Page 89 - Hydrogeology Principles and Practice

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72 Chapter Two

The Carboniferous strata include the massive,
well-fissured karstic limestones that give large sup-
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plies of up to 10 m s from springs in the Mendip
Hills (see Section 2.7), South Wales and, to a lesser
extent, Northern Ireland. Later, rhythmic sequences
of massive grits, sandstone, limestone, shale and coals
produce minor supplies from fissured horizons in
sandstone and limestone and form aquifers of local
significance, particularly from the Yoredale Series
in northern England and in the Midland Valley of
Scotland.
The Old Red Sandstone is the principal aquifer unit
of the Devonian Period and includes sandstones,
Fig. 2.33 Exposure of Triassic Sherwood Sandstone (Otter
marls and conglomerates that yield small supplies
Sandstone Formation) at Ladram Bay, South Devon. The Otter
from sandstones in the Welsh borders. In Scotland,
Sandstone Formation comprises predominantly fine- to medium-
grained red-brown, micaceous, variably cemented, ferruginous the Upper Old Red Sandstone is of much greater
sands and sandstones, with occasional thin silt and conglomerate significance and around Fife and the southern flank of
lenses. Frequent, well-developed bedding planes show a gentle
the Moray Firth consists of fine- to medium-grained
2–4° east-south-east dip, with, as shown in this cliff face, cross-
sandstones, subordinate mudstones and conglomer-
bedding also typically present. Cementation along bedding planes
ates with good intergranular permeability yielding
exerts a small but potentially significant control in promoting
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horizontal groundwater movement (Walton 1981). Two supplies of up to 10 m s . Widespread outcrops
prominent fissure openings along cemented bedding planes are of fine- to medium-grained Lower and Middle Old
visible to the left, in the lower one-third and upper two-thirds, of
Red Sandstones, in places flaggy, with siltstones,
the face shown.
mudstones and conglomerates as well as interbedded
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lavas give borehole yields ranging from 10 m s
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in the Borders to 10 m s in Ayrshire and parts
a desert environment and much of the fine- to of Strathmore. In Northern Ireland the principal
medium-grained cross-bedded sandstones are soft, Devonian lithology is conglomerate with some sand-
compact rock that is only weakly cemented. Ground- stone, subordinate mudstone and volcanic rocks but
water can flow through the intergranular matrix but these are indurated and poorly jointed with small
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the presence of fractures enhances the permeability borehole yields of 10 m s where secondary per-
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considerably giving good yields of up to 10 m s of meability is present.
good quality. Above the Triassic sandstones, the Silurian, Ordovician and other Lower Palaeozoic
Mercia Mudstone confines the underlying aquifer and Late Precambrian sedimentary facies pre-
although local supplies are possible from minor dominate in southern Scotland, the north-east and
sandstone intercalations. Up to 300 m of Triassic north-west of Ireland and Wales and consist of great
sandstone occurs in the Lagan Valley and around thicknesses of highly indurated and tectonically
Newtownards in Northern Ireland where it is deformed shales, mudstones, slates and some lime-
intruded by many basalt dykes and sills with yields of stones and sandstones (Fig. 2.34). Some groundwater
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up to 10 m s obtained from a fine- to medium- may occur in shallow cracks and joints that produce a
grained sandstone. Smaller isolated Permian sands subsurface permeable zone in which perched water
and sandstones are found in north-west England and tables may occur that support occasional springs and
south-west Scotland. At the base of the Permian, and shallow boreholes providing small yields.
overlain by red marls, the Magnesian Limestone
forms a sequence of massive dolomitic and reef
Metamorphic rocks
limestones that are important for water supply in the
north-east of England where typical yields range up The Lower Palaeozoic and Precambrian crystalline
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to 10 m s . basement rocks of the Highlands and Islands of