESE. Yet another lynchetted bank extends WNW from a point 5m
beyond that lower entrance grave's south west edge.
In addition to the surviving visible prehistoric remains, recent pollen
samples from near Innisidgen Carn have indicated a vegetal history in
this area involving clearance of early oak-hazel woodland, considered to have
occurred during the prehistoric period. The clearance was succeeded by
grassland with some cereal cultivation, followed later by the growth of
The north western sector of the Civil War breastwork runs behind the coastal
cliff of Innisidgen Hill, adjacent to the lower edge of the prehistoric field
system. The breastwork survives as an earth and rubble bank, up to 1.5m wide
and 0.7m high, with a ditch, up to 1.75m wide and 0.2m deep, along its
landward side. Occasional larger rubble blocks, up to 0.4m across, are visible
in the bank surface, the remains of a rough facing. The breastwork is visible
over a distance of 520m, following an angular course combining several almost
straight lengths closely following the line of the coastal cliff from
Innisidgen Hill to Helvear Hill, with only minor breaks in the bank due to
coastal erosion and one break of 30m to the north east of the lower entrance
grave due to the creation of a recent slipway through the cliff.
The breastwork incorporates three contemporary gun batteries in its line,
130m-150m apart. The north western battery is located 25m north west of the
lower entrance grave, the central battery is below Innisidgen Carn, and the
south eastern is midway between Innisidgen Carn and Block House Point. The
batteries are visible as trapezoidal levelled platforms, 6m-18m wide and 3m-5m
from front to rear, defined to each side by a sharp angle seawards in the
breastwork bank, which extends along the forward edge of the battery and is
increased in size, up to 3m wide and 1.4m high, with further traces of facing
blocks. Due to their extreme cliff-edge situations, parts of the bank around
the north western and south eastern batteries have been truncated by the
eroding cliff edge.
At the south east end of the monument, the breastwork across the landward side
of Block House Point is broken by the constricted entrance to an approach way,
flanked by extensions of the breastwork bank, to a blockhouse situated on the
tip of the point. The blockhouse survives on a slight raised grassy knoll on
the Point and is visible as a rectangular structure defined by earth and
rubble walls on its north west, north east and south west sides, with a
levelled internal area measuring 6m north west-south east by 5.5m north east-
south west; the south east side is open. The turf-covered walls, 1m wide, rise
to 1.9m high above the surrounding ground level and 0.8m above the blockhouse
interior. The breastwork, batteries and blockhouse in this monument form part
of an integrated system of Civil War coastal defences which survive
extensively around St Mary's and include breastworks bordering potential
landing places and near important settlements and installations, coupled with
a system of batteries commanding complementary fields of fire over the waters
around much of the island's coast. The breastwork in this monument provides
cover over this relatively low-lying portion of coastal cliff, while the
batteries command fields of fire over Crow Sound, the principal approach for
shipping from the east into the Isles of Scilly. The blockhouse commanded a
wide outlook over this important stretch of water.
Beyond this monument, other broadly contemporary prehistoric field systems and
settlement sites are recorded near the coast at Bar Point, from 275m to the
north west, while prehistoric funerary cairns are located on Helvear Down,
120m south west of the monument's north west end and on Helvear Hill, 110m
south west of the monument's south east end. The Civil War batteries
complement the fields of fire of larger, more elevated batteries situated on
Helvear Hill to the south and above Bar Point to the north west. Further
lengths of breastwork also survive along the coast in both directions.
All English Heritage signs, plinths and signposts, boundary markers
and modern gravel surfaces are excluded from the scheduling, although the
ground beneath is included.

The site of the monument is shown on the attached map extract.
It includes a 2 metre boundary around the archaeological features,
considered to be essential for the monument's support and preservation.

Source: Historic England

Reasons for Scheduling

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'
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.

