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When a group of pebbles have been long exposed to attrition by the waters of a powerful stream, especially where the current is intermittent, and where a large quantity of sand has been worked or blown into the stream by freshet or wind storm, they may become rounded by the erosive action of the water or by the abrasive power of the sand, as well as by the attrition consequent upon their sharp contact with one another. This is exemplified in the case of boulders in a river bed, it having been noted in certain streams on the Navajo Reservation that while the upstream sides of the boulders were polished and rounded, and even sometimes faceted and etched, but little change was observable on the downstream sides. This has been tested experimentally, holes an inch in depth having been drilled in opposite sides of sandstone boulders, and on examination five years later in five different localities where this had been done, the deepest hole remaining on the upstream sides measured but four-tenths of an inch, while in one locality the holes had entirely disappeared, and yet so trifling was the effect of the water on the down-stream side that a blue-pencil mark had not been washed away. Of course, the erosion of quartzite and limestone boulders tested in this way proved to be a much slower process, amounting to less than one-hundredth of an inch annually. Another important consideration in the shaping of pebbles by river-water is the swiftness of the current, it having been noted, for instance, that those which have been washed down the steep slopes of the Navajo Mountain and the edge of the Black Mesa are somewhat better rounded than those that have been borne along for a much greater distance by less swift-flowing water.
That striated, faceted, or polished pebbles are always of glacial origin, or that those of glacial origin usually offer these characteristics is far from the fact; indeed, it may rather be said that they are generally missing. The fluvio-glacial drift is much more widespread than ground moraine, and the pebbles found in the former rarely present these aspects; indeed, it has been noted that in an hour's search through the glacial drift of Connecticut, only a single such specimen may be met with. On the other hand, many pebbles of this type have been found under conditions plainly showing that the striation was due to other causes, in some instances, as with those occurring in conglomerates, to pressure and differential movement. |
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Precious Stones Guide Vol 8
>> About the Natural Shaping and Polishing of Precious Gem Stones and Pebbles
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