Tilton and Holwell - An Introduction
The Lower Jurassic rocks - the Lias Group - (often simply referred to as the Lias) of Leicestershire were all deposited under marine conditions, and following standard geological practice, have traditionally been divided into three: lower, middle and upper. The 'lower' Lias Group, apart from limestones at the base, consists predominantly of mudstones. It forms the low rolling scenery which is traversed as one travels east from Leicester along the A47, although it is here overlain by extensive spreads of boulder clay and other Pleistocene drift.
The 'middle' Lias Group consists of a relatively thick lower sequence of argillaceous silts (and siltstones) and fine sandstones (the Dyrham Formation), overlain by the relatively thin sandstone and ironstone of the Marlstone Rock Formation. Where the latter is at all substantial, as in the area to be visited, the 'middle' Lias Group forms a fine escarpment, marked by a line of springs at its base. Coming from Leicester, the first sight of this escarpment is the distinctive wooded outlier of Billesdon Coplow. Tilton on the Hill, as the name suggests, sits right atop this much-incised escarpment.
The 'upper' Lias Group (the Whitby Mudstone Formation) again consists predominantly of mudstones, and again forms relatively lower and rolling country to the east of Tilton. This is seen as one drops rapidly down the dip slope from Tilton village towards the railway cutting locality. The overlying (and more resistant) Middle Jurassic rocks (Northampton Sand Formation) cap two local hills as outliers - Whatborough Hill, and Robin-a-Tiptoe Hill, but not the hill nearest to the cutting - Colborough Hill - which is entirely Upper Lias.
The old railway cutting to be visited lies just over one and a half kilometres to the east of Tilton village (and to the east of the hamlet of Halstead), along the unclassified road that is essentially the main road between Leicester and Oakham (county town of Rutland) - a road that is thus both narrow and busy! Parking is one of the major problems for parties visiting the cutting. A minibus can park right in the entrance to the site, and one perhaps two cars in front of the adjacent field gate (without blocking it). Otherwise improvisation is called for!
The cutting to the south of the road bridge is a geological SSSI, and is owned and managed as a nature reserve by the Leicestershire and Rutland Wildlife Trust. (Brocks Hill Environment Centre, Washbrook Lane, Oadby, Leicestershire, LE2 5JJ (phone: (0116) 02720444)). Permission to visit should be obtained from them. Management of the reserve to maintain its geological and other interest is an ongoing process. Money was originally obtained a few years ago from the then Nature Conservancy Council for the original re-establishment of the faces using a JCB. Since then, further monies from what is now English Nature, together with the large and largely voluntary effort of the Leicestershire and Rutland Wildlife Trust have ensured continued easy access, and further JCB work to further refurbish the faces.
Tilton Railway Cutting Nature Reserve, Halstead, Tilton on the Hill, Leicestershire.
Grid Reference (entrance to southern extremity): SK76130560 - SK76550482.
The lowest beds are seen at the first of the recently cleared exposures (at the northern end of the cutting), and higher beds come down to the floor of the cutting further south, as a result of the gentle southerly (perhaps south-easterly) dip and the gentle upward slope of the cutting floor.
The succession (see diagram) starts with somewhat ferruginous argillaceous silt (and siltstone) and fine-grained sandstones of the lower part of the 'middle' Lias Group (Dyrham Formation): ammonites (Amaltheus subnodosus, and A. margaritatus) of the subnodosus Subzone of the margaritatus Zone have been found (J.G. Martin - pers. comm. , details in MS by Don Blake which contains pers. comm. identifications by M.K. Howarth).
The base of the Marlstone Rock Formation (in the sense used here) is an irregular pebble bed with mudstone/siltstone clasts. This presumably represents a non-sequence, which if the lower part of the Marlstone has been correctly attributed to the spinatum Zone, would represent the whole of the gibbosus Subzone at least. The Marlstone Rock Formation can be divided into two, informally named, lithostratigraphic units with a gradational contact: (i) a ferruginous Sandstone Member below (the term 'Sandrock' has been applied, but should be discouraged as there is a unit (now replaced) of the same name in the 'lower' Lias Group in north Leicestershire), and (ii) a chamosite (berthierine) oolite Ironstone Member above. The latter was formerly quarried nearby (and extensively in north Leicestershire/south Lincolnshire). The Sandstone Member is particularly noted for two (and a weak third) bands full of the brachiopods Tetrarhynchia tetraedra (a large ribbed rhynchonellid) and Lobothyris punctata (a smooth terebratulid), which seem to represent overwhelmed life assemblages (Hallam 1962). These and other brachiopods, bivalves, and belemnites occur throughout the Marlstone. The Ironstone Member, although largely an oolite, contains cross-bedded banks of crinoid debris. Howarth (1980) showed that the base of the Toarcian Stage occurs within the Ironstone Member, not, as formerly believed, abruptly at the 'Transition Bed' at the top. Dactylioceras semicelatum (and thus the eponymous subzone) has been proved to 0.9 metres below the top of the Marlstone, and the base of the tenuicostatum Zone (and thus of the Toarcian) has been taken at 2.5 metres below the top (Howarth 1992). Rare Pleuroceras have been found in the Ironstone and top of the Sandstone Members in the Tilton area.