Entrance graves are funerary and ritual monuments dating to the later
Neolithic, Early and Middle Bronze Age (c.2500-1000 BC). They were constructed
with a roughly circular mound of heaped rubble and earth, up to 25m in
diameter, the perimeter of which may be defined by a kerb of edge-set slabs
or, occasionally, coursed stone. The mound contains a rectangular chamber
built of edge-set slabs or coursed rubble walling, or a combination of both.
The chamber was roofed by further slabs, called capstones, spanning the walls.
The chamber was accessible via a gap in the mound's kerb or outer edge and
often extends back beyond the centre of the mound. Excavations within entrance
graves have revealed cremated human bone and funerary urns, usually within the
chambers but on occasion within the mound. Unburnt human bone has been
recovered but is only rarely preserved. Some chambers have also produced
ritual deposits of domestic midden debris, including dark earth typical of the
surface soil found in settlements, animal bone and artefact fragments.
Entrance graves may occur as single monuments or in small or large groups,
often associated with other cairn types in cemeteries. They may also occur in
close proximity to broadly contemporary field boundaries. The national
distribution of entrance graves is heavily weighted towards the Isles of
Scilly which contain 79 of the 93 surviving examples recorded nationally, the
remaining 14 being located in western Cornwall.
Regular field systems form one of several types of early field layout which
occur close to entrance graves, and which is known to have been employed in
the Isles of Scilly from the Bronze Age to the Roman period (c.2000 BC-AD
400). They comprise a collection of field plots defined by boundaries laid out
in a consistent manner, along two dominant axes at approximate right angles to
each other. This results in rectilinear fields which may vary in size and
length:width ratio within the field system. The fields are bounded by rubble
walls or banks, often incorporating edge- or end-set slabs. Regular field
systems may be associated with broadly contemporary settlement sites such as
stone hut circles. Some regular field systems on the Isles of Scilly have a
distinctive association, rarely encountered elsewhere, whereby certain field
boundaries directly incorporate or link prehistoric funerary monuments,
including some entrance graves. Although no precise figure is available,
regular field systems form one of the principal forms of prehistoric field
system which survive in over 70 areas of the Isles of Scilly. They provide
significant insights into the physical and social organisation of past
landscapes and give evidence for the wider context within which other
nationally important monuments were constructed.
Civil War fieldworks on the Isles of Scilly are earthworks which were raised
during military operations between 1642 and 1651 to provide temporary
protection for infantry or to act as gun emplacements. The earthworks, which
may have been reinforced with revetting or palisades, consist of earth and
rubble platforms or banks and ditches. Three main types of Civil War fieldwork
have been recognised on the Isles of Scilly: breastworks, batteries and
platforms, which could be deployed separately or in combination to form a
defensive complex.
Breastworks are earth-and-rubble banks, up to 4m wide and 1.7m high but
generally much smaller, which on the Isle of Scilly run beside the coastal
cliff edge and are usually accompanied by a ditch along their landward side.
Sixteen surviving breastworks are recorded on the islands.
Batteries are a levelled area or platform, generally up to 20m across,
situated on a hilltop or terraced into a slope to serve as a gun emplacement.
They vary considerably in size and shape and are usually partially or wholly
enclosed by a bank, occasionally incorporating one or two outer ditches.
Twenty batteries survive on the Isles of Scilly, several connected by
breastworks. Adjacent to some batteries are examples of the third fieldwork
type, platforms. These are partly terraced into, and partly out from, sloping
ground and represent sites of lookouts and temporary buildings. Eight such
platforms, measuring up to 12m by 8m in size, are known to survive on the
islands. The fieldworks were designed to defend the deep water approaches to
the islands, especially St Mary's where most examples are found. Fieldworks
are also known from Tresco, Bryher, Samson, St Agnes and Gugh. The
circumstances of their construction are recorded in contemporary historical
documents which indicate most were produced by the Royalist forces which
controlled the islands for the entire Civil War period except during 1646-8.
The Civil War fieldworks of the Isles of Scilly form a major part of the 150
surviving examples of fieldworks recorded nationally. They present an
unusually complete system of fortifications from this period, both in the
surviving range of fieldwork types represented and in the surviving pattern of
their strategic disposition.
The fieldworks and fieldwork complexes were occasionally associated with other
classes of defensive monument, including blockhouses. Blockhouses are small,
strongly-built defensive structures, built from the late 14th to mid 17th
centuries and designed to house guns and protect the gunners and ammunition
from attack. Blockhouses vary considerably in form, construction and ground
plan but were typically sited as forward defences to cover anchorages,
harbours, other defences and their approaches. Of the 27 blockhouses with
extant remains recorded nationally, three are located on the Isles of Scilly,
each of a different design, built during separate periods and for differing
purposes, demonstrating well the diversity of this class of defensive

The prehistoric entrance graves and field system in this monument have
survived well. Despite some disturbance evident at both entrance graves, each
displays clear evidence for its mode of construction, including unusual
features such as the asymmetrical chamber in the south east entrance grave and
the graded wall coursing along the chamber of the north west entrance grave.
These features and their good overall survival have resulted in their frequent
mention in reviews of this monument class. Their conjunction with a broadly
contemporary field system illustrates well the unusual and distinctive
integration of such funerary monuments into the farming landscape during the
prehistoric period on the Isles of Scilly. The value of the relationships
between the prehistoric features in this monument is amplified by the pollen
evidence for the environmental context within which they were built. Further
buried environmental data will typically survive within the considerable
lynchet deposits of the field system.
The Civil War breastwork, batteries and the blockhouse in the monument have
survived well as an inter-related complex of fieldworks, despite some
encroachment by the coastal cliff. Their situation and the survival of
extensive documentation giving the historical context in which they were built
demonstrate clearly the strategic methods employed by the Civil War military
forces and the function of fieldworks within them. This is emphasised by the
survival nearby of a series of complementary breastworks and batteries
flanking this important maritime approach, of which the fieldworks in this
monument formed an integral part.

