Reasons for Designation
During the medieval and early post-medieval period, tin ore extracted from
mines was taken to stamping mills to be crushed, using heavy iron-shod stamps
attached to the lower end of vertical wooden posts called lifters, which were
raised using a water driven rotating axle. Thus raised, the stamps fell under
gravity onto the ore, crushing it between the stamp's head and a hard slab of
rock called the mortar stone. There were two types of stamping machinery. The
first, known as dry stamps, involved the crushing of the ore without use of
water, and appears to have been employed throughout much of the medieval
period until the introduction of wet stamping in the 16th century. Wet
stamping utilised a constant flow of water to carry the tin crushed by the
stamps through a fine grate into a channel, to be carried in suspension to a
settling pit from where it could be collected for dressing. Dressing involved
separating the lighter waste minerals from the heavier cassiterite (tin oxide)
using water. Much of this work was carried out in sloping rectangular or
triangular shaped boxes called buddles where, to prevent premature
sedimentation, shovels were used to agitate the contents.
The original number of stamping mills on Dartmoor is unknown, but at least 60
survive. Those with associated dressing floors are relatively rare. All well
preserved examples are considered to be of natural importance.
The two stamping mills 200m and 175m north of Norsworthy Bridge survive well
and together contain a rich array of archaeological evidence relating to the
crushing and processing of tin ore. The mill 175m north of Norsworthy Bridge
contains a variety of worked stones, which together illustrate that this mill
functioned for a considerable time. By contrast, the mill 200m north of
Norsworthy Bridge may have been relatively short lived, but unusually for
Dartmoor, a triple head of stamps was employed.
Details
The monument, which falls into two areas of protection, includes two stamping
mills together with their leats, a dressing floor and part of a tinwork
situated on either side of the River Meavy. The northern stamping mill
includes a sub-rectangular building measuring 7.3m long by 3.7m wide denoted
by a substantial drystone wall standing up to 1.7m high. Within the mill are
at least six mortar stones, one of which has been reused at least four times
as an axle bearing stone. Some of the mortar stones have more than two hollows
indicating that they have been reused. The pattern of hollows on the stones
indicates that double stamps were used within this mill. Two stones within the
building have rectangular sockets cut into one side, whilst a third stone has
a complete rectangular slot cut into its upper face. These types of stone are
known from grist and crazing mills, where they are considered to have formed
part of the support for the machinery. It is therefore possible that this
building was also at one time used for crushing tin using horizontally
rotating stones. The water supply to power the machinery within the mill was
provided by a leat carrying water from the nearby River Meavy and this can be
traced leading 90m northward from the building. The floor where the crushed
tin was dressed lies south of the mill, and within this area four rectangular
hollows probably represent the remains of buddles.
The southern mill lies on the western side of the River Meavy and survives
as a 4.6m long by 3m wide rectangular building denoted by drystone walling.
The wheelpit is attached to the northern side of the building and measures 4m
long by 0.94m wide. The wheelpit wall is up to 1.2m high adjacent to the mill
and 2.5m high on the eastern side. A single mortar stone with three hollows on
one face sits on top of the slope east of the mill and indicates that a triple
head of stamps were employed. The leat carrying water from the nearby river
survives as an earthwork measuring up to 1m wide and 0.35m deep, with the
associated bank thrown up during its construction being up to 3m wide and 0.7m
high.
A short distance west of the mill is a rectangular earthwork which may
represent the site of a broadly contemporary building. The interior of this
structure measures 5m long by 2.5m wide and is denoted by a rubble bank
standing up to 1.8m wide and 0.8m high. This building and the mill sit within
a 3.5m deep gully formed by earlier tinworking at this location.
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.
