The region over which triassic rocks outcrop in England stretches across the island from a point in the south-western part of the English Channel about Exmouth, Devon, north-north-eastward, and also from the centre of this band along a north-westward course to Liverpool, thence dividing and running north-east to the Tees, and north-west to Solway Firth.
In central Europe the trias is found largely developed, and in North America it covers an area whose aggregate length is some 700 or 800 miles.
The beds, in England, may be divided as follows;
| Average Thickness. | ||
| Keuper— | Red marls, with rock-salt and gypsum | 1000 ft. |
| Lower Keuper sandstones, with trias sandstones and marls (waterstones) | 250 ft. | |
| Dolomitic conglomerate | ||
| Bunter— | Upper red and mottled sandstone | 300 ft. |
| Pebble beds, or uncompacted conglomerate | 300 ft. | |
| Lower red and mottled sandstone | 250 ft. |
The Keuper series is introduced by a conglomerate often calcareous, passing up into brown, yellow, or white freestone, and then into thinly laminated sandstones and marls. The other subdivisions are remarkably uniform in character, except in the case of the pebble beds, which in the north-west form a light red pebbly building stone, but in the central counties becomes generally an unconsolidated conglomerate of quartzose pebbles.
The following tabulated form, due to Edward Hull, Esq., M.A., shows the comparative thickness and range of the Triassic series along a south-easterly direction from the estuary of the Mersey, and also shows the thinning away of all the Triassic strata from the north-west towards the south-east of England, which Hull was amongst the first to demonstrate.
| Thickness and Range of the Trias in a S.E. direction from the Mersey. | |||
| Names of Strata. | Lancashire and West Cheshire | Staffordshire. | Leicestershire and Warwickshire. |
| Keuper Series— | |||
| Red marl | 3,000 | 800 | 700 |
| Lower Keuper sandstone | 450 | 200 | 150 |
| Bunter Series— | |||
| Upper mottled sandstone | 500 | 50 to 200 | absent |
| Pebble beds | 500 to 750 | 100 to 300 | 0 to 100 |
| Lower mottled sandstone | 200 to 500 | 0 to 100 | absent |
The formation may be looked upon as almost equally permeable in all directions, and the whole mass may be regarded as a reservoir up to a certain level, from which, whenever wells are sunk, water will always be obtained more or less abundantly, This view is very fairly borne out by experience, and the occurrence of the water is certainly not solely due to the presence of the fissures or joints traversing the rock, but to its permeability, which, however, varies in different districts. In the neighbourhood of Liverpool the rock, or at least the pebble bed, is less porous than in the neighbourhood of Whitmore, Nottingham, and other parts of the midland counties, where it becomes either an unconsolidated conglomerate or a soft crumbly sandstone. Yet wells sunk even in the hard building stone of the pebble beds, either in Cheshire or Lancashire, always yield water at a certain variable depth. Beyond a certain depth the water tends to decrease, as was the case in the St. Helen’s public well, situated on Eccleston Hill. At this well an attempt was made, in 1868, to increase the supply by boring deeper into the sandstone, but without any good result. When water percolates downwards in the rock we may suppose there are two forces of an antagonistic character brought into play; there is the force of friction, increasing with the depth, and tending to hinder the downward progress of the water, while there is the hydrostatic pressure tending to force the water downwards; and we may suppose that when equilibrium has been established between these two forces, the further percolation will cease.
The proportion of rain which finds it way into the rock in some parts of the country must be very large. When the rock, as is generally the case in Lancashire, Cheshire, and Shropshire, is partly overspread by a coating of dense boulder clay, almost impervious to water, the quantity probably does not exceed one-third of the rainfall over a considerable area; but in some parts of the midland counties, where the rock is very open, and the covering of drift scanty or altogether absent, the percolation amounts to a much larger proportion, probably one-half or two-thirds, as all the rain which is not evaporated passes downwards. The new red sandstone, as remarked, may be regarded, in respect to water supply, as a nearly homogeneous mass, equally available throughout; and it is owing to this structure, and the almost entire absence of beds of impervious clay or marl, that the formation is capable of affording such large supplies of water; for the rain which falls on its surface and penetrates into the rock is free to pass in any direction towards a well when sunk in a central position. If we consider the rock as a mass completely saturated with water through a certain vertical depth, the water being in a state of equilibrium, when a well is sunk, and the water pumped up, the state of equilibrium is destroyed, and the water in the rock is forced in from all sides. The percolation is, doubtless, much facilitated by joints, fissures, and faults, and in cases where one side of a fault is composed of impervious strata, such as the Keuper marls, or coal measures, the quantity of water pent up against the face of the fault may be very large, and the position often favourable for a well. An instance of the effect of faults in the rock itself, in increasing the supply, is afforded in the case of the well at Flaybrick Hill, near Birkenhead. From the bottom of this well a heading was driven at a depth of about 160 feet from the surface, to cut a fault about 150 feet distant, and upon this having been effected the water flowed in with such impetuosity that the supply, which had been 400,000 gallons a day, was at once doubled.
The water from the new red sandstone is clear, wholesome, and pleasant to drink; it is also well adapted for the purposes of bleaching, dyeing, and brewing; at the same time it must be admitted that its qualities as regards hardness, in other words, the proportions of carbonates of limes and magnesia it contains, are subject to considerable variation, depending on the locality and composition of the rock. As a general rule, the water from the new red sandstone may be considered as occupying a position intermediate between the hard water of the chalk, and the soft water supplied to some of our large towns from the drainage of mountainous tracts of the primary formations, of which the water supplied from Loch Katrine to Glasgow is perhaps the purest example, containing only 2·35 grains of solid matter to the gallon. Having besides but a small proportion of saline ingredients, which, while they tend to harden the water, are probably not without benefit in the animal economy, the water supply from the new red sandstone possesses incalculable advantages over that from rivers and surface drainage. Many of our large towns are now partially or entirely supplied with water pumped from deep wells in this sandstone; and several from copious springs gushing forth from the rock at its junction with some underlying impervious stratum belonging to the primary series.