The Cornbrash, which succeeds (so called also from a district in Wiltshire, favourable to corn), is a light grey, fine-grained limestone, often so hard as to need blasting. It abounds in fossils. Among them are Avicula Echinata, Ostræa Sowerbyi, Clypeus Ptotii, Ammonites Macrocephalus, A. Herveyi, Nucula Variabilis, Astarte Minima, Trigonia (of four kinds), Modiola (of four kinds), Myacites (five kinds), Cypricardia, Corbicella Bathonica, Pholadomya (two kinds), Cardium (three kinds), Pecten (six kinds), and several more. [95b]
The great Oolite (so named from the Greek Oon, an egg, referring to the number of small stones, like fish-ova, found in it) is divided into Oolite clays and O. limestone. The clays are mottled green and bluish, with bands of ironstone, and concretions of lime. They indicate a shallow sea, as contrasted with the Oxford clay. Fossils are not numerous, but Rhinconella Concinna, Gervillia Crassicosta, Modiola Ungulata, Ostræa Gregaria, O. Sowerbvi, O. Subrugulosa, Perna Quardrata, Trigonia Flecta, and Palate of Fish are found. [95c] These beds correspond to the so-called Forest Marble of the South of England.
The Oolite limestone beds consist of white soft limestones, having at intervals bands of marly clay. This formation burns well, and makes good lime. Its chief fossils are Serpula, Rhynconella, Terebratula, and T. Intermedia, Avicula Echinata, Corbicella, Lima Rigida, Lucina, Modiola Imbricata, Myacites Calceiformis, Mytilus Furcatus, Ostræa Sowerbyi, Pecten Vagans, Pteroperna plana, Trigonia, T. costata, T. flecta, T. striata, T. undulata. [95d]
The Estuarine deposit, underlying the great Oolite limestone, is composed of light blue, green, and purple clays, intersected by soft bands of sandstone, and having at its base a band of nodular ironstone. It is not very fossiliferous, but the following are found:—Rhynconella Concinna, Modiola Imbricata, Ostræa Sowerbyi, Monodonta. [95e] The sandstone bands contain plant-markings in considerable numbers. As its name implies, this formation
was produced as the bed of an estuary or tidal river.
The next lower formation is the Lincolnshire limestone. This enters largely into the making of what is called “the Cliff,” which is the high land running south from Lincoln (visible from Woodhall) to the west of the Witham Valley and the Fens. It is a hard building stone, though once the muddy bed of a sea. It is sub-divided into the Hibaldstow and Kirton beds, so called because these strata are exposed in those parishes. Dipping to the east, it underlies the Fens and other upper strata to be found in the Woodhall well. It abounds in fossils, there being as many as 340 species classified, [96a] and consists, indeed, very largely of the hard parts of shells and corals compressed into a solid mass. To a Lincolnshire person, it is sufficient to say of this stone that our grand Cathedral is mainly built of it. We can only give here a few of the more frequent species of fossils:—Three kinds of Echinus, Coral (Thecosmilia gregarea), Serpula socialis, Lima (five kinds), Ostræa flabelloides, Pecten (two kinds), Hinnites abjectus, Astarte elegans, Cardium Buckmani, Ceromya Bajociana, Cyprina Loweana, Homomya Crassiuscula, Isocardia, Cordata, Rhynconella (four kinds); among the bivalves are Avicula Inequivalvis, and A Munsteri, Lima (three kinds), Lucina Bellona, Modiola Gibbosa, Mytilus Imbricatus, Pholadomya (two kinds), Trigonia costata; of univalves, Natica (two kinds), Nerinea Cingenda; of fishes, Strophodus (two kinds). [96b] This is a most useful stone for building purposes. The so-called “Lincoln stone” is largely used in our churches; whilst the “Ancaster quarries,” which also belong to this formation, are famous. The commissioners appointed in 1839 to report on the building stones of England, for the new houses of Parliament, stated that “many buildings constructed of material similar to the Oolite of Ancaster, such as Newark and Grantham churches, have scarcely yielded to the effects of atmospheric influences.” (“Old Lincolnshire,” vol. i., p. 23.) The well-known Colly-Weston slates are the lowest stratum of this rock. The fine old Roman “Newport Arch,” which for some 700 years has “braved the battle and the breeze,” a pretty good test of its durability, is built of this stone.
The base on which this lowest Oolite lies is the Northampton Sands, an irony stratum of red ferruginous sand and sandstone, the upper portion of it also being called the Lower Estuarine deposit. It is from this stratum so many springs arise in various parts of the county, as already mentioned, and among them the Woodhall well water. Its fresh water conditions show it to have been the bed of a great river, but a tidal river, as among the fossils which it contains some are marine shells. In this formation is found the iron-stone, which is worked at Lincoln. Its commoner fossils are Lima duplicata, L. Dustonensis, Hinnites abjectus, Astarte elegans, Cardium Buckmani, Modiola Gibbosa, Ammonites Murchisoniæ, Belemnites Acutus. Its ferruginous layers are (as given by Capt. Macdakin), [97a] Peroxide bed, clay ironstone, hard carbonate of iron, hard blue carbonate peroxidised band, blue ferruginous sand, ironstone nodules, bed of coprolites with iron pyrites.
And this brings us to the Lias formation, in which lies the lower part, amounting to rather more than a third, of the Woodhall wells. It is divided into the upper Lias of clay and shale; the middle Lias of Marlstone rock bed, clay, and ironstone; and the lower Lias of clays, ironstone clays, limestones. [97b] This formation, with a thickness of from 900 to 1,000 feet, runs across England from Yorkshire and Lincolnshire down to the coast of Dorset. The upper Lias of the Woodhall wells also helps to form the slope from 100 feet to 150 feet in thickness of the escarpment called “the Cliff,” to the west of the Witham Fens, and to the north of Lincoln, by Fillingham, and so on. Lincoln itself stands on Lias beds, with a capping of the lower Oolite limestone. [97c] It contains many Belemnites, Lucina, Ammonites Bifrons, A. Serpentinus, A. Communis, A. Heterophillus, Nucula Hammeri, Pleuromyæ, [97d] and especially the Leda Ovum,
which distinguishes it from other strata. The middle Lias, which underlies the upper, contains Ammonites Spinatus, A. Margaritatus (in great abundance), Rhinconella, Icthyosauri, Plesiosauri, and fossil wood, etc. [98a] The lower Lias contains Ammonites Capricornus, with many pyrites, A. Ibex, A. Jamesoni, A. Armatus, A. Oxynotus, A. Obtusus, A. Semicostatus, A. Bucklandi, A. Angulatus, A. Planorbis, Gryphœa incurva very abundant, and fossils of many other kinds. [98b] This brings us to the base of the Woodhall Spa wells. For a full list of the fossils so far found at Woodhall, the reader is referred to Appendix II. at the and of this volume.
These strata are shewn in the diagram given at the head of this chapter.