Article 64

Knowledge Of A New Continent – Britain Joined Together In The Devonian Period.

In his Chapter 6, Peter Toghill recalls that the Devonian Period saw remarkable contrasts in the geological development of Britain. In the south around the margins of the Rheic Ocean (now between Britain and Gondwana yet further to the south) marine Devonian sediments of Gondwana advanced northwards and merged into the sediments of Old Red Sandstone formed on the coastal plains of Wales and England, which were in turn receiving eroded sediments from the Caledonian mountains further to the north, these being eroded in an arid climate at 10-15 degrees south of the equator, but with periods of heavy rainfall causing flash floods. Over Scotland and the far north-east of England great thicknesses (up to 10 km) of Old Red Sandstone sediments accumulated in lakes within inter-montane basins, river valleys and flood plains. These lakes were by now supporting a wide range of primitive fish and land plants were colonising the surrounding marshy areas, and insects and arthropods were appearing on land and winged insects were taking to the air in the late Devonian Period. Meanwhile great thicknesses of lava (up to 3 km thick) were erupting from volcanoes in the Midland Valley area and the southern Highlands where cauldron subsistence occurred. This volcanic activity also expressed the ‘newer’ granites of Scotland which formed during the late Silurian and early Devonian Periods. There is also evidence that the Dalradian rocks metamorphosed during the early Ordovician and folded up as the Caledonian Mountains, and were not adjacent to the Midland Valley until the late Devonian as fragments are absent from the early Red Sandstone of the Midland Valley. Tear faulting along the Highland Boundary Fault could have brought the two areas together, and this fault together with the Great Glen Fault may be a terrane boundary. By the end of this period, the relief had become low enough for the Rheic Ocean to spread its shallow continental shelf seas over many areas at the start of the Carboniferous Period.

Again, in his Chapter 7: Tropical Seas And Coral Swamps -The Carboniferous Period, Peter Toghill recalls that the Carboniferous was so named in 1822 by Conybeare and Phillips, to include the coal-bearing (carbonaceous) strata in Britain; and that it is now estimated that this period of time lasted around 73 million years from 363 to 290 Ma. By the end of the Devonian, the Caledonian Mountains had been eroded down to lower altitudes and the Old Red Sandstone Continent was invaded by the shallow-self seas of the Rheic Ocean in a world-wide (eustatic) rise in sea level, caused by melting ice sheets around the south pole and/or by an increase in plate tectonic activity at mid-ocean ridges, the increased volume of such ridges displacing oceanic water onto adjacent continental shelves. The British area, 360 Ma was astride the equator and within a single continent within a single surrounding ocean but relatively close to an area of that Ocean now called the Tethys Sea which was warm and laying down tropical carbonates which now form the Carboniferous Limestone of the Pennines and other areas. Later these shallow seas were invaded by deltas formed by rivers flowing in from adjacent high ground. The sandstones formed in these deltas became Millstone Grits of the now northern English moors. The then humid climate and these deltas began to support swamps and tropical rain forests while these rapidly changing deltas, repeated inundations of the swamp areas, and the burial and decay of the luxuriant vegetation led to the eventual formation of numerous coal seams and of the now famous Coal Measures. This tripartite division into Carboniferous Limestone, Millstone Grit and Coal Measures can be seen over most of Britain, except in the south-west of England where deeper seas of the Rheic Ocean, produced a sequence of sandstones and mudstones, but without lime stones or conspicuous coal seams, while in Scotland, the division into Carboniferous Limestone and Millstone Grit is also less obvious and volcanic lavas and ashes are common.

During the Carboniferous, marine life continued to evolve with goniatites (ancestors of ammonites) being useful as zone fossils, with corals and brachiopods being common in early limestones, and with non-marine bivalves being useful for dating the Coal Measures. Land plants evolved to provide large trees which provided the Coal Measures. Amphibians and early reptiles evolved in humid forests and spiders and giant flying insects were features of the dense vegetation. Thereafter, a major cooling occurred with glaciation over the continents of Gondwanaland which then included the south pole, perhaps because the plant growth of the Carboniferous had removed significant amounts of carbon dioxide from the global atmosphere. Thereafter, Peter Toghill’s Chapter 7 provides further details of the Carboniferous Period which is sub-divided into the Dinantian Series which roughly equates to the Limestone, the Namurian to the Millstone Grit and the Westphalian to the Coal Measures, while it is unlikely that rocks of the Stephanian occur anywhere in Britain. However, while the details of this subdivision need not concern us here, Peter Toghill explains them under the subheads, the stratigraphical framework, the Carboniferous rock sequences of Britain, Devon and Cornwall, the south-west province, the Millstone Grit and Coal Measures, North Wales, The English Midlands, The Pennines and northern England, and Scotland, before summarising all of it as follows. 

The Carboniferous Period, 363 -290 Ma, saw an initial spreading of a warm and shallow tropical sea over the eroded remnants of the Caledonian Mountains and the Old Sandstone Continent. This sea deposited Carboniferous Limestone over many parts of Britain, but in the northern Pennines, limestones are part of cyclical sequences which also contain clastic sediments caused by rapid sea-level changes, while in south-west England, deeper Rheic Ocean sediments without limestones form the Carboniferous Culm Measures. Again, in Scotland the early Carboniferous lagoonal limestones and fluvial sandstones also contain great thicknesses of basaltic lavas, and this volcanicity continued throughout the Carboniferous. Yet again, the early Carboniferous seas were invaded by rivers and deltas forming the Millstone Grit during the middle of the period, while during the later Carboniferous these supported luxuriant swamps and rain forests, while these deltas and their swamps were repeatedly covered by marine and lagoonal sediments caused by rapid changes in sea level. The thus buried peat deposits quickly decayed to form coal seams between the sandstones and shales of the Coal Measures. Towards the end of the period, the Variscan orogeny (see later) caused the now British area to rise above sea level and the change from a humid to an arid climate caused the formation of continental sediments.