Reasons for Designation
Bodmin Moor, the largest of the Cornish granite uplands, has long been
recognised to have exceptional preservation of archaeological remains. The
Moor has been the subject of detailed archaeological survey and is one of the
best recorded upland landscapes in England. The extensive relict landscapes of
prehistoric, medieval and post-medieval date provide direct evidence for human
exploitation of the Moor from the earliest prehistoric period onwards. The
well-preserved and often visible relationship between settlement sites, field
systems, ceremonial and funerary monuments as well as later industrial remains
provides significant insights into successive changes in the pattern of land
use through time.
Streamworking formed one of the chief methods of mining throughout the history
of the tin industry, and was the predominant method before the 17th century.
The method used the high specific gravity of tin ore, cassiterite, to separate
it from surrounding `waste' such as other minerals, earth and grits. It
involved diverting and channelling a water supply across the ground to be
worked, carefully controlling its speed of flow to remove waste material while
retaining the tin ore. The ore-bearing ground was put into the channel for
separating, usually from the upstream or upslope side. The tin ore and larger
grits left behind were dug from the channel floor for secondary ore-dressing
nearby, while larger rubble was dug out and heaped along the channel's
downslope side, creating a linear spoil mound. Once a channel became too wide
to control the speed of water flow, the rubble soil was used to define a new
one, and so the streamworking advanced upstream or upslope until there was no
further ore-bearing ground that was economical to work.
The end result of streamworking is a broad steep-sided gully from which an
often considerable volume of waste material has been removed; in the floor of
the gully, various patterns of linear rubble dumps reveal the method and
sequence of working, with the earlier dumps sometimes masked beneath silts
washed down from the later working. Outside the streamwork gully may be leats
and reservoirs which served its water supply.
Streamworking was applied to two types of ore-bearing deposit, both separated
from the parent lodes by weathering. In alluvial deposits, tin ore
accumulated with other eroded materials in valley floors, becoming partly
concentrated by natural sorting in the water-rich environment. Here the
valley floor stream provided the water supply necessary from streamworking.
In elluvial deposits, tin ore occurs in more poorly sorted subsoils weathered
directly over the parent lodes, or on slopes and in hollows down which they
have drifted under gravity and subsoil slumping. Exploitation of these by
streamworking often required a considerable catchment area, and used leats and
small reservoirs. Streamworks provide our main source of evidence for the
methods employed in tin mining and its scale in the landscape during the early
and post-medieval periods, aspects for which historical documentation is
scanty and inadequate.
The elluvial streamwork at Gonamena has survived well. It retains a good range
of features, of which some; the north-south channel, its dumps, leat and
reservoirs, demonstrate the basic methods of streamworking while others; the
prospecting pits, the eastern extension, its reservoir, water course and
relationship with the earlier dams, clearly illustrate the development and
changes in the streamwork through time. The streamwork contains one of the
broadest and deepest of known streamwork gullies and is unusual in combining
two such complementary gullies, working elluvial deposits in the hollow and
over the adjacent lode itself. Further details of the streamwork's earlier
phase working will be preserved beneath the later silts on the floor of its
southern sector.
Details
The monument includes an early post-medieval tin mine, of a type called a
streamwork, near Gonamena on the lower western slopes of Caradon Hill on south
eastern Bodmin Moor.
The Gonamena streamwork is visible as a large and deep gully which, overall,
extends along roughly 800m of the lower western flank of Caradon Hill. It
results from the exploitation of tin ore, cassiterite, which had been detached
from its parent lodes by weathering to form an elluvial deposit both overlying
the lodes and accumulated in a slight trough north east of the hamlet of
Gonamena. The methods used to extract the ore involved careful manipulation of
water to flush away unwanted soil, clays and grits, leaving behind the heavier
tin ore. The resulting physical remains include: a deep channel along the
course of the former elluvial deposit, from which the tin ore has been removed
and considerable quantities of waste material have been washed away; dumps of
the larger waste rubble in the channel floor which also directed the water
used to work the ground immediately upslope; and remains of water-courses,
called leats, and reservoirs which brought and concentrated water to operate
the streamwork.