The 'Transition Bed' is not a bed in the normal sense of the term, but merely a weathered zone at the top of the ironstone, although it is a band with distinctive abundant and diverse fauna: brachiopods, bivalves, small gastropods, belemnites, the ammonites Dactylioceras and Tiltoniceras, etc. (Wilson and Crick 1889).
The mudstones of the 'upper' Lias Group (the Whitby Mudstone Formation) are currently quite well-exposed, and within the cutting all lie within the falciferum Zone (exaratum and falciferum Subzones). They yield the typical ammonites (harpoceratids and dactylioceratids) relatively abundantly, and often 3-D preserved. Towards the base (Bed RGC13 of the section) there are some paper shales with early diagenetic calcareous concretions. These beds are analogous to, and roughly coeval with, the famous Posidonia Schiefer of southern Germany. Like their German counterparts, the concretions are often crowded with the enigmatic tiny gastropod, Coelodiscus minutus. (For papers on the significance of these beds, see papers by Jenkyns.) Higher in the succession (beds RGC15 and RGC16) the mudstones contain quite abundant phosphatic/ ferruginous/calcareous ooliths - an horizon that seems very widespread in the East Midlands (see paper by Horton et al, 1980). Recently (2003) the Whitby Mudstones have yielded a partial skeleton of an ichthyosaur.
Holwell, near Melton Mowbray, Leicestershire.
(Grid Reference (generalised): SK74 24)
The geological interest of Holwell is spread around a number of exposures, exposure that represents all that remains of the extensive former ironstone workings. The principal exposures (all RIGS) are:
1. Holwell Mineral Line Cutting (Grid Reference:
SK73452415 - SK73422465)
2. Brown's Hill Quarry Nature Reserve (Grid Reference (centre of face): SK74182345). Owned and managed by Leicestershire and Rutland Wildlife Trust, this is managed as the principal recreational/educational geological resource.
3. North Quarry Nature Reserve (Grid Reference: SK74132365 - SK74452400). Also owned by the Trust, this is managed as the principal research geological resource.
We shall visit the last two.
The succession is very similar to that seen at Tilton, except that no typical Dyrham Formation is exposed, although it is undoubtedly present in the area. The 'middle' Lias Group - as seen - consists of oolitic ironstone above ferruginous sandstone, and as at Tilton, and following conventional usage (see for example Hallam 1968, Clements 1989), both are here regarded as members of the Marlstone Rock Formation. Recently (2002) the British Geological Survey has published a revised version of the 1:50,000 Melton Mowbray sheet, along with a 'sheet explanation' (Carney et al 2002) in which the Sandstone Member is still called by the unfortunate name, 'Sandrock', and included within the Dyrham Formation. Above the Marlstone, a good sequence of the Whitby Mudstone Formation ('upper' Lias Group) is again present.
The Sandstone Member of the Marlstone Rock Formation is a little thicker than at Tilton, and is most fully exposed in the Mineral Line Cutting. The top of the Member is best seen at Brown's Hill. The Ironstone Member is notably thicker than at Tilton, and can be best seen in Brown's Hill Quarry, and less well so in North Quarry. The most interesting feature - very well seen at Brown's Hill - is the 'belemnite pavement' remanié layer at the top of the ironstone. There is no evidence of the '"Transition Bed" fauna' here, nor of any of the other obviously Toarcian elements that have been recorded from the Ironstone Member at Tilton. It seems likely that a major non-sequence occurs between the Marlstone Rock and Whitby Mudstone Formations at Holwell (perhaps representing the greater part of the tenuicostatum Zone).
Essentially the same 'members' for the Whitby Mudstone Formation can be recognised here as are seen at Tilton ('lower or yellow clays member'; 'paper shale member'; 'upper or main clays member'; 'phosphatic oolite member'), although again they are all distinctly thicker. The lower part of the Whitby Mudstone Formation is best seen in Brown's Hill Quarry, and a less weathered and more complete sequence (up to the 'phosphatic oolite member') can be seen in North Quarry. In general, at Holwell, the Formation is a more fossiliferous succession than at Tilton - this is particularly true of the 'total palaeontology' of the 'main clays member' (beds RGC.15-20). The palaeontology of the 'yellow clays member' (beds RGC.8-13) also warrants more detailed studies. The 'paper shales member' (bed RGC.14) tends to be rather weathered at Brown's Hill (and therefore rather disappointing), less so (and more promising) in North Quarry.
As a result of this faunal richness, these localities represent a good place to investigate the widespread early Toarcian oceanic anoxic event (OAE) here represented by the 'paper shale member' - what happened to the faunas going into and coming out of the OAE? In this respect, Holwell is probably rather better than Tilton. Some preliminary work is under way.
Department of Geology
University of Leicester
revised to 20. viii. 04