Conclusions in Regard to Gravel Beaches.
1. Much of the coarse material that is contributed to the beach is eliminated by wave action.
2. All sand in the original mixed feed that is finer than 5.0 mm. is quickly eliminated. Generally additions are being made to this for some distance from the point where the feeding takes place; for a river carries most of the fine matter out to sea and the wave action soon returns it to the beach where it is quickly reduced by impact and ground up.
3. There is a constant decrease of grade in the beach material in the direction of drift along the beach.
4. Pebbles that are larger than ½ inch in diameter are worn flat on these beaches.
5. Pebbles that are smaller than ½ inch in diameter are rounded because the wave action is sufficiently strong to suspend pebbles of this size and they do not slide along the beach. If wave action were less intense flattening would continue with pebbles of a smaller size. Since a long time and much wear is required for the round shape to supersede the flat form some remnants of the flattened shape will be found in pebbles far smaller than ½ inch diameter.
6. Until the gravel is very fine practically all the smaller grades on a beach owe their smallness to impact, not to abrasion (in other words fragments have been chipped off them). This is proved by the general angular form of the smaller pieces. Under the conditions that obtain on Hawkes Bay beaches pebbles are able to fracture those smaller ones that have one fifth of their diameter.
7. Abrasion takes place with extreme slowness when a beach is composed of little pebbles finer than 3.4 mm. in diameter. This is proved by the highly polished surfaces of all the pebbles in beaches composed of such material.
8. The almost complete absence of grains smaller than 0.42 mm., after the destruction of that brought down by the feed, except for
a small amount that is subsequently introduced, proves that sand is not formed by beach action.
9. The remarkably even grade of the finer gravel or coarse sand on beaches is thus shown to be due to:—
Elimination of the coarsest material by wave action.
Destruction of the sand in a mixed gravel by grinding.
The action of impact which in the Hawkes Bay beaches, in consequence of the heavy surf, maintains the limits of the grades between the proportions of 5 to 1. That this is due to impact is clearly demonstrated by the fact that the finest grade in any beach sample is always angular. This bears out in a precise manner the results of experimental abrasion in the Deval machine.
Abrasion which acts far more rapidly with coarse than fine and thus with the lapse of time tends to make the difference between the coarse and the fine grade less and less.
These conclusions are opposed to those of Vaughan Cornish (loc. cit. p. 541) who ascribes the even grading to “the washing away of the particles below a certain critical size.” It is clear that this explanation fails in regard to the Hawkes Bay beaches which grade with a progressive decrease throughout their length. The question of washing away of particles will be considered under the heading of “sand.”