Tertiary Cycles of Sedimentation and Igneous Activity

Abstract

The Cretaceous-Tertiary boundary is the only division between eras in the British post-Cambrian succession which is not bound up with a period of mountain-building. For one thing the British area was a long way from the Mesozoic geosyncline and hence from the orogenic folding of it (the Alpine mountain-building, Chapter 13), so was not directly involved as it was with the Caledonian and Armorican orogeneses. But perhaps a more important point is that the vast Cretaceous marine transgression, which was world wide, followed by the abrupt regression which ushered in the Tertiary, had a catastrophic effect upon the life of the time. At least it seems reasonable to ascribe to that cause the changes in fauna which make Cretaceous and Tertiary as clearly distinct, in their fossil content, as any other two eras. The ammonites and belemnites, the great sea reptiles (ichthyosaurs, plesiosaurs, mosasaurs) and several other characteristic Cretaceous groups of animals were quite suddenly extinguished at the end of the Cretaceous. On the land the dinosaurs died out and left the field free for the rapid expansion of the mammals in the Tertiary. There was also a big change in plant life, the true flowering plants, including grasses, spreading and becoming dominant in the Tertiary; but this was a much less dramatic change which had begun well back in the Cretaceous.