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NORTHAMPTON SANDS AND LOWER ESTUARINE
These beds consist of sand and sand-rock, containing much iron-ore, and having a thickness of from 20 to 80 feet.
Mr. S. Sharp has noted the following divisions of the beds in the Northampton district :4. White or grey sand,' and sandstone sometimes quarried for build
ing-stone, containing a plant-bed (Lower Estuarine Series) 12 3. Thin beds of ferruginous sandstone and shelly calcareous beds
very variable, being sometimes entirely calcareous, at others
30 2. Coarse oolitic limestone
4 1. fronstone-beds containing Rhynchonella variabilis, R. cynocephala,
Amongst other fossils from the Northampton Sands are:Avicula Braamburiensis, Gervillia acuta, G. Hartmanni, Hinnites abjectus, Lima duplicata, Ostrea gregaria, Pecten personatus, P. demissus, Astarte elegans, Cardium Buckmani, Ceromya Bajociana, Cucullæa cancellata, Trigonia costata, Terebratula perovalis, Nerinæa cingenda, Belemnites giganteus, and Ammonites Murchisonæ.
Amongst the localities where the beds may be observed are Kingthorpe, Northampton, Duston (slaty beds), Blisworth, and Gayton.
The Ironstone-beds yield a number of fossils, many of them, as Mr. Sharp points out, cast, as it were, in iron, so that the introduction of the ore must have taken place subsequently by infiltration and replacement.
The ore yields 40, and sometimes 55, per cent. of pig
1 Clay-beds in this series have been worked for Terra-cotta in the neighbourhood of Stamford. Concretionary masses of sandstone called • Pot-lids' are met with in the sands.
iron. It is worked at Duston, Blisworth, Gayton, Wellingborough, Cranford, Stamford, &c.
The Northampton sands include the Lower Estuarine series of Mr. Judd, which embraces the brown and white sands with argillaceous beds and plant-remains that occur above the fossiliferous Ironstone-beds.
In the northern part of Oxfordshire and south Northamptonshire, the Northampton Sands are considered by the Geological Survey (chiefly through Mr. Judd's investigations) to be the equivalent of the lower zone of the Great Oolite and part of the Inferior Oolite; while in the northern part of Northamptonshire and in Lincolnshire they include the Lower Estuarine series, and occur beneath the Collyweston Slates and Lincolnshire Limestone (Inferior Oolite).
The Northampton Sand forms a rich soil.
Origin of the Northampton Sand.—The following conclusions as to the origin of the Northampton Sand and Ironore, which have been so carefully worked out by Mr. Judd, may be introduced here, as they may serve to throw light on the origin of similar deposits of other periods :-)
We find, in what is now the Midland district of England, and at a period separated by a long interval of time from that of the last deposit in the area, the Upper Lias Clay, that a number of considerable rivers, flowing through the Palæozoic district lying to the north-west, formed a great delta. Within the area of this delta the usual alternations of marine, brackish-water, and terrestrial conditions occurred, and more or less irregular accumulations of sand or mud, in strata of small horizontal extent, took place. Subsequently, and probably in consequence of the gradual depression of the area, the conditions were changed, and in an open sea of no great depth, by the abundant growth of coral reefs and the accumulation of dead-shell banks during enormous periods of time, the materials of the great deposits of the Lincolnshire Oolite limestone were formed. On a re-elevation of the area the former estuarine conditions were also reproduced, and similar deposits, but of an argillaceous rather than an arenaceous character, were formed. Confining our attention to the earlier
of these two estuarine series, that of the Northampton Sand, we must imagine the beds as being carried down to great depths in the earth by the deposition upon them of the superincumbent strata. But at the same time another most important cause has come into operation, namely, the passage through some portions of the rock of subterranean water containing carbonate of iron in solution. By this agent carbonate of iron was deposited in the substance of the rock, while portions of the siliceous and other materials were dissolved ; and these entering into new combinations, were in part re-deposited in the mass of the rock in the form of politic grains, and in part, probably, carried away in solution. During the existence of the beds under a great pressure of overlying rocks, they would likewise become consolidated and jointed. These metamorphic processes would probably take place with extreme slowness, and may possibly still be going on, where the rock remains deep-seated in the earth; by their means portions, greater or less, of the sandy strata, but always those resting immediately on the impervious Upper Lias Olay, would be gradually converted into solid and jointed rock beds, composed principally of carbonate of iron. The next stage in the course of alteration in these rocks would commence when, by the action of denudation, portions of them were brought again near to the surface, so as to be traversed by the atmospheric waters, entering them as rain and passing away from them as springs. The action of this water is to remove the carbonic acid and soluble salts, to change the protoxide of iron into hydrated peroxide, and to redistribute it in such a manner as to produce the remarkable cellular structure of the rock, and also the mammillated, botryoidal, and sculptured surfaces. Finally, by mechanical, as distinguished from chemical, subaërial denudation, the beds of Northamptonshire iron-ore nearest the surface are disintegrated and broken up, and the softer and less ferruginous portions to some extent carried away in suspension, and thus deposits, composed of the harder and denser materials, formed, constituting the bed usually worked as an iron-ore. (J. W. Judd.)
