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The stratigraphical succession of the Cardiff district ranges from Silurian to Lower Jurassic, while structurally the rocks have been affected by Caledonian, Hercynian and Alpine movements. Caledonian folding is relatively weak but powerful Hercynian (Asturian) folding and faulting took place about the end of the Westphalian; the elongate South Wales Coalfield Basin being formed at this time. Mesozoic strata, up to the Liassic, are also folded and faulted by movements which may have been as late as the Miocene.
Silurian rocks which occur in the Usk and Rumney Inliers consist of sandstones, siltstones and shales (often calcareous) as well as some limestones. The argillaceous rocks often weather deeply and degenerate to clay with rock lithorelicts, consequently they pose problems in foundations and cuttings, e.g. on the east side of Cardiff.
The Old Red Sandstone, both Lower and Upper divisions are present, is made up of marls, sandstones and conglomerates. Some of the sandstones are aquifers and provide water in commercial quantities. The marls, especially where steeply inclined are liable to slipping, as happened for example, in the Brynglas (M4) Tunnel at Newport.
The Carboniferous Limestone surrounds the coalfield and consists mainly of limestone and dolomite (see also below).
The Millstone Grit does not contain the gritty sandstones of the Pennines and is made up mainly of strong siliceous sandstones and shales. The Coal Measures show the usual lithology; a
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