. Bulletin. Natural history; Science. "The mile-wide mutterings of unimagined rivers" John Clark wmmmi^t-r'- ^;::^^^^s^H':^^w^^^'^'-'^''^^-^. Kipling's Explorer would feel lost in our modern world: there are no unimagined rivers left to explore. But fie need only step back into tfie past to explore tfie unknown again. Wfiat were thie landscapes of tfiirty million years ago? Or forty million, or one fiundred million? Where were the mountains, and what rivers inexorably wore them down? What continents enjoyed tropic balm, where were the deserts, and where and when did ice ages chill th
. Bulletin. Natural history; Science. "The mile-wide mutterings of unimagined rivers" John Clark wmmmi^t-r'- ^;::^^^^s^H':^^w^^^'^'-'^''^^-^. Kipling's Explorer would feel lost in our modern world: there are no unimagined rivers left to explore. But fie need only step back into tfie past to explore tfie unknown again. Wfiat were thie landscapes of tfiirty million years ago? Or forty million, or one fiundred million? Where were the mountains, and what rivers inexorably wore them down? What continents enjoyed tropic balm, where were the deserts, and where and when did ice ages chill the Earth? It is our privilege to search for answers to these questions, to ask the rocks and try to understand what they may tell us. For example, consider the picture of the northeastern Colorado plains, with a sinuous ledge of sandstone sweeping across them. That ledge was once the course of a meandering stream, and the sandstone is the sand and gravel that formed its bed. How wide was the stream, and how deep? As wide as the broken ribbon of rock, about 100 feet, and as deep as the thickest cross-beds, about 4 feet. How fast did the current flow? About 5 miles an hour, judging from the size of the pebbles that it carried. And where were its headwaters? In a particular section of the Colorado Front Range, just at the Wyoming-Colorado border, because only there do granites of the types found in its pebbles occur together. A little arithmetic shows us that this 35-million-year-old river carried about 1,500 cubic feet of water per second, when it was running bank-full. That would be smaller than the Fox and the Desplaines Rivers, and somewhat larger than the Du Page. The stream was, naturally, both swifter and cleaner than our rivers in northeastern Illinois. Studies like this, of every 35-million- year-old stream deposit from Colorado through Montana, make possible maps of ancient geography. From them we can estimate, roughly, the runoff and the rainfall of the past. Falls of as
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