Abrasional shaping of basalt pebbles was studied in nine rivers and on 14 high- and low-energy beaches of Tahiti-Nui. Measurements were made on four size classes, ranging from 16 to 256 mm. Surf ranged from 10-ft combers at Punaauia to few-inch ripples at Point Venus.

Roundness was measured by sharpest-corner/inscribed circle, adapted from Wentworth (1922); fluvial pebbles had mean roundness of .38 whereas beach pebbles averaged .45-.50 in low-energy areas, and .60 in high-energy areas.

Maximum-projection sphericity (Sneed and Folk, 1958) averaged .68 for river pebbles, and .60 for beach pebbles. A new measure of oblateness-prolateness, OP = L/S (L-I/L-S - .50), is introduced; river pebbles averaged OP = +0.2 (neutral), and beaches OP = -1.7 (oblate), with greatest difference between the two environments shown by the smallest pebbles (16-32 mm). Using isotropic rock of uniform basaltic composition, there is no doubt that, on Tahiti, accumulations of beach pebbles distinctively have lower sphericity and more discoidal form compared with river pebbles.

The above generalities are modified by some complicating factors. On some high-energy beaches consisting almost entirely of coarse gravel (e.g., Papenoo and Punaauia), large pebbles (128-256 mm) have lowest sphericity (.55) whereas small pebbles average .68; apparently the waves are so large that they can slide the large pebbles, but toss the small ones randomly. On some low-energy beaches (e.g., Maraa, Arue, Venus), the smallest pebbles (16-32 mm) are nearly coin-shaped (sph. = .54, OP = -5, strongly oblate) because they are able to be slid by the gentlest waves, whereas large pebbles relatively are immobile and do not achieve typical beach shapes (sph. = .67, OP = +1, weakly prolate). Thus the particular pebble size at which most extreme oblateness and lowest sphericity occur is a m asure of wave energy.

Where pebbles are scattered on a dominantly sandy beach (Mahaiatea, Papeiha), abnormally great concentrations of low-sphericity discs occur. On sand beaches with gravel cusps (Taaone), discs are scarce in the gravel cusps but abundant in the sandy intervening zones. In a few areas discs tend to be thrown far back on the beach, and rods and equants accumulate at the beach foot. Despite the definite evidence of shape-sorting in some localities, the great increase of discs in the beach pebbles as a whole, compared with the river pebbles as a whole, must be the result of surf abrasion. The change in shape is accomplished within distances of a few feet to a few hundred feet of the river mouth.

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