The oil window
The oil window
It is a few million years later—perhaps three, perhaps five. Sediment has been pouring onto a Silurian sea floor that will, much later, be sliced into by a different sea and become the rugged cliff-fringed coastline of central Wales. It has been pouring in so thick and fast that our pebble stuff is now some two kilometres or more down below that sea floor. This is quite rapid burial, even by geological standards, and one can blame changing geography for that. To produce a lot of sediment, there is need for a lot of erosion, and also for the production of something that can be eroded—that is, uplands and mountains on land. On Earth, such production of topography is supplied by the marvellous machine of plate tectonics. And at that time, the ocean between Avalonia and Scotland, that we call the Iapetus Ocean, had just about closed, and those two landmasses were just beginning to nudge into each other. Soft collision, it’s called, when the pressure from the adjoining continents is just enough for sections of crust to begin to be pushed up and (to compensate) pushed down in different places—but not enough for the wholesale crumpling that goes with the creation of great mountain ranges. Thus, the landmass that was then in, and just south of, what is today South Wales was driven upwards, while the floor of the sea that then covered Wales was forced downwards. The resultant flood of sediment was Nature’s means of trying to restore equilibrium. Here, the particular pattern of squashing of the pebble stuff is linked with those enormous, mysterious movements of continents hundreds and thousands of kilometres away. And mysterious they certainly were, for on the heels of the soft collision should have followed the hard collision and mountain-building. But it didn’t. The mountain-building did take place—but only eventually, and not until many millions of years later. The Welsh mountains are quite a bit younger than they should be—and so that story will have to wait. Another story developed in splendid isolation from such tectonic violence. We can showcase it now.
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