Upper Lias.—The Upper Lias is chiefly represented by a thin bed of clay, with some characteristic fossils. It occurs on the hills of Fenny Compton and elsewhere, and there is evidence to show that it formerly capped the range of the Edge Hills adjacent, occupying its natural position above the marlstone, or Middle Lias, of which they are mainly composed. From Fenny Compton to Harbury, a good descending section may be obtained from the marlstone (rock bed), through the underlying clays and marly beds, through the “Lima Beds” and White Lias, to the New Red Marls at Harbury.
Marlstone or Middle Lias.—The Marlstone (rock bed) is largely quarried on the Avon and Burton Dassett Hills. It forms a good building stone, more or less indurated, of a green or yellow brown colour, sometimes ferruginous. It forms a conspicuous range of hills of moderate height of which Edge Hill is the highest, from which it strikes southward towards Oxfordshire. The plain below is occupied by the underlying division of the Lower Lias. In this county the marlstone contains very few fossils, and those chiefly brachiopodous shells belonging to the genus Terebratula. In most cases elsewhere the Marlstone proper, or highest zone, is very fossiliferous, and abounds in marine shells, which are usually well preserved. The sandy beds immediately below are rarely exposed, but crop out in a lane near Bitham House, where as usual they contain many fossils. The inferior clays and marls are not visible except in some brick pits near Fenny Compton and along the line of railway. These are very full of fossils in the zone of Ammonites Jamiesoni and Ibex, here nearly one hundred feet thick, and especially at one horizon in a coarse, hard, stony band which contains numerous corals towards the upper part of the cutting, near the station.
Lower Lias.—For the most part this formation spreads over the portion of the country on the north-east, east, south-east, south and south-west of Warwick. A remarkably fine section is exposed in the railway cutting near Harbury Station. This portion of the series is also largely quarried at Rugby, and in other places south and south-east of Stratford. The strata consist of beds of blue clay or shale interstratified with beds of blue rubbly and argillaceous limestone, much quarried for hydraulic lime. One good section of the lime-yielding beds occurs at Messrs. Greaves and Lakin’s Quarries at Stockton and Harbury. The lowest zones of the Lias are largely quarried at Wilmcote, and may be seen at the remarkable outlier of Brown’s Wood, near Henley-in-Arden, and at another (Copt Heath), near Knowle. These two last are of special interest, because they shew the lowest beds of the Lias (in connection with and passing into the Rhætics,) resting immediately upon the New Red Marls. The thickness of the Lower Lias in the county is above 600 feet; but only the inferior zones of Ammonites angulatus and A. planorbis are laid open to any great extent. The best sections of the Lima beds (A. angulatus zone) occur in the railway cutting at Harbury, Stockton lime quarries, and the extensive quarries at Newbold near Rugby. Fossils are not very numerous, but the following occur:—Gryphea incurva, Rhynchonella variabilis, Ammonites angulatus, Pecten, various species, Lima gigantea, and bones and teeth of Plesiosaurus and Ichthyosaurus. Fish are comparatively rare, two or three only were found at Harbury and a very few near Rugby.
The higher ground round Wilmcote and Binton is also capped by these Lima beds; but the district is more or less affected by small faults, so that certain beds in one contiguous quarry are absent in another. The lower limestones (insect beds) are largely worked in this locality, and are of much economical value. With the exception of remains of insects and fragments of plants, the fossils are entirely marine, Ammonites planorbis and A. Johnstoni, being abundant and characteristic. Crustacea belonging to the genera Astacus and Eryon, the latter of great size are not unfrequently met with. Small fishes, Pholidophorus Stricklandi, and the larger Dapedium and Tetragonolepis more rarely occur. A fine example of the latter is preserved in the Warwick Museum. The large Enaliosaurians are well represented by some fine specimens of Ichthyosaurus and Plesiosaurus; the P. megacephalus in the Museum at Warwick being nearly entire, measuring 14 feet 4 inches in length. Large masses of driftwood and a few fronds of ferns are sometimes met with. But perhaps the most interesting and remarkable fossils are the insects. Twenty-four families and genera were determined more than twenty years ago, since which time many important additions have been made. The Coleoptera and Neuroptera are most numerous. Small beetles are not unfrequently found entire. Among these may be noted the families Buprestidæ, Elateridæ, Carabidæ, and others.
There are also remains of Orthoptera, Homoptera, Libellulidæ, and some Diptera. Many of the Neuroptera were evidently of gigantic proportions, but most of the insects were of small size, and like the associated plants, are indicative of a temperate climate. They are most nearly allied to forms which now inhabit North America. There are few extinct or unknown genera among them.
Rhætic Series.—The highest beds referred to this series consist of certain hard, fine grained limestones, which, from their ordinary white colour, have been termed White Lias. They occupy a considerable area south and south-east of Warwick. They constitute a purely local deposit, and are confined for the most part to this county and Somersetshire. They are often close-grained and hard limestones, and make a useful building material and a good lime. Their colour is mostly white, with a yellow tinge, and occasionally pink and grey. Some geologists consider these beds to belong to the “Rhætic Series,” others to the passage beds between the Lias and the latter, while others still class them with the Lias.
The undisputed Rhætic rocks lie between the White Lias and the Triassic Marls. In Warwickshire they are rarely exposed, and then much reduced in bulk, compared with their development in Gloucester and Glamorgan. They may be seen to a limited extent below the White Lias in the railway cutting at Harbury, where a band of yellowish sandstone contains the small bivalved crustacean, Estheria minuta; and also at the small outlier of Brown’s Wood, and at Stooper’s Wood, near Wooton Wawen, where this sandstone occurs with inferior shelly limestones and sandy bands, containing the usual Rhætic fossils, e.g., Cardium Rhæticum, Avicula contorta, Pleurophorus elongatus, Pecten valoniensis, and Schizodus cloacinus. The nearest exposure of the Rhætic to Birmingham occurs round the fringe of an outlier of Lower Lias resting on the Upper Red Marl near the village of Knowle. This outlier is about a mile and a half long, by half a mile broad. Its highest beds at Copt Heath contain Ammonites planorbis. The beds referred to the Rhætic include a stratum of yellow micaceous sandstone full of Schizodus cloacinus, which, though usually in the form of casts, is sharp and well defined. The bone bed, though no where exposed, is probably present in its normal position. A fine section, with numerous characteristic Rhætic fossils was exposed on the railway cutting at Summer Hill, between Stratford and Alcester. Rhætic black shales were passed through at Snitterfield, in making a tunnel in connection with the new reservoir for Stratford.
Glacial and Post Tertiary Deposits.
BY H. W. CROSSKEY, LL.D., F.G.S.