It has been a quiet 20 million years for the pebble: an interlude, at somewhere around 3–4 kilometres under the sea floor. The rock has still been crystallizing, but only very slowly. The water has by now mostly been squeezed out, so little fluid has flowed through that rock. At this depth it is hot, well above 1008°C. The pebble-form is sterile, lifeless. The time is now a little under 400 million years ago. We are in the Devonian Period. Above, at the Earth’s surface, changes have been taking place, but as far as they affected the pebble they could be on another planet. In the sea, the graptolites have been going through an evolutionary rollercoaster, with explosions of diversity separated by bad times, when they only just survive. Soon, one of those bad times will be terminal, and they will disappear from the open seas, never to return. By contrast, the fish are beginning to thrive both in the sea and in rivers and lakes. The land is greening, almost explosively, as plants evolve furiously. None of this affects the future pebble. But something soon will. The sea above has been gradually shallowing, filled in with sediment from the encroaching land. Eventually, it changed, some few million years ago, into a vast coastal plain, traversed by rivers. We are about at the time, now, when that lowland is about to rear up to form a range of mountains that—although much reduced from their early glory—can still be climbed today. What took them so long? For the Iapetus Ocean to the north, which, 50 million years ago, was more than 1000 kilometres across, had effectively disappeared 20 million years ago, the ocean plate sliding beneath the northern continent of Scotland and north America. But on Avalonia, the effect was as if these continents had just slid neatly into place, with only minor distortion of the Avalonian crust (and, in truth, these landmasses did approach each other partly from the side, rather than headon). Did the mountain-building force still come from the north, perhaps as some mysteriously delayed intensification of the vice-like grip that held these landmasses together?
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