Source: Historic England


Books and journals
Ashbee, P, Ancient Scilly, (1974)
Ashbee, P, Ancient Scilly, (1974)
Ashbee, P, Ancient Scilly, (1974)
Ashbee, P, Ancient Scilly, (1974)
Ashbee, P, Ancient Scilly, (1974)
Ashbee, P, Ancient Scilly, (1974)
Ashbee, P, The chambered Tombs on St Mary's, Isles of Scilly, (1963), 9-18
O'Neil, BH St J, Ancient Monuments of the Isles of Scilly, (1960)
O'Neil, BH St J, Ancient Monuments of the Isles of Scilly, (1960)
Thomas, C, Exploration of a Drowned Landscape, (1985)
Thomas, C, Exploration of a Drowned Landscape, (1985)
Thomas, C, Exploration of a Drowned Landscape, (1985)
Ashbee, P, 'Cornish Archaeology' in Bant's Carn, St Mary's: An Entrance Grave Restored And Reconsidered, , Vol. 15, (1976), 11-26
Ashbee, P, 'Cornish Archaeology' in Bant's Carn, St Mary's: An Entrance Grave Restored And Reconsidered, , Vol. 15, (1976), 11-26
Ashbee, P, 'Cornish Archaeology' in Bant's Carn, St Mary's: An Entrance Grave Restored And Reconsidered, , Vol. 15, (1976), 11-26
Ashbee, P, 'Cornish Studies' in George Bonsor: An Archaeological Pioneer From Spain On Scilly, , Vol. 8, (1980), 53-62
Dimbleby, G W, 'Cornish Studies' in A buried soil at Innisidgen, St Mary's, Isles of Scilly, (1977), 5-10
Dimbleby, G W, 'Cornish Studies' in A buried soil at Innisidgen, St Mary's, Isles of Scilly, (1977), 5-10
Evans, J G, 'Cornish Studies' in Excavations at Bar Point, St Mary's, Isles of Scilly, 1979-80, , Vol. 11, (1983), 7-32
Fowler, P J, Thomas, A C, 'Antiquity' in Lyonesse Revisited; the early walls of Scilly, , Vol. 53, (1979), 175-189
Saunders, A D, 'Cornish Archaeology' in Harry's Walls, St Mary's, Scilly: new interpretation, , Vol. 1, (1963), 85-91
Scaife, R G, 'Cornish Studies' in A history of Flandrian vegetation in the Isles of Scilly, , Vol. 11, (1984), 33-48
Scaife, R G, 'Cornish Studies' in A history of Flandrian vegetation in the Isles of Scilly, , Vol. 11, (1984), 33-47
Scaife, R G, 'Cornish Studies' in A history of Flandrian vegetation in the Isles of Scilly, , Vol. 11, (1984), 33-47
AM7 & Ancient Monuments Terrier for SI 352,
consulted 1994, Parkes, C, AM 107 for Scilly SMR entry PRN 7454, (1988)
consulted 1994, Parkes, C, AM 107 for Scilly SMR entry PRN 7454.01, (1988)
consulted 1994, Parkes, C, AM 107 for Scilly SMR entry PRN 7454.04, (1988)
consulted 1994, Parkes, C, AM 107 for Scilly SMR entry PRN 7455, (1988)
consulted 1994, Parkes, C, AM 107 relating to Scilly SMR entry for PRN 7453.01, (1988)
Parkes, C, AM 107 for Scilly SMR entry PRN 7454, (1988)
Parkes, C, AM 107 for Scilly SMR entry PRN 7454.03, (1988)
Parkes, C, AM 107 for Scilly SMR entry PRN 7456, (1988)
Parkes, C, AM 107 relating to Scilly SMR entry PRN 7453, (1988)
Parkes, C, AM 107 relating to Scilly SMR entry PRN 7453.02, (1988)
Rees, S E, AM7 scheduling documentation for CO 1031, 1975,
Saunders, AD, Recent Fortifications and Defences Works Assessment, 1992, Unpubl. draft, p.97; Innisidgen bl.
Title: 1:2500 Ordnance Survey Map; SV 9212
Source Date: 1980

Title: 1:2500 Ordnance Survey Maps; SV 9112 & SV 9212
Source Date: 1980

Source: Historic England

Other nearby scheduled monuments is an independent online resource and is not associated with any government department. All government data published here is used under licence. Please do not contact for any queries related to any individual ancient or schedued monument, planning permission related to scheduled monuments or the scheduling process itself. is a Good Stuff website.