Crinoids at Hartington
Stephen K Donovan (The Netherlands)
Much of the secondary railway route in Derbyshire, from Buxton south to Ashbourne, was closed in the 1950s and 1960s. Today, only the northern section is still in use as a railway, providing a route for major limestone quarry traffic (Roberts and Emerson, 2018). But the remainder of the line, from about 2.25km north of the closed Hurdlow station (Rimmer, 1998, p. 102), all the way to Ashbourne – a distance of about 27.5km – is now open as a cycle path called the Tissington Trail. This is part of the High Peak Trail north of High Peak Junction, which is south of Parsley Hay, and provides excellent access. For a map, see http://www.peakdistrict.gov.uk/__data/assets/pdf_file/0009/90486/hptisstrails.pdf.
The interest of this route for the geologist is that most of it is through the Carboniferous limestones (Mississippian) of the Derbyshire plateau. The beauty of the scenery combines with the accessibility of exposures in railway cuttings to provide much of interest to the geologist on foot or bicycle. The northern part of the route, from south of the site of Hurdlow station, through Parsley Hay (with cycle hire and a cafe) to Hartington, is described in a brief field guide by Simpson (1982, pp. 102-107).
My interest in these limestones is for their fossil crinoids. These are commonly difficult to see in the massive beds of limestone, which, over many years, have developed a surface patina that conceals internal features such as fossils. As this is a national park, there is no recourse to hammering, but there are other ways to prospect for fossils. For example, I have been exploring for rare, thin mudstone horizons with included ossicles (Donovan, 2018) (ossicles are the small calcareous elements embedded in the dermis of the body wall of echinoderms, including crinoids, forming the endoskeleton) and loose cobbles that have been etched on the surface by many years of weakly acidic rain (Donovan and De Winter, in press). The latter are analogous to some building stones in cities where limestones in facing stones and bollards have been etched (Donovan, in press, figs 1b, 2a, 3b, h, 5b, 6a, c).
For the avid palaeontologist armed with a camera, there are other, perhaps less expected sources of data that can be found it you are alert. Consider, for example, the site of the closed station at Hartington. Much of the lineside space formerly filled by wooden platforms and buildings, and sidings (Sprenger, 2013) is now filled by trees, but one structure, the signal box (Fig. 1 (top)), has been preserved.
On summer weekends and holidays, the brick base is open as a cafe, serving drinks, snacks and ice creams, which can be enjoyed at one of the several picnic tables. There is ample space for parking cars and, in consequence, it is a popular starting point for cyclists. The facilities are completed by a relatively new toilet block (Fig. 1B). It is the external walls of this latter building that fascinated me.
The sandstone blocks at the corners (Fig. 2A) have been decorated and are certainly attractive. Unfortunately for the geologist, this has destroyed the surface manifestations of any internal sedimentary structures, such as bedding. That is, their effect is entirely decorative.
Much more informative are the fresh, broken surfaces of the limestones in the walls (Fig. 2B to H). These have not been sculpted and expose their inner secrets, and will continue to do so until surface weathering obscures them. The dominant fossils are my beloved crinoids, which are recognisable from sections broken through fragments of stems (= pluricolumnals). (Crinoid stems are made up of interlocking discs called “columnals” and each of these columnals are perforated down their centre, creating a lumen or cavity down the centre of the stem. Such axial canals contain nerves and other of the animal’s organs.) At least one block exposes the external features of some pluricolumnals (Fig. 2B), including what appears to be part of an attachment (= ‘root’).
The other blocks are similar in their fossil content and demonstrate three important features shown by crinoids in the Mississippian limestones of Derbyshire (Donovan, 2013a). The first is immediately apparent: the fossils cannot be collected. Calcitic fossils like crinoids in such crystalline, well-cemented limestones can only be developed by mechanical means, such as a pin in a pin-vice, and abundant patience. With Mississippian pluricolumnals being abundantly available from muddier successions, such as at Clitheroe in Lancashire, nobody is likely to invest the necessary time and effort.
Which is doubly unfortunate because the second feature of note is their large size (Fig. 2C-H) – many of these columnals have a diameter greater than 10mm. These are giants amongst crinoids. Although broader pluricolumnals are known from the Mississippian, up to about 50mm in diameter (Donovan, 2013b), any columnal of 10mm or more is notable. Certainly, no extant crinoid has such a girth of stem. They are also unexpected when compared with other occurrences of crinoid pluricolumnals and columnals in Derbyshire limestones. Most of these are smaller than 10mm in diameter, and include occurrences in chert (= screwstones; Bouman and Donovan, 2015), mudrocks (Donovan, 2018) and surface etchings (Donovan and De Winter, in press).
Thirdly, there is a sameness of form. All of these large pluricolumnals are circular, with a moderately broad, central lumen. This may indicate that they represent only one or a few species and I tentative lump them together in one columnal morphotaxon (that is, a species determined by the gross features of the stem), Cyclocyclicus (col.) sp sensu lato. It also demonstrates a conservatism of stem geometry, which contrasts with the range of forms known from many older horizons. While the Mississippian is known as the ‘Age of Crinoids’ (Kammer and Ausich, 2007), it is also an age of conservatism in the geometry of the crinoid stem.
In conclusion, the public conveniences at the former railway station site at Hartington in Derbyshire are constructed of (presumably) local stone, particularly Mississippian limestones rich in crinoid stems. These numerous specimens demonstrate three common features of crinoidal limestones of the Derbyshire plateau. Pluricolumnals in well-cemented, crystalline limestones are difficult to collect. Their large diameter is a feature of the massive limestone beds. And the circular columnals with a circular lumen, Cyclocyclicus (col.) sp sensu lato, are dominant, albeit morphologically conservative.
Bouman, R.W. & Donovan, S.K. 2015. Biodiversity of Mississippian (Lower Carboniferous) crinoids from Bradford Dale, Derbyshire, U.K. Proceedings of the Yorkshire Geological Society, 60: 293-302.
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Donovan, S.K. 2013b. Giant crinoid stems from the Lower Carboniferous (Mississippian) of northwest England. Proceedings of the Yorkshire Geological Society, 59: 211-218.
Donovan, S.K. 2018. Taphonomy of a Mississippian crinoid pluricolumnal, Newton Grange, Derbyshire, UK. Proceedings of the Yorkshire Geological Society, 62: 59-63.
Donovan, S.K. (in press). Urban geology: Mississippian in the Mainstreet. Geology Today.
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Rimmer, A. 1998. The Cromford & High Peak Railway. Reprint of 2nd edition. Oakwood Press, Usk, Monmouthshire.
Roberts, A. & Emerson, F. 2018. The Limestone Quarries of Buxton. Buxton Civic Association, Buxton.
Simpson, I.M. 1982. The Peak District. Rocks and Fossils, volume 3. Unwin Paperbacks, London.