The Gonamena streamwork combines two main elements: a north-south channel
extending 675m, following the elluvial deposit in the trough along the hill's
western foot, and an eastward extension from the north of that channel, which
tapers in width and depth over its 260m length to follow elluvial deposits
over the parent lodes and possibly exploit upper levels of the lodes
themselves by cutting an open trench along them, called a stockwork.
The north-south channel is considered to have formed the initial phase of the
streamwork, during which the lodes underlying the eastern extension will have
been revealed. The channel has a gentle `S'-curve and ranges generally from
40m to 70m wide, its steep sides descending up to 16m to the streamwork floor.
A stream runs roughly centrally along the floor, meandering around remains of
dumps and modifying their surface expression by silting. Where surface scrub
and later deposits allow inspection, traces of linear dumps are visible, about
2m wide and 0.25m high, lying at an angle to the streamwork's long axis and
with their downstream ends towards the centre of the channel. The southernmost
185m of the channel has been modified and narrowed by the intrusion into its
western side of revetted dumps and a finger dump, beyond this scheduling, from
the mid- to later 19th century Gonamena Mine. North of those dumps and within
this scheduling, further spoil from that later mine forms a rubble spread over
the west of the streamwork floor.
The eastward extension of the streamwork tapers from 140m wide and 16m deep on
the west side where it meets the north-south channel, to a rounded eastern tip
10m wide and about 4m deep. The sides of the eastern extension drop steeply to
the floor which here contains a pattern of linear dumps, some over 1m high,
both parallel with and angled to the long axis of the extension. Terraced out
from the extension's northern slope, near its junction with the north-south
channel, is a rounded reservoir up to 25m across, which is considered to have
served an ore-processing area on the streamwork floor below after
streamworking had progressed eastward past this point. Another later feature
which may have supplied water for ore-dressing on the floor is a narrow leat
which originates against the northern side of the extension near its upper
end, cutting through several dumps along its course and revetted variously by
coursed rubble and edge-set slabs.
Beyond the northern edge of the streamwork are several closely related or
later features. Extending at least 135m north west from the northern end of
the north-south channel is a leat, now dry, which directed water to the
streamwork; the final 35m of the leat beside the head of the streamwork may
have been modified by the ditch of a post-medieval hedgebank. A small upright
slab within that hedgebank ditch as it descends into the head of the
streamwork may be a post-medieval common land boundary-marker. Around the head
of the streamwork's north-south channel is a dense scatter of prospecting pits
used for seeking underlying lodes after the end of work on the streamwork
channel. The pits, 1m-6m across, 0.2m-2m deep and linked by their spoil heaps,
occupy a well-defined zone 20m-40m wide around the upper end of the
streamwork. Towards the east of the eastern extension, its northern crest
crosses a group of three closely-spaced and parallel dams whose earthen banks
extend 20m-50m to the north, each with a slightly sunken reservoir hollow 3m-
4m wide behind the dam and two with clear central breaks which formerly held
sluice gates. Their truncation by the streamwork extension rendered them
useless in their original form and confirms their earlier date, most probably
to store water to work the north-south channel. However after truncation they
were modified by a bank linking the western two dams behind the crest of the
extension's scarp, forming a smaller reservoir contemporary with the
extension.
Only 40m beyond the southern end of this monument, the upper valley of the
River Seaton contains a broadly contemporary streamwork exploiting the
alluvial concentration of tin ore along the valley floor, though much of that
has been disturbed or overdumped by the spoil and processing areas of the
intensive 19th century mining in the vicinity.
Working of this monument is likely to have begun during the medieval period
but its earliest documentary reference relates to the `tynworks' at Gonamena
in 1662, and, in 1690, to the `Gunnamanna streamworks' and the `Mill Bounds',
all now called `Gunnamanna new pitched'. Exploitation may have continued into
the 18th century but had ceased by the 19th century, the streamwork being
depicted to its present extent on the earliest Ordnance Survey drawings of
1805.
The electricity supply cables and their poles, all modern gates, fences and
the surface of the embanked track across the south of the streamwork are
excluded from the scheduling, although the ground beneath them is included.
MAP EXTRACT
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.
