| NORTHAMPTONSHIRE
SLATESTONES, Dr D.S. Sutherland, Geology Department, University of Leicester. |
| In central
England there is little metamorphic slate of the type found in say, the
Cambrian tectonically cleaved mudstones of Wales; the small inliers of
late Precambrian Swithland Slate in Leicestershire are the nearest. But
certain fissile sedimentary rocks from the Mesozoic outcrops have been
selected and trimmed for roofing material since the Middle Ages. |
| The calcareous, sandy "slates" of
Collyweston, from the Lincolnshire Limestone Formation, are well known.
However, the succession of Middle Jurassic rocks in Northamptonshire (see
Fig.1) includes other horizons of sandy limestones and calcareous sandstones
of relatively fissile character, and also flaggy limestones perhaps from
near-surface outcrops, which have been used as slates in the past, and
can turn up in archaeological contexts. |
| Great Oolite Group |
Oxford Clay
Kellaways sand and clay
Cornbrash
Blisworth Clay
Blisworth Limestone
Rutland Formation (Upper Estuarine
Series) |
| Lincolnshire Limestone
Formation |
Upper
Lower: Collyweston
Grantham Formation (Lower Estuarine
Series) |
| Northampton Sand Formation |
Variable Beds
Ironstone |
| Lias |
Upper
Middle - Marlstone Rock Bed |
Figure 1: The Middle and Lower Jurassic
Succession in Northampton.
|
1. BLISWORTH
(GREAT OOLITE) LIMESTONE
This limestone,
of Bathonian age, is younger than the Lincolnshire Limestone. There is
little geological information regarding slates from this formation, which
has a wide outcrop in eastern and southern Northamptonshire, but fissile
rock was locally available, as "paving" beds within the succession, or
as frosted rock near the surface. |
| Thompson (1927)
quotes Sharp, describing a quarry
west of Oundle where "Pendle" - the quarrymen's term for any fissile limestone
- occurs at the top of the section, and "splits into thin flags or
slates". Certainly the Blisworth Limestone provided a source of slates
on a local basis, and perhaps in the past certain sources were better known.
Kerr, for example (1925, p.106) records
the use of "slatstone" from "Jerdele", (which archaeologist John Williams
suggests would be Yardley Chase), in rebuilding the Middle Gatehouse of
Higham Ferrers castle in 1431. (The castle no longer exists; some stone
is believed to have gone to Kimbolton). Blisworth Limestone in Northamptonshire
is cream-coloured, including micritic and biosparite varieties, seldom
oolitic, and weathering grey; it commonly contains oysters in flat pieces,
which would contribute to a fissile character. |
2 UPPER ESTUARINE
LIMESTONE
Morton
(1712) mentions that slates from "Pichely" [Pytchley] were used for the
roof of the vicarage in that village, and geologically it is likely that
the source here was the Upper Estuarine Limestone. |
| This limestone, again geologically
younger than the "Collyweston" of the Lincolnshire Limestone, occurs as
a rib, on average about 5 feet thick, within the clays below the Blisworth
(Great Oolite) Limestone. Locally it can be sandy and flaggy, but also
shelly limestones occur, and fissile rock from this geological horizon
is often found to contain many small round bivalves (Placunopsis), as well
as oysters and echinoid debris. Slates of this type are recorded from various
archaeological sites, in Northampton for instance (Williams,
1979, p.327; Williams et al., 1985, p.71). Richardson
(1925, p.143) lists a section in the Limekiln Quarry on Hopping Hill, near
Duston, west of Northampton, where a "Slate Bed" of calcareous sandstone
occurs in the Upper Estuarine beds below the Great Oolite Limestone. The
outcrop, though thin, occurs around Kettering, Wellingborough and Northampton
(including Duston), and thickens south-west of Towcester towards Helmdon,
where, still including laminated limestone, it was also used as a building
stone. Thompson (1906, p.298) noted
that the Manor at Sulgrave was roofed with stone slates, "probably supplied
from Helmdon". |
3 LINCOLNSHIRE LIMESTONE FORMATION
- COLLYWESTON SLATES
The best-known
source of slatestone in the county is Collyweston, where a sandy limestone
or calcareous sandstone from the basal beds of the Lincolnshire Limestone
has been mined for centuries. |
| Old shafts
remain in the village of Collyweston and near Easton-on-the-Hill.
