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| The south-west end of the Park Tunnel |
Arriving at the Phoenix Park Park & Ride with Paul May, to start our recce for a field trip in Nottingham 2 weeks later - to explore its Triassic geology and the various building stones used in its historic architecture – I didn’t think too much about the underlying geology, when we were working out how to buy our tickets.
Phoenix Park tram stop is set on the Permian Edlington Formation, but on the route to the city centre - crossing various faults that throw the geological formations out of sequence - we passed over the Cadeby Formation, the Lenton Sandstone Formation and alluvium laid down by the River Leen before continuing on the Triassic Chester Formation until we reached Old Market Square.
Starting at an elevation of 58 m, we reached a low point of 36 m at the tram depot in the Leen valley and then rose to a high point on the Chester Formation of 82 m at the High School stop, before dropping down to 44 m at Old Market Square.
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| The Geology of the East Midlands |
Having both enjoyed the up and down journey on the tram into Nottingham city centre, we firstly visited the Nottingham Tourist Centre, where the member of staff who helped us with our enquiries was exceptional, we located the public toilets and then headed up to Derby Road to find the entrance to the Grade II Listed Park Tunnel – a geological site that is on the front cover of the Geologists’ Association guide to the East Midlands.
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| Triassic sandstone at the north-east entrance to the Park Tunnel |
During the 3 years spent in Nottingham as an undergraduate geologist, I visited Ye Olde to Trip to Jerusalem a few times – which I took advantage of at the end of a very long day out in 2019 - but I didn’t make an effort to explore any more of the geology of Nottingham and I was therefore looking forward to seeing this.
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| The Park Tunnel |
The Park Tunnel was built in 1855 for the 5th Duke of Newcastle, by Thomas Chambers Hine, to allow horse drawn carriages access between the Park Estate and Derby Road. The original requirement was for a maximum gradient of 1 in 14, but the actual gradient of 1 in 12 is too great for horse-drawn carriages so it was never used as originally intended.
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| The Chester Formation in the central cutting |
Passing through the car parking area to the block of offices and apartments and the partly brick lined first section of the tunnel, the first exposure of the Chester Formation, which was formerly called the Nottingham Castle Sandstone Formation, is seen in the cutting where a set of steps that lead up to The Ropewalk.
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| The steps to the Ropewalk |
In Nottinghamshire, the formation comprises pinkish red or buff-grey, medium to coarse grained, pebbly, cross-bedded, friable sandstone. Near the base of the Park Tunnel section, cross-bedding occurs in sets nearly 2 m thick, bounded above and below by near horizontal erosional surfaces.
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| The unlined section of the Park Tunnel |
In the thicker beds, trough cross-bedding can be clearly seen and the three dimensional geometry of the cross-stratification within the sandstone is well displayed in the sides and arched roof of the tunnel. Towards the top of the section, thinner sets of cross-bedding predominate and the foresets mostly dip ENE, indicating current flow in that direction when the sediment was deposited.
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| Examining the Chester Formation |
Sediment was derived from erosion of contemporary Triassic uplands, notably the Variscan mountains that lay to the south of what is now Britain. Deposition took place in a huge braided river system that spread across the semi-arid inland basins occupying much of southern, north-western and north-eastern England, with various sized pebbles of liver coloured metaquartzite and white vein quartz being very common.
Continuing through the second tunnel, which has not been lined, I found quite a large piece of sandstone and, not having my Estwing hammer with me to break it, I threw it to the floor and was very surprised to see it disintegrate completely into a pile of sand.
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| A disintegrated lump of sandstone |
Diagenetic studies suggest that the sandstone was originally cemented by authigenic carbonates, sulphates and halite, but these were subsequently dissolved by circulating meteoric groundwater to produce the weakly cemented sandstone with extensive secondary porosity now seen.
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| A specimen of Bulwell stone |
Further along Tennis Mews, we stopped to examine the Bulwell stone, which has been used for boundary walling and various retaining walls above the Chester Formation. In one place, part of the masonry had fallen away and I obtained a small sample to add to my rock collection. With a hand lens I can see the rhombohedral crystals of dolomite, which are characteristic of this rock and give a sparkle to the stone, when light is reflected from the crystal faces.
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| A Bulwell stone boundary wall |
Having had a good look at the various sedimentary structures and lithologies, which makes this such a good field trip location, we then retraced our steps and ascended the steps up to The Ropewalk and continued our recce at Newcastle Drive.
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| Bulwell stone retaining walls at the Park Tunnel |
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