There have been made public many papers on the roundness of gravel, but no work has so far been done on various sizes of gravels over a long distance in field. However, in order to make clear the relations between roundness and transportational agency it is necessary to investigate these factors. The purpose of this paper is to clarify the change in roundness of various sizes of pebbles over a distance and the relations between roundness and size in particular stations.
The investigation was made in the river-bed of the Tanla River extending over about 60km. from Hikawva to Futako-Tamagawa and 12 sampling stations were selected there. The mathod of measuring is as follows: First samples are divided into four clesses, those of 61_??_. 32_??_. 16_??_. and 8_??_mm. in diameter respectively, and next, about 150 graywacke pebbles of each class were photographed and then the roundness was measured by the method developed by Wadell. The results are as follow:
(1) The number of pebbles necessary for measuring mean roundness was determined in the following way. At Ome, where the frequency of roundness seems to be most complex, the relations among number and mean roundness and confidence interval were elucidated (Tab. 1). The table shows that the smallest number with small confidence interval is approximately 150.
(2) The roundness of pebbles is not always the function of transported distance as shown in fig. 2. Where the detritus from valley walls are mixed with transported gravels in the upper reaches, the roundness decreases abruptly. The location where the roundnesss suddenly decreases is different according to pebble size. For the size of detrital materials differs according to the density of crack or joint spacing which differs as the outcropping locality differs.
(3) In the lower reaches there are locations where the roundess of pebbles increases abruptly. The location where the roundness increases abruptly lies in comparatively upper streamn in case of larger pebbles and in comparatively lower reaches incase of smaller pebbles. It is probable that laorger rabbles change first the mode of heir downward shifting by decrease of the gradient of river course and friction acts on them earlier than it does on smaller pebbles.
(4) The relation between size and roundness is not always expressed as y=mxn, where y is roundness, x is size, n is a coefficient, and. in is a constantas shown in fig. 3. In the upper reaches where the detritus from the valley walls are mixed with river gravels, no relation like this can be found or the converse relation is observed. At each sampling station in the river-bed from Ome to Futako-Tanzagawa, roundness of pebbles of 8_??_4mm. in size is remarkably lower than that of larger pebbles. This fact suggests that pebbles of a size of 8_??_4mm. are transported by saltation, while the larger pebbles by rolling or sliding in the reaches above-mentioned.
