Article 62

Knowledge Of The Cambrian, Ordovician, And Silurian Periods.

In his Chapter 2, Animals In Abundance – The Cambrian Period, Peter Toghill summarises our knowledge of what is now the British Isles at a time when it consisted of two areas that were up to 7000 km apart on either side of the widening Iapetus Ocean before coming together again as this Ocean closed; that this ocean did widen, is indicated by the lack of andesitic volcanicity in any of the marine sequences; that life in Cambrian times existed in shallow-shelf areas on either side of this ocean; but that its width resulted in faunal provinces; and that the exceptional preservation of fossils suggests a greater variety of Cambrian life forms than we often find fossilised. In his Chapter 3: Volcanoes Of The Iapetus Ocean–Ordovician Period, he summarises our knowledge that this period contains great thicknesses of volcanic rocks (particularly andesites); that these indicate the continued but erratic closure of the Iapetus Ocean with subduction zones on either side; that while southern Britain moved steadily northwards covering a distance of 3000 km during the Ordovician from near the Antarctic Circle to around 30 degrees South, Scotland moved only a little from its equatorial latitudes. On the Scottish continental shelf shallow-water sediments and old oceanic crust merged south into deeper water greywackes and black shales while this northern margin of the Iapetus was affected by the Ordovician Grampian orogeny, which folded up the Grampian Highlands and caused high-grade metamorphism while on the southern margin of the Iapetus, huge volcanic islands erupted vast thicknesses of Ordovician lavas and ashes, interbedded with richly fossiliferous deep- and shallow-water sediments; that some animals were entombed by volcanic ash falls; that the Welsh and Lake District basins merged south into a shelf which occupied parts of the Welsh borders; and that the southern shoreline of the Iapetus is traceable through Shropshire.

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