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A Building Stones Guide to Central Manchester
Third Edition (2014)
Four self-guided walks through the city centre
Now available to purchase |
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Newsletter - June 2019
The full, illustrated newsletter is available as a pdf for download.
Text extracts are given below.
MGA/GeoLancashire field excursion to Thurns Head Quarry and Healey Dell, near Rochdale
Sixteen people met at Cowm Water Ski Centre car park at Whitworth where we were greeted by Arthur
Baldwin, our leader for Thurns Head. A highlight was magnificent examples of soft sediment deformation
giving rise to load casts on a metre scale. Fragments of flaggy sandstone left by the quarrymen displayed
examples of ripple bedding and abundant examples of Lockeia sp., the trace fossil of the 'foot anchor' of
a non-marine bivalve.
Evidence of the work of man was also
widespread, with examples of large thin ripple
marked slabs set vertically in rows in the
ground as fences and a paved stone road,
probably for bringing stone down into the
valley on wooden sledges.
Two erratics were observed, one of coarse-grained quartz gritstone likely to be local Rough Rock, and
the other a finely crystalline igneous rock with twinned potassium feldspars. Granitic erratics like this one
are fairly common locally, and probably came from the Lake District.
In the afternoon the group, less one or two who had to leave early, assembled at the car park at Healey
Dell where we met Ron Powell, our leader for the afternoon. Ron was part of the team who had the site
registered as an LGS (Local Geodiversity Site, previously known as RIGS). He is currently preparing a
geological trail around the Dell and he used the same route for our excursion. Two faults, one at either
end of the reserve, bring the older Upper Namurian, Rough Rock, into juxtaposition with the younger
Westphalian, Lower Coal Measures. The more southerly fault is visible in the gorge where the dip of the
bedding in Westphalian mudrocks has been increased adjacent to the well-exposed fault plane, having
been 'dragged' in that direction by the upward movement of the Rough Rock.
In the river bed we saw examples of potholes, formed when water born small pebbles and sediment swirl
to erode flask shaped holes in the bedrock. We also saw a variety of sedimentary structures, such as
foresets, in the cliffs on the side of the gorge where the Rough Rock is well exposed.
We followed the River Spodden downstream through a narrow gorge, past a succession of waterfalls, to
the visitor centre where some were led astray by the attractions of the café. Those remaining examined a
road-cutting exposure of non-marine shales which contained a bed of siderite nodules. A 7.5 kg nodule
did not effervesce with HCl and, using Archimedes principle, the SG was found to be 3.4 compared with
fully cemented limestone and sandstone about 2.7 and pure siderite 4.0. It is likely that these iron-rich
nodules were the ore used in a nearby 12th or 13th Century bloomery.
The leaders were thanked for the hard work they had put in to make the day such a success. The industrial archaeology in Healey Dell is very extensive and a field excursion to examine it in the context of the
local geology is being planned.
Peter del Strother
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Jura beige fossil identified
A new library and NHS building in Market Street, Altrincham is decorated with attractive fossiliferous
stone. On enquiring with the architect I learned that it was 'Jura Beige honed limestone from Germany'.
Further online enquiries revealed that its source is the Titting-Kaldorf region of Upper Bavaria. It comes
from the same quarries as the Jura Grey, seen in the Bridgewater Hall and several other locations
described in the recently revised Building Stones Guide to Central Manchester. Details are here.
Many of the fossils are clearly the same in the 'Beige' as in the 'Grey' and it is difficult to distinguish the
difference in colour without seeing them side-by-side. However, there is one 'fossil?' which occurs in
several stones which looks quite different and I have no idea what it might be. Note that the stone colour
is truly beige, not as gray as it appears in the image.
If you live nearby, or visit Altrincham, it is worth a walk round the outside of the building to examine the
range of fossils displayed.
Without revealing what has been suggested I would be interested to hear any further suggestions as to
what it might be. Email your thoughts to info@mangeolassoc.org.uk
Fred Owen
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