Transactions of the Geological Society, 1st series, vol. 3/On the Geology of Cambridgeshire
|←On the Geology of Barbadoes||On the Geology of Cambridgeshire
|On a Bed of Trap in Birch-hill Colliery→|
By the Rev. J. Hailstone, F.R. & L S. Woodwardian Professor in the University of Cambridge.
The upland parts of Cambridgeshire consist of chalk hills, being part of that great range which traverses the island in a south-easterly direction, from Dorsetshire to the Yorkshire coast. At their northern extremity they appear to rest upon an extensive bed of blue clay, provincially called gault. They are composed of both the varieties of chalk; of the upper containing the common black flint in abundance; and the lower or grey chalk, which contains little or none. If a line be drawn from Royston by Balsham to Newmarket, it will pretty exactly define the limits of both the varieties; the hills to the eastward being composed of the upper beds, while those to the west consist of the lower or grey chalk. Further to the east, on the borders of Suffolk and also of Essex, the chalk disappears under a thick bed of clay, which occasions a corresponding difference in the soil and its produce. To the west, a succession of hills composed of beds of grey chalk with wide intervening vallies of gault occur; till on the extremity of the county, at Gamlingay and Potton, a tract of sand comes in, evidently connecting the strata of Cambridgeshire with those of Bedfordshire. And here the features of the former county undergo a manifest alteration, its high elevations subsiding by degrees into the sand hills of the latter.
Upon some of the highest hills near Cambridge, a deposit of gravel and loose stones in horizontal layers, has lately been found, resting immediately upon the chalk. This gravel differs in so many respects from the red ferruginous gravel found dispersed in patches over the gault in the subjacent flat, that I think it must be considered as a deposit of a different epoch. It contains numerous fragments of strata belonging to the oolite series, which occur in the neighbouring counties of Northampton and Rutland, surrounding Cambridge on the west and north-west. Pieces of basaltic rocks are sometimes found, but these are not very common. These fragments are of all sizes, and worn down in different degrees. Some are pebbles entirely rounded; others have their edges merely blunted. Some appear so tender and so little capable of resistance, that it is difficult to conceive how they have been transported without being entirely destroyed. The prevailing material of these masses of gravel, is the pale blue or light grey variety of flint, with numerous traces of the alcyonium or other similar bodies in its substance.
According to my observation, this variety of chalk flint is not so common in the southern parts of this great chain, whilst in its continuation through Lincolnshire and Yorkshire, scarce any other is to be met with. The two principal deposits of the gravel of which I have been speaking are to be seen on the summit of Gogmagog hills, and on Harston hill about five miles to the south of the former. The height of these hills may be estimated at 800 feet above the river at Cambridge.
Harston hill has been examined by Mr. Warburton, Secretary of the Geological Society, who has obligingly communicated the result of his observations to me. Mine were principally made at the pit on Gogmagog hills; and as the contents of the two pits seem to differ in some respects, I have great pleasure in subjoining his remarks in the same terms as they were conveyed to me. His conjectures as to the cause of these alluvial deposits, will be read with great interest by Geologists.
“ The hill at Harston is of a conical form, and situated about a quarter of a mile S. E. of the village, and about five miles S. W. of Cambridge. I selected from the rubble which is deposited on the summit of the hill the following specimens.
1. Rounded pebbles of very hard chalk, scarcely softer than the Antrim limestone. These compose three-fourths of the mass of the rubble.
2. Angular masses of striped flint. These are numerous and very large; one cylindrical piece that I measured being one-third of a foot in diameter and one foot long.
3. Fragments of septaria.
4. Fragments of a shelly limestone.
5. Ochreous balls, resulting probably from the decomposition of pyritous nodules.
6. Angular or scarcely rounded pebbles of trap or greenstone.
7. Organic remains, viz.
- a small fusiform belemnite;
- a large belemnite;
- a small gryphite;
- a large oyster; vide fig. 7 8: 8. Pl. 8. Townshend; some bones and teeth.
The appearance of these specimens is not very inviting; but they are not without interest.
The pebbles of hard chalk are probably the remnants of the bed which immediately covers the green sand and the gault, of which these are either fragments rounded by attrition, or they are the nodules peculiar to the lower chalk, (as may be seen in Wiltshire,) washed out of the bed itself, which is disintegrated.
No. 2 is probably a variety of common flint; specimens of this substance are not uncommon, in which there is an appearance, as in a Scotch pebble, of alternate layers of deposition. The action of air and moisture might render these natural divisions more visible, just as slates are obtained by the exposure of the blocks of fissile stone to the weather.
As other beds, besides the London clay, contain septaria, we cannot say from what bed the fragments of this substance are derived; nor will any of the specimens previous to No. 6, furnish any data for guessing the nature and direction of the current, which has heaped together this mass of confusion.
The mass of greenstone nearest to Cambridge is found in the toadstone beds of Derbyshire, to some specimens of which the pebble No. 6, bears a close resemblance. Few of these pebbles weigh less than 8 ounces.
The large oyster is the same with fig. 7 and 8 of Pl. 8 of Townshend's work, and belongs to the bed which underlies the coral rag.
The large belemnite is peculiar, I believe, to the lower oolite.
The bassetings of the three last mentioned beds, that is, of the toadstone, the coral rag and the lower oolite, are found (at least in England) only in a direction west of Cambridge; so that we are led to explain the accumulation of these alluvia by the agency of a powerful current flowing from west to east.
The rubble rests on the summit of a conical hillock, the sides of which consist of naked grey chalk; than which one can hardly suppose a situation more unfavourable for the accumulation of alluvial matter; at any rate why is not this found in as great abundance on the Hanks as on the summit of the hillock? This looks like the partial destruction of an alluvial level by some subsequent cause, the discovery of which I leave to more learned members of the Society.”
