Ordovician

Definition and stakes

1911 Encyclopædia Britannica (1911)

Its formations are much more widespread than those of any other system since the Ordovician. They appear at the surface in great areas in the interior, in the south-west and about many of the western mountains. In many places in the west they rest on what appear to be Ordovician beds, but without unconformity. The explanation of the apparent conformity of the strata from the Cambrian to the Pennsylvanian in some parts of the west, with no fossils defining with certainty any horizon between the Ordovician and the Mississippian, is one of the open problems in the geology of the United States.
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Kingsley Garland Jayne and Edgar Prestage 1911 Encyclopædia Britannica (1911)

The Tertiary deposits cover the plain of the Tagus and are found in other low-lying areas near the coast. Of the Lower Palaeozoic rocks the Ordovician appears to be the most widely-spread. Large areas have been referred to the Cambrian, but it is only at Villa Boim, about 6 m. W.S.W. of Elvas, that Cambrian fossils have been found. The Ordovician beds have yielded fossils in several places, Vallongo and Bussaco being amongst the best-known localities. The succession is similar to that of Brittany and Spain.
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O. J. R. Howarth, H. R. Mill, J. A. Howe, A. Mawer and W. A. Phillips 1911 Encyclopædia Britannica (1911)

Next in point of age comes the Ordovician system, which is well developed upon the Shropshire border and in the Lake District. In the same two areas we find the Silurian rocks, shales and limestones with grits and flags. In N. and S. Devon are the Devonian limestones, grits and shales; the corresponding Old Red Sandstone type of the system (marls and sandstones) being exposed over a large part of Herefordshire, stretching also into Shropshire and Monmouth.
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