I know that this got linked to yesterday, but these are only a few minutes from my house, so I went during lunch today and took pictures:
The spot where these are at is right at the modern tide line, but I would have expected it to be much lower in the past (the tide line). Its also a pretty different material type the petroglyphs are in. Most of our beach rock is either basalt or coral. But in this particular spot its a combination of some sandstone and some conglomerate. There is also some very weird erosion right off shore, that isn't like any other coastal erosion (more channelized, more smooth than basalt or fossil corral). Its a pretty unique spot considering the entire coast, because sandstone of any kind is actually pretty rare here.
Pokai bay is basically the main mouth of the Waiʻanae valley, and unlike many of our beaches where it has coarse, coral sand, this is smooth rock sand. There is an Hawaiian old fish pond near by and a heiau at the bay.
This is really fascinating and the article was a good read. I had no idea carvings this adjacent to the ocean could last that long. On that fact alone, this area should be protected and recorded as best we can. What I know of preservation, my first instinct upon reading that a historian was pushing sand into the petroglyphs to highlight them was horror! But duh, what the hell is she doing that the ocean hasn't been doing for centuries. And the glyphs still remain! So cool, thanks for sharing.
So I think they spend almost all their time under sand. I don't think many people realize how seriously and constantly the beach reconstructs itself.
When they do appear, its for a few weeks, then a few decades under sand. IF they were facing constant wave action, they would be gone very quickly (I think). Its not particularly hard rock they are carved in.