THE NEW RED SANDSTONE.
This formation has been already alluded to at pp. 5 and 8; it is, next
to the chalk and lower greensand, the most extensive source of water
supply from wells we have in England, and although the two formations
mentioned occupy a larger area, yet, owing to geographical position, the
new red sandstone receives a more considerable quantity of rainfall,
and, owing to the comparative scarceness of carbonate of lime, yields
softer water.
The new red sandstone is called on the Continent "the Trias," as in
Germany and parts of France it presents a distinct threefold division.
Although the names of each of the divisions are commonly used, they are
in themselves local and unessential, as the same exact relations between
them do not occur in other remote parts of Europe or in England, and are
not to be looked for in distant continents. The names of the divisions
and their English equivalents are:
1. Keuper, or red marls.
2. Muschelkalk, or shell limestones (not found in this country).
3. Bunter sandstone, or variegated sandstone.
The strata consist in general of red, mottled, purple, or yellowish
sandstones and marls, with beds of rock-salt, gypsum pebbles, and
conglomerate.
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.
------------------+----------------+---------------+---------------
| Lancashire | | Leicestershire
Names of Strata. | and | Staffordshire.| and
| West Cheshire. | | 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.