A Field Study of the Roundness of River Pebbles

The investigation was made in the river bad of the Asa River and Iruma River. The method of measuring is as follow : First river gravels were analysed mechanically, and next pebbles are divided in four classes, those of 64-. 32-. 16-. And 8-mm. in diameter respectively and than the roundness of about 150 graywacke pebbles of each class was measured by the method developed by Wadell. The results are as follow : (1) The roundness diagrams show that the action of chipping takes place when gravels are rolled in the upper reaches and grinding takes place in the down reaches. It is therefore considered highly improbable that the rigor in natural stream transportation is sufficient for splitting or crushing. (2) The author attributes great importance to the longitudal slope of the river bed : the less it is, the weaker abrasion process. For these reasons, he attributes swift rounding to high velocity