A great deal

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A great deal of land out there had not been there when the carbonate rocks sloped away to ocean-crustal deeps in Ordovician time. California, Oregon, Washington, British Columbia had appeared where tl1ere was no continental structure of any kind. Up and down the western margin, in fact, there was an unaccounted-for swath of land averaging four hundred miles wide. There was also the whole of Alaska. How did all that country come to be where it is? What compressed the western mountains? If Europe were on the international date line, these conference room rotterdam questions would have a ready answer, but inconveniently it was not.
No one was enthusiastic enough to suggest a hit-and-run visit from China. Where, then, since Ordovician time, had the North American continent acquired nine hundred million acres of land? There was an answer in the concept of microplates, also known as exotic terranes. New Guineas, New Zealands, New Caledonias, Madagascars, Kodiaks, Mindanaos, Fijis, Solomons, and Taiwans had come over the sea to collect like driftwood against the North American craton. The first such terrane identified was Wrangellia, named for the stratovolcanic Wrangells, some of the Fujis of Alaska. Dozens of other exotic terranes have since been named-Sonomia, Stikinia, the Smartville Block. If a piece of country is possibly exotic and possibly not-if it is so enigmatic that no one can say whether it has come from near or far-it is known as suspect terrane. I returned one time from a visit to the country north of the Tanana River, in easternĀ conference room amsterdam interior Alaska, where streams that resemble gin come down from mountains and into the glacial Yukon. A geologist in New Jersey welcomed me home with an article from Nature which described the Alaskan region of the upper Yukon. “The terrane is probably composite,” said Nature, “with nappes of upper Palaeozoic oceanic assemblages thrust across a quartzo-feldspathic and silicic volcanicrich protolith of probable Precambrian to known Palaeozoic age and of unknown continental affinity.” I was appalled to discover that that was where I had been, and mildly disturbed to learn that terrain long familiar to me had now become suspect.