John
Morton's Natural History of Northamptonshire (1712, p.109) gives one
of the earliest descriptions of the method of obtaining the slates "which
being sprinkled with Water and exposed to Frosts, do readily cleave into
thin and eaven Plates as are fit for covering the Roofs of Houses". The
sandy limestone is underlain by soft sands which could be dug away along
underground galleries or "foxholes", enabling the removal of the overlying
blocks. A geological section at a Collyweston Slate-mine is given by Woodward
(1894, p.197; see also Sharp, 1873),
the slate bed being quoted as 3 feet 3 inches thick. |
| Woodward also
describes the working of Jurassic slates for roofing (op. cit., p.482-485),
but for the Collyweston Slates he is quoting an earlier account by Judd
(1875).The rock was also worked near Kirby Lodge, Deene (Taylor, 1963,
p.64). Kerr (1925), for instance, records the use of 11,000 "stone slattes"
from "Kyrgby" for Higham Ferrers castle in 1462. At the old Collyweston
Slate Pits west-north-west of Kirby Lodge the pale brown sandy limestone
is 3 feet 6 inches (just over a metre) thick (Taylor, op. cit., p.64).
Typically the rock in the ground is cross-bedded, and fissile along the
cross-bedding. Taylor (op. cit., p.43) describes the rock as containing
"30 per cent or more of quartz grains and sparse flakes of white mica",
with shell fragments, and cemented by calcite. |
| A thin section
of Collyweston Slate at Leicester University (LEIUG 119511) is more sandy
(a fine-grained calcareous sandstone, rather than a sandy limestone), the
sand grains being angular and 0.1 mm in size. Muscovite flakes, and shell
fragments, are dispersed but lie flat on the bedding and cause the rock
to split along this direction. The Lincolnshire Limestone as a whole has
a restricted distribution in Northamptonshire, occurring only in the north-east
(and not everywhere are the basal beds suitable for slates); it becomes
thinner southwards, and is not present south of Kettering (Sylvester-Bradley
and Ford, 1968). |
4 NORTHAMPTON SAND FORMATION - DUSTON
SLATES
The Northampton Sand Formation has
been quarried or mined for stone slates since at least 1712 at Duston and
Harleston Heath, to the west of Northampton, Pitsford and Weston Favell,
the latter probably near surface rather than underground, to the north
and east respectively. In 1870, the Duston Stonepit was described as ‘very
ancient and large some 40 feet deep, in which two beds of “white pendle”,
each two or three feet thick occur between upper and lower sandstones’
(Sharp 1870, 370). It is not known
when they ceased working but by 1906 they were no longer used (Thompson
1906). |
| In the Duston area, several quarries,
delves and mines were in existence, working a variety of building stones.
The lower Pendle bed was called (incorrectly) Collyweston Slate by the
quarrymen and was mined in a similar way (Sharp
1870, 370), although it is not clear whether it was a frosting slate. There
is some confusion about the names and locations of the various workings
but the correct situation is shown in the map. At Pitsford and Weston Flavell,
the slates were suitable for use straight from the ground without frost
treatment (Morton 1712, 109). It seems
that in the East Midlands only here and at Duston was the Northampton Sand
sufficiently sandy and fissile to be useful as slates. |
|
1 Duston Ironstone
Quarry
2 Limekiln Quarry. Slate is in the
Upper Estuarine Limestone
3 Hopping Hill Brickworks
4
5 Field Pit
6 Top Pit New Duston, Old Duston
Pit (Sharp)
7 Tennant’s Quarry = Sharp's Old
Slate Quarry Close. Northampton Sand formation
8 Richardson's Slate Pit Plantation.
Northampton Sand formation |
9 Cotter Quarry,
Harlestone (The quarry is a quarter mile further south than shown.)
10 Larger Quarry
11
12
13
14
15 Harpole Sandpit |
|