The Inferior Oolite consists of buff-coloured sandy colitic and iron-shot limestone, with occasional beds of compact limestone. It is generally darker in colour than the Great Oolite.
At Leckhampton Hill it admits of the following divisions :6. Upper Ragstone and Clypeus bed 38 feet-Zone of Ammonites Par5. Lower Ragstones with C. Plotii.
kinsoni. 4. Upper (flaggy) Freestone )
Humphresianus. (unfossiliferous) 3. Oolite Marl 2. Lower Freestone
Murchisona. 1. Pea Grit
The Pea-grit (according to Mr. Hull) is composed of flattened spheroidal masses about the size of a pea, and
fourth or one fifth of an inch in diameter. Many of the ovules consist of layers of carbonate of lime, aggregated around some organic or inorganic fragment: some appear to be small rolled fragments of limestone. This division is very fossiliferous.
The Lower Freestone yields the building-stone quarried at Bourton, Broadway, Guiting, Stanway Hill, Cleeve Cloud, Leckhampton Hill, Painswick Hill, Birdlip, Sheepscomb Hill, Hellcomb, Lyreford, Brockhampton, and Longborough.' The freestone is largely composed of comminuted shells. The Upper Freestone, also quarried, is oolitic. The fossils include Echinoderms, Nucleolites (Clypeus) sinuatus, Acrosalenia, Hemipedina ; Molluscs, Gryphæa Buckmani, Pholadomya fidicula, Trigonia costata, Ceromya concentrica, Ostrea Varshii, Myacites musculoides, Terebratula fimbria, T. globuta, T. plicata, Rhynchonella tetrahedra, R. spi
Beds 5 and 6 have locally, in the Cotteswold Hills, been divided into palæontological zones by Mr. Lycett and Dr. Wright: thus in descending order they are represented by the Pholadomya Grit, the Trigonia Girit (T. costata), the Gryphite Grit (G. subloba), the Chemnitzia Grit (C. procera): all these have been collectively grouped as the Spinosa
1 The stone when first removed can usually be cut by the saw; it hardens upon exposure.
stage, characterized by Rhynchonella spinosa. No. 3 has been termed the Fimbria stage, being characterized by Terebratula fimbria.
The beds which rest on the Pea-grit are, according to Mr. Hull, frequently pierced by Lithodomus attenuatus : the upper bed of the Ragstone is also bored by Lithodomi.
Burton Bradstock is so well known as a locality for Inferior Oolite fossils, that it may be interesting to give the following section afforded by the quarries in the zone of Ammonites Parkinsoni which was taken at this locality by Dr. Wright : 5. Coarse ferruginous iron-shot oolite, containing many fossils :
Ammonites Parkinsoni, A. Truellei, A. subradiatus, Ancylo-
3 0 4. Thin-bedded oolitic limestone, with few fossils
1 6 3. Brachiopoda-bed ; a rich shelly polite, containing immense numbers of Terebratula sphæroidalis
1 0 2. Thin-bedded oolitic limestone; few fossils
1 8 1. Coarse brown colitic limestone
The quarries at Halfway House, between Sherborne and Yeovil, and those near Bradford Abbas, have yielded many fossils.
The two well-defined divisions of the Inferior Oolite of the south of England are thus described by Dr. Holl:
Upper Ragstone, consisting of light-coloured, coarse
grained, thin-bedded or flaggy oolite, containing few
fossils, and those chiefly in the form of casts. Lower Ragstone, consisting of hard, brown, ferruginous
limestone, often much speckled with ovoid grains of
peroxide of iron, and abounding in fossil remains. In the neighbourhood of Bath the Inferior Oolite is about 60 feet in thickness.
Prof. Hull observes that this formation, which in the Cotteswold Hills attains a thickness of 264 feet, is represented in the neighbourhood of Woodstock by only 5 to 10