geologically accepted answer would Dr. Alley and the ranger have given the tourist? A. The river used to be much wider before the desert formed, and so cut a wide canyon, but the river has narrowed as the drying occurred, and now cuts only a narrow canyon. B. The canyon is really the same width at the top as at the bottom, but the well-known “optical illusion” of distant things appearing smaller causes it to look as if the canyon is narrowing downward. C. The river used to be much wider because it was not steep, and water spreads out when running slowly (a little tap feeds a big bathtub…); then, as the Rockies were raised, the river steepened and narrowed, so it used to cut a wide canyon and now cuts a narrow one. D. The river cuts down, and that steepens the walls of the canyon, which fall, topple, slump, creep or flow into the river to be washed away, thus widening the canyon above the river. E. The river used to meander, cutting a wide swath, but now runs straight, cutting a narrow swath. The tourist suggested that the river has gotten narrower over time. Dr. Alley asked the tourist whether he would ever consider going out on a particular narrow pillar of rock (already teetering dangerously and separated from the walls of the canyon by a huge crack) with a few hundred of his closest friends, and jumping up-and-down vigorously. Predictably, the tourist said "of course not, it might fall over." Dr. Alley then pointed out the many places where rocks clearly had fallen off the cliffs and moved downhill, at which point the tourist quickly switched his opinion to the "down-cutting" river explanation, with the ranger thoroughly enjoying the show. Points Earned: 1/1 Correct Answer: D Your Response: D
14. In the photo above, Sam Ascah is standing on sand and gravel in a pothole, where a stream swirls during the short but intense thunderstorms of Zion National Park. And next to that stream, the other picture shows the sandstone and the hang-on-so-you-don't-fall-over-the-cliff chain along the trail. A likely interpretation of these features is: A. The Park Service carefully cut little grooves behind the chain before they hung it, so that it would look cute and slide well, and they cut the potholes so that hikers would have something to look at. B. The potholes and the grooves behind the chain were gnawed by giant marmots. C. The potholes and the grooves behind the chain were gnawed by giant beavers. D. The stream swirled rocks around and cut the potholes, and even bounced up the cliff to cut the notches behind the chain. E. The grooves behind the chain have been cut over decades by motion of the chain as hikers grabbed it, and the potholes were cut by water swirling rocks around during the rare floods over much longer times. The chain really has hung there for decades, and has been scraped against the cliff dozens of times per day each summer, slowly wearing into the easily broken sandstone. The stream does swirl rocks around and slowly wear down the potholes. The potholes were there beside the cliff when the trail was established, and haven’t changed too much over decades.
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