Thus far Mr. Warburton. With respect to the hard chalk pebbles it may be proper to remark, that in some parts of the chalk formation a harder bed is found, of a close grain and compact texture, which might very well supply the material from which these pebbles have been formed. This bed may be seen at Sudbury in Suffolk, and I have also observed it in some parts of the Yorkshire wolds. The striped variety of flint is also to be met with in the ordinary chalk of the same hills. I have before observed that a distinction must be made betwixt this deposit and the ordinary gravel found at a lower level; and in fact throughout the Isle of Ely such a distinction is universally admitted, the one being called the white gravel and the other the red. The same distinction is known in Dorsetshire, as I learn from De Luc's Travels, vol. ii. p. 77, in a passage particularly illustrative of these two deposits.
It would be unnecessary to trouble the Society with any observations on the chalk bed with flints, as I am not aware that they present any thing new, and their phenomena may be studied in other parts of the island to greater advantage. I pass therefore to the lower beds or grey chalk, which composes by far the greatest part of the hills of Cambridgeshire. These beds, as is well known, contain no flints, but, not uncommonly, dispersed masses of the radiated pyrites, globular or kidney-form. It is considerably harder than the common chalk and its colour is usually some shade of grey. It is well known in this county under the name of clunch, and is the material from which the best lime is burnt. Some of the beds are hard enough to serve the purpose of building stone, and are quarried and shaped in blocks for that purpose. It also endures the fire well, and, like the Ryegate stone in London, is much esteemed for the backs of grates and other similar applications.
This stone is dug in the greatest quantities at Reach, a small hamlet in the parish of Burwell, situated on the skirts of the fen country precisely where the Devil's ditch terminates in that direction. The excavations at this place are immense.
Clunch, when burnt, affords a lime in such universal esteem that the crude material is sent from hence for that purpose as far as Peterborough and other distant places, within reach of the water carriage of that level district.
The bedding of a chalk hill is difficult to ascertain, on account of the great number and irregularity of the rifts and joints intersecting the stone in all directions. By careful observations however, made in different places, I am enabled to state that the general direction of the. beds is from the N.E. to the S.W. and that they have a gentle inclination to the S.E. Their direction consequently coincides with the line that I have mentioned above, as dividing the upper from the lower chalk.
In one of the pits at Reach a bed of clunch occurs, which differs from the ordinary sort and presents some remarkable appearances; the mass itself is much harder, and stuck full of concretions of a yellow indurated marl: outwardly they are of a green colour arising from the oxide of iron: they are in general kidney-shaped and of all sizes, from a hazel nut to an ordinary potatoe. The shape of the bed also deserves notice; its general thickness is about fifteen inches, which it preserves for near thirty yards, as appears in the section of the quarry; it then diminishes at each end to a thin edge, and at length totally disappears. At a short distance in front, there happens to be left by the quarry-men an insulated butt, where this bed is not found at the level where it might be expected; the conclusion therefore is, that it has terminated also in this direction in a similar manner: thus, as to its figure, putting the practical geologist in mind of the mineral deposit called a pipework in the lead countries. I must however observe that in an extensive pit at Kneesworth on the other side of the county, the same bed occurs again, where it preserves an uniform thickness and direction like the other beds with which it is associated. In general, I take this to be the bed which Mr. Townsend notices as hard and rubbly, and found in descending the hills from Everly to the vale of Pewsey. It has also been observed by Mr. Warburton at Marlborough.
I have never been able to observe any strong line of separation betwixt the clunch and the succeeding stratum of gault on which it rests. I believe they pass by degrees into each other. The lower beds of clunch become more sandy, and gradually assume the nature of an argillaceous loam. In the next observable stage of transition, the mass assumes a greenish grey colour, and a plentiful admixture of dark green sand is uniformly dispersed through its substance. At the same time it contains numerous irregular dark brown nodules of a ferruginous indurated marl. At length these foreign matters disappear, the mass becomes uniform and ends in the bluish clay or argillaceous marle called gault. This occurrence of green sand in the confines of the two beds was first noticed by Mr. Warburton at the brick pits near the Castle hill, from which he inferred that it always takes place under the same circumstances: an inference which is borne out by the testimony of the most experienced brickmakers about Cambridge.
I am at a loss where to class the bed of sand and sandstone which lies over the gault at Ely, and forms in some measure the elevation on which that city stands. It contains particles of green sand, but not in sufficient quantity to be characteristic. It is principally a concrete of siliceous sand, with small rounded fragments of ironstone and quartz pebbles: the bed is from eight to twelve feet in thickness, and perhaps an alluvial deposit.
It is unnecessary to trouble the Society with an enumeration of all the organic remains that are to be found in the clunch or chalk beds, they are in general the same as occur in other parts of the kingdom where the chalk formation presents itself. Of the remains of fish we find their teeth, bony palates, and in many cases their brown scales not much altered. Cornua ammonia occur and some bivalves apparently of the chama and mytilus genera. Of the former genus I have found the haliotidea very distinct. The anomia terebratula occurs in the beds at Reach. In the vegetable kingdom the fruit cones of Cherry Hinton deserve a particular notice. As early as Woodward's time this fossil had attracted the attention of naturalists, and two very perfect specimens in his collection appear to put their vegetable origin beyond doubt. In confirmation of this opinion, I beg to refer to a fossil in the collection which I have the honor to transmit, which I met with at the same pit, and seems to be the impression of a branch of some vegetable of the fir tribe, with the linear leaves surrounding it.
- A Character of Moses, vol. 1, page 98.