The main feature that characterised the Cambrian period all over the world was the evolution of advanced forms of marine life. This is made apparent in sedimentary rocks laid down in the region now occupied by the British Isles. At the same time there was a general rise in relative sea level that took place throughout most of the Cambrian period, only steadying during the final few million years.
The rise in sea level led to the steady erosion and submergence of large areas of what had been the old Pre-Cambrian land surface, unconformably covering over the folded Pre-Cambrian rocks with shallow-water marine sediments. In Scotland, for example, the basal Laurentian Cambrian marine beds can be seen to unconformably overlie the Pre-Cambrian Torridonian Sandstones.
The area now occupied by England and southern Ireland was part of a small crustal plate called Avalonia, being situated close to the then south pole - as can be seen on Plate Migration Map 3.
The main, more-detailed, Cambrian map of the British Isles and Ireland has a diagonal SW-NE dividing line representing many thousands of kilometres of Iapetus Ocean between Avalonia to the south-east and Laurentia to the north-west.
Scotland and north-western Ireland were a coastal part of the completely separate Laurentian plate which was rapidly migrating northwards. To the south-west is marked the Avalonian Plate, with the shallow sourthern margin of the Ran Sea covering what is now England and Southern Ireland, depositing grits, sandstones and silts.
At this time the distance between Avalonia and Laurentia was several thousand kilometres, as shown on Plate Migration Map 3. The plate more immediately adjacent to Avalonia was Baltica, which was separated from Avalonia by the smaller Ran Sea. The Laurentian Plate was already moving away during the Cambrian and would continue to do so during the Ordovician. It would not move back to collide with Avalonia, and fix Scotland onto England and southern Ireland until the end of the Silurian. Thus the Scottish Cambrian beds were deposited thousands of kilometres away from the Welsh and English ones, and display characteristics more associated with the present Greenland and North American continent than Wales or England.
A low-lying Avalonian land area locally called Pretannia is shown on the map, bordered to the north-west by two unstable, geosynclinal sinking regions where silts, sands, and grits were deposited to a great thickness. Trilobites and brachiopods were amongst the rich fauna found in the rocks of the period. The Pretannia coastline follows the general structural trends of the Cambrian in Avalonia.
The steadily sinking sea floor in the Welsh Basin often resulted in sediment instability, with characteristic turbidite and greywacke formation. As well as the grits and sands illustrated on the detailed map, which show evidence of the north-easterly flowing currents, there are sedimentaries displaying graded bedding; the lower portion of each bed containing increasingly coarse particles towards its base. Sediments disturbed by storms or seismic events flowed towards the basin centre in the form of turbidity currents; upon reachig the deeper, flatter sea floor, they spread out and settled, with the coarser fraction dropping out first, followed later by the settling out of increasingly finer particles.
Although normally common in geosynclinal environments, volcanic rocks are rare in the British and Irish Cambrian sequences. Deposits across the Iapetus Ocean in the Laurentian coastal region of what is now Scotland, were primarily quatrzitic sands, sequentially followed by dolomitic limestones.
South of Scotland, and back across the Iapetus Ocean, in Avalonia, owing to the great variety of Cambrian strata, it is generally accepted that the Cambrian should be simply divided into Upper, Middle, and Lower, as shown in the upper diagram below:
The lower diagram below shows the approximate relationship between the Welsh sequence of rocks and the north Scotland Cambrian rocks of Laurentia. The difference at that time was caused by the still huge gap across the Iapetus Ocean. Off the Laurentian coast, predominantly siliceous marine sediments were followed by calcium carbonate limestones. These comprise the rocks now called the Durness Quartzite and the overlying grey-coloured, well-bedded Durness Limestone.