|All trial excavations were carried
out with a Caterpillar 428C back hoe (Fig 12). A 600 mm bucket was found
to be satisfactory for most of the work but in the unconsolidated hillwash
at Bull Farm where the trench walls tended to collapse, a 1000 mm bucket
was used. Excavations were carried out to a metre or so in depth, so shuttering
was not needed.
|The proportion of the excavated
material which was potentially suitable for roofing was estimated visually.
Samples were then split and dressed using hand tools to determine how easily
it could be converted to roofing and to estimate the overall recovery from
rock to product. At Bull Farm, where bedrock is exposed, rock was also
extracted by hand using bars and chisels to confirm that larger slabs could
be obtained and to compare the results with those won by mechanical excavation.
|At Park Wood a series of twelve
trenches were formed in a north easterly direction from the old quarry
to the crest of the hill (Fig 13). The excavations revealed that the fissile
shelly material could be worked as a single bench with initial overburden
and waste stock piled to the north west until space became available for
back-filling. It was estimated that the final back-filled ground level
would be very similar to the existing, assuming that not more than 20%
was removed for roofing and allowing for bulking up of the waste.
|Non-destructive techniques of survey,
such as radar, would not help in this sort of investigation. While it could
detect a change in soil type (eg a grave) or a horizontal break in a uniform
granite, it would not be able to distinguish between a thinly bedded sandstone
slate and a variably broken sandstone.
|The investigations established that
mechanical excavation would only be essential for the initial removal of
overburden and establishing a working bench. However, a front end loader
or small back-hoe could be used to work the loose material without unacceptable
damage to the rock. The bedrock below would need to be worked by hand tools
to maximize the size of slabs and the overall recovery of useable rock.
|The area underlain by fissile shelly
rock was about 0.5 hectares in extent. Assuming a depth of 0.5 metres of
which 10% would be suitable for roofing, it was estimated that there would
be a volume of about 250 sq m of useable rock equivalent to about 8,000
sq m of roofing 30 mm thick on average. Assuming the coverage on the roof
to be 35% because of double lapping, this would be sufficient to cover
an area of about 3,000 sq m; more than enough for the whole roof of Pitchford
|At Bull Farm the ground in The Quarry
was much steeper than that at Park Wood and the loose material had slumped
downhill to form a thicker cover over the bedrock. This had been removed
over a substantial area during previous quarrying, exposing bedrock of
both shelly and fine grained sandstone (Figs 14 and 15). Assuming 10% recovery
from an accessible area of 2,500 sq m by 1.5 metres depth, about 4,000
sq m of roof could be covered, more than enough for Pitchford Church and
to complete the re-roofing of Pitchford Hall. However to obtain large-size
slates, the consolidated rock at a low level would have to be exploited.
This would probably be more difficult and result in a lower recovery than
from Park Wood, where the weathered, looser material was close to the surface.
There is no set ratio in quarrying to decide the cost-effectiveness of
removing a quantity of overburden to access a particular amount of slate.
It is determined more by the potential output and its likely price.
|Conclusions from the investigation
|Sufficient suitable rock existed
to re-roof Pitchford Church including the chancel. There would also be
enough to complete the re-roofing of Pitchford Hall. The bulk could be
obtained from the loose material close to the surface in Park Wood but
some bedrock might need to be worked to ensure that the size mix on the
existing roof was maintained.
|At the quarry at Bull Farm the overburden
was thicker and while it also contained disturbed shelly rock it was generally
unsuitable for roofing, being too small and too thick. It was underlain
by fissile, shelly and fine grained fissile sandstone bedrock and both
types were exposed at the quarry floor. Elsewhere they were covered by
up to five metres of overburden.
|Both sources could be readily exploited
using mechanical excavation to remove the overburden and hand tools to
extract from the bedrock.
|It seemed probable that most of
the roofing material obtained in the past from the quarries along the two
hills came from the beds of loose rock. This suggested that if they were
reworked into the consolidated rock below there would be ample reserves
to re-roof all the other buildings covered with Harnage stone as this became
necessary. An opportunity therefore existed to extend the quarrying latterly
into previously worked ground to establish a stockpile of roofing stone
for future needs and thus secure a supply of this unique type of roofing.