Experimental Abrasion of Pebbles: 2. Rolling by Current

The abrasion of pebbles rolled in a large concrete basin by a revolving current, both on a sandy and on a pebbly floor, was studied-a setup believed to be a substantial improvement on the customary tumbling-mill experiments. One of the most significant results is that, on a sandy floor, abrasion is less than on a pebbly bottom under similar conditions. The difference increases with size of the rolling pebble and can be four or five times less for medium pebble sizes. Hence reports on measurements of roundness in nature should always be accompanied by statements not only of the size of the pebbles, of current velocities, and of type of rock investigated but especially as to the nature of the bottom. On a sandy bottom, weight and velocity have only slight influence on the percentage of abrasion per kilometer. With increasing roundness, there is a small reduction in the rate of abrasion. It was found that abrasion on a pebbly floor is reduced by 10-15 per cent by the introduction of sand. The abrasion on a pebbly floor increases in proportion to the square of the velocity. Over the natural range of stream velocities it is doubled for fine gravel. Pebble weight is of even greater importance, the percentage of loss at low velocities being proportional to the diameter. It increases three to four times between fine and coarse gravel. Increasing roundness, from sharp-cornered to subrounded, reduces abrasion significantly; beyond subroundness, abrasion is reduced much less. As a result of these relations, coarse gravel can lose four to five times as much in percentages as can fine gravel at equal velocities. Twenty-five times as much loss per kilometer is possible under extreme conditions of velocity, size, and bottom cover. The abrasion processes are shown to be as follows: splitting (= breaking), crushing, chipping, cracking (superficially), and grinding. Sharp-edged material rolled on a pebbly floor chips during the first 2-10 km. of transport, losing up to five times as much as by cracking, which is the main process later on. On a sandy bed, only grinding takes place. Roundness measurements by Cailleux's method tend to give values that are too high because of parallax, different investigators obtaining different results. Cailleux's method could be somewrhat improved by using the middle instead of the longest axis. It should be realized that a pebble will pass relatively very swiftly through the first roundness class and still quite fast through the second and third, then more slowly, until, beyond about the fourth class (= roundness 200), the passage from class to class, on an unchanging floor, will require greater and greater distances of rolling until the ultimate shape is attained. The conclusion is reached that splitting is rarely the result of impact, except in poorly consolidated or fissile rock, but breaking does occur and must be attributed to weathering, especially frost action, on pebbles while they form part of alluvial deposits alongside the stream course. Poser and Hovermann studied rounding in rivers of the Harz. The present investigation shows that abrasion of the graywackes they studied is hastened at the start because the solifluction material is superficially weathered. In the experiments on a pebbly floor the abrasion was about four times as much per kilometer as in nature. This indicates that in these natural strearrs much of the bed rray be covered